On Fri, Nov 05, 2004 at 11:38:22PM -0800, Adam J. Richter wrote: > The following patch implements a facility for invoking a user > level helper program whenever a program attempts to access a > nonexistent file on a given tmpfs file system. It can provide > functionality like autofs or devfs. At 1708 bytes of object code, it > should result in smaller implementations when it can be used, and > may avoid some potential recursion problems. A patch that shrinks > devfs by much more than the size of this facility depends on it, > so I would like to get it into the kernel soon.
What's the status of this patch - do you still plan to push it into
the kernel?
> Documentation/filesystems/lookup-trap.txt provides cookbook
> examples of how to use lookup traps with scripts or short C programs
> to for a variety of uses. For example, here a way to create a create
> a simple automounter for remote file systems:
>
> % cat > /usr/sbin/tmpfs-automount
> #!/bin/sh
> mkdir $3
> mount -t nfs $3:/ $1/$3
> ^D
> % chmod a+x /usr/sbin/tmpfs-automount
> % mkdir /auto
> % mount -t tmpfs -o helper="/usr/sbin/tmpfs-automount /auto" x /auto
>
>
> I will soon post a small devfs replacement based on this
> facility. The lookup traps and the devfs patch are an evolution of
> the shrunken devfs reimplementation that I posted almost two years
> ago, which I have been running continuously on a number of computers
> ever since, although the use of tmpfs is only a few days old.
>
> The implementation is small. The total increase in object
> code size on x86 is 1708 bytes (mm/shmem.o change + fs/userhelper.o +
> fs/lookuptrap.o), and I've made it a configuration option, so people
> who don't want it will only be victimized by 180 object bytes of
> changes I had to make outside of the ifdef's to mm/shmem.c (those
> changes are counted in those 1708 byte tally, by the way).
>
> By line count, if you don't count the Documentation
> subdirectory, the devfs replacement and lookup traps taken together
> are actually a net reduction of more than 1800 lines of code from the
> kernel. The lookup traps patch taken by itself is a net increase of
> 533 lines of code, most of which is in two files outside the tmpfs
> sources to facilitate possible use by other file systems. In
> comparison, fs/autofs/*.[ch] is more than triple that line count.
>
> Over the past few days, I have posted earlier versions of
> these patches on linux-fsdevel, originally under the name "trapfs."
> This latest patch incorporate feedback expressed by a few people on
> that list. Greg Kroah-Hartmann prefered the facility being a tmpfs
> option, which I saw some advantages to as well. Mike Waychison and
> Matthew Wilcox got me to take a harder look at a user level race
> condition that I mentioned in the original posting, which eventually
> led to an apparent solution that is incorporated into this version.
>
> Finally, I should mention a limitation and couple of
> deficiencies that I'm aware of with respect to lookup traps to save
> everyone some discussion time.
>
> 1. As Mike Waychison pointed out, this facility in tmpfs
> cannot be used to trap accesses to the root of a file system or create
> directories and then trap accesses when a program attempts to "cd" in.
> Such functionality could provide a more consistent and informative
> user interface for an automatic mounter of a small to moderate number
> of remote file systems, in the style of autofs. The good news is that
> this patch provides a facility in fs/userhelper.c that could be called
> by another file system to provide functionality on
> inode_operations->follow_link() events similar to what this patch does
> for inode_operations->lookup() under tmpfs, hopefully allowing such an
> implementation to be smaller.
>
> 2. Like sysfs and ramfs, tmpfs uses a struct inode and a
> struct dentry for every file system node, consuming something like 500
> bytes per node. A backing store patch similar to the sysfs backing
> store patch could dramatically reduce the memory used by the
> tmpfs-based devfs, implementation. the devfs application of ,
> although devfs file systems tend to have fewer nodes, since they are
> created on demand, and devfs memory consumption pales in comparison to
> sysfs. I think that at some point in the future, it might be useful
> to implement some kind of release of struct inode's for device files
> at least, similar to the "sysfs backing store" patch that is supposed
> to be on its way into the stock kernel.
>
> 3. Until the trapfs helper exits, it is impossible to
> control-C out of the access that invoked the helper. This is a
> deficiency of the synchronous call_usermodehelper interface. Every
> kernel facility that uses call_usermodehelper has this problem. There
> are a number of ways to fix synchronous call_usermodehelper, and I
> surely expect trapfs to use whatever solution is implemented.
>
> Any feedback would be welcome.
>
> __ ______________
> Adam J. Richter \ /
> [EMAIL PROTECTED] | g g d r a s i l
>
>
> --- linux-2.6.10-rc1-bk14/include/linux/shmem_fs.h 2004-10-18
> 14:54:55.000000000 -0700
> +++ linux/include/linux/shmem_fs.h 2004-11-05 23:24:20.000000000 -0800
> @@ -1,8 +1,10 @@
> #ifndef __SHMEM_FS_H
> #define __SHMEM_FS_H
>
> +#include <linux/config.h>
> #include <linux/swap.h>
> #include <linux/mempolicy.h>
> +#include <linux/fsuserhelper.h>
>
> /* inode in-kernel data */
>
> @@ -22,11 +24,15 @@
> };
>
> struct shmem_sb_info {
> + int limited; /* 0 = ignore max_blocks and max_inodes */
> unsigned long max_blocks; /* How many blocks are allowed */
> unsigned long free_blocks; /* How many are left for allocation */
> unsigned long max_inodes; /* How many inodes are allowed */
> unsigned long free_inodes; /* How many are left for allocation */
> spinlock_t stat_lock;
> +#ifdef CONFIG_TMPFS_LOOKUP_TRAPS
> + char *helper_shell_command;
> +#endif
> };
>
> static inline struct shmem_inode_info *SHMEM_I(struct inode *inode)
> @@ -34,4 +40,6 @@
> return container_of(inode, struct shmem_inode_info, vfs_inode);
> }
>
> +extern int __init init_tmpfs(void); /* early initialization for devfs */
Adding this hook probably belongs to the devfs-replacement patch,
which would be applied on top of the lookup-trap patch.
> +
> #endif
> --- linux-2.6.10-rc1-bk14/include/linux/fsuserhelper.h 1969-12-31
> 16:00:00.000000000 -0800
> +++ linux/include/linux/fsuserhelper.h 2004-11-05 23:24:21.000000000
> -0800
> @@ -0,0 +1,15 @@
> +#ifndef _LINUX_FS_HELPER
> +#define _LINUX_FS_HELPER
> +
> +#include <linux/dcache.h>
> +#include <linux/rwsem.h>
> +
> +/*
> + Note: call_fs_helper releases and retakes dentry->d_parent->d_inode->i_sem.
> +*/
This comment style (which is used in your patch all over the place)
does not match the style used in the rest of the kernel.
> +extern void call_fs_helper(char **comand_str_ptr,
Typo: command_str_ptr (and in the actual definition this parameter is
named "command_ptr").
> + struct rw_semaphore *command_rwsem,
> + const char *event,
> + struct dentry *dentry);
> +
> +#endif /* _LINUX_FS_HELPER */
> --- linux-2.6.10-rc1-bk14/include/linux/lookuptrap.h 1969-12-31
> 16:00:00.000000000 -0800
> +++ linux/include/linux/lookuptrap.h 2004-11-05 23:24:22.000000000 -0800
> @@ -0,0 +1,12 @@
> +#ifndef _LINUX_LOOKUPTRAP_H
> +#define _LINUX_LOOKUPTRAP_H
> +
> +#include <linux/fs.h>
> +#include <linux/dcache.h>
> +
> +extern struct dentry *trapping_lookup(struct inode *dir,
> + struct dentry *dentry,
> + struct nameidata *nd,
> + char **shell_command_ptr);
> +
> +#endif /* _LINUX_LOOKUPTRAP_H */
> --- linux-2.6.10-rc1-bk14/mm/shmem.c 2004-11-04 23:32:44.000000000 -0800
> +++ linux/mm/shmem.c 2004-11-05 23:24:23.000000000 -0800
> @@ -14,6 +14,9 @@
> * Copyright (c) 2004, Luke Kenneth Casson Leighton <[EMAIL PROTECTED]>
> * Copyright (c) 2004 Red Hat, Inc., James Morris <[EMAIL PROTECTED]>
> *
> + * User level helper for directory lookups:
> + * Copyright (C) 2004 Adam J. Richter, Yggdrasil Computing, Inc.
> + *
> * This file is released under the GPL.
> */
>
> @@ -46,6 +49,8 @@
> #include <linux/mempolicy.h>
> #include <linux/namei.h>
> #include <linux/xattr.h>
> +#include <linux/lookuptrap.h>
> +#include <linux/parser.h>
> #include <asm/uaccess.h>
> #include <asm/div64.h>
> #include <asm/pgtable.h>
> @@ -135,7 +140,20 @@
>
> static inline struct shmem_sb_info *SHMEM_SB(struct super_block *sb)
> {
> +#ifdef CONFIG_TMPFS
> return sb->s_fs_info;
> +#else
> + return NULL; /* compiler optimization */
> +#endif
> +}
> +
> +static inline int shmem_have_quotas(struct shmem_sb_info *sb_info)
> +{
> +#ifdef CONFIG_TMPFS
> + return sb_info->limited;
> +#else
> + return 0; /* sb_info will be NULL. */
> +#endif
> }
>
> /*
> @@ -194,7 +212,8 @@
> static void shmem_free_blocks(struct inode *inode, long pages)
> {
> struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
> - if (sbinfo) {
> +
> + if (shmem_have_quotas(sbinfo)) {
> spin_lock(&sbinfo->stat_lock);
> sbinfo->free_blocks += pages;
> inode->i_blocks -= pages*BLOCKS_PER_PAGE;
> @@ -357,7 +376,7 @@
> * page (and perhaps indirect index pages) yet to allocate:
> * a waste to allocate index if we cannot allocate data.
> */
> - if (sbinfo) {
> + if (shmem_have_quotas(sbinfo)) {
> spin_lock(&sbinfo->stat_lock);
> if (sbinfo->free_blocks <= 1) {
> spin_unlock(&sbinfo->stat_lock);
> @@ -678,7 +697,7 @@
> spin_unlock(&shmem_swaplist_lock);
> }
> }
> - if (sbinfo) {
> + if (shmem_have_quotas(sbinfo)) {
> BUG_ON(inode->i_blocks);
> spin_lock(&sbinfo->stat_lock);
> sbinfo->free_inodes++;
> @@ -1081,7 +1100,7 @@
> } else {
> shmem_swp_unmap(entry);
> sbinfo = SHMEM_SB(inode->i_sb);
> - if (sbinfo) {
> + if (shmem_have_quotas(sbinfo)) {
> spin_lock(&sbinfo->stat_lock);
> if (sbinfo->free_blocks == 0 ||
> shmem_acct_block(info->flags)) {
> @@ -1269,7 +1288,7 @@
> struct shmem_inode_info *info;
> struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
>
> - if (sbinfo) {
> + if (shmem_have_quotas(sbinfo)) {
> spin_lock(&sbinfo->stat_lock);
> if (!sbinfo->free_inodes) {
> spin_unlock(&sbinfo->stat_lock);
> @@ -1598,7 +1617,7 @@
> buf->f_type = TMPFS_MAGIC;
> buf->f_bsize = PAGE_CACHE_SIZE;
> buf->f_namelen = NAME_MAX;
> - if (sbinfo) {
> + if (shmem_have_quotas(sbinfo)) {
> spin_lock(&sbinfo->stat_lock);
> buf->f_blocks = sbinfo->max_blocks;
> buf->f_bavail = buf->f_bfree = sbinfo->free_blocks;
> @@ -1650,6 +1669,19 @@
> return shmem_mknod(dir, dentry, mode | S_IFREG, 0);
> }
>
> +#ifdef CONFIG_TMPFS_LOOKUP_TRAPS
> +static struct dentry * shmem_lookup(struct inode *dir,
> + struct dentry *dentry,
> + struct nameidata *nd)
> +{
> + struct shmem_sb_info *sbinfo = SHMEM_SB(dir->i_sb);
> +
> + return trapping_lookup(dir, dentry, nd, &sbinfo->helper_shell_command);
> +}
> +#else
> +# define shmem_lookup simple_lookup
> +#endif
> +
> /*
> * Link a file..
> */
> @@ -1663,7 +1695,7 @@
> * but each new link needs a new dentry, pinning lowmem, and
> * tmpfs dentries cannot be pruned until they are unlinked.
> */
> - if (sbinfo) {
> + if (shmem_have_quotas(sbinfo)) {
> spin_lock(&sbinfo->stat_lock);
> if (!sbinfo->free_inodes) {
> spin_unlock(&sbinfo->stat_lock);
> @@ -1688,7 +1720,7 @@
>
> if (inode->i_nlink > 1 && !S_ISDIR(inode->i_mode)) {
> struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
> - if (sbinfo) {
> + if (shmem_have_quotas(sbinfo)) {
> spin_lock(&sbinfo->stat_lock);
> sbinfo->free_inodes++;
> spin_unlock(&sbinfo->stat_lock);
> @@ -1840,7 +1872,7 @@
> #endif
> };
>
> -static int shmem_parse_options(char *options, int *mode, uid_t *uid, gid_t
> *gid, unsigned long *blocks, unsigned long *inodes)
> +static int shmem_parse_options(char *options, int *mode, uid_t *uid, gid_t
> *gid, unsigned long *blocks, unsigned long *inodes, substring_t *helper)
> {
> char *this_char, *value, *rest;
>
> @@ -1894,6 +1926,9 @@
> *gid = simple_strtoul(value,&rest,0);
> if (*rest)
> goto bad_val;
> + } else if (!strcmp(this_char,"helper")) {
> + helper->from = value;
> + helper->to = value + strlen(value);
> } else {
> printk(KERN_ERR "tmpfs: Bad mount option %s\n",
> this_char);
> @@ -1909,32 +1944,73 @@
>
> }
>
> +#ifdef CONFIG_TMPFS_LOOKUP_TRAPS
> +static int
> +maybe_replace_from_substr(char **target, substring_t *substr)
> +{
> + char *str;
> +
> + if (substr->from == NULL)
> + return 0;
> +
> + if (substr->from == substr->to)
> + str = NULL;
> + else {
> + str = match_strdup(substr);
> + if (!str)
> + return -ENOMEM;
> + }
> + kfree(*target);
> + *target = str;
> + return 0;
> +}
> +#endif /* CONFIG_TMPFS_LOOKUP_TRAPS */
> +
> static int shmem_remount_fs(struct super_block *sb, int *flags, char *data)
> {
> struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
> unsigned long max_blocks = 0;
> unsigned long max_inodes = 0;
> + substring_t helper_str;
>
> - if (sbinfo) {
> + if (shmem_have_quotas(sbinfo)) {
> max_blocks = sbinfo->max_blocks;
> max_inodes = sbinfo->max_inodes;
> }
> - if (shmem_parse_options(data, NULL, NULL, NULL, &max_blocks,
> &max_inodes))
> + helper_str.from = NULL;
> + if (shmem_parse_options(data, NULL, NULL, NULL, &max_blocks,
> &max_inodes, &helper_str))
> return -EINVAL;
> +
> +#ifdef CONFIG_TMPFS_LOOKUP_TRAPS
> + if (maybe_replace_from_substr(&sbinfo->helper_shell_command,
> + &helper_str) != 0)
> + return -ENOMEM;
> +#endif
> +
> /* Keep it simple: disallow limited <-> unlimited remount */
> - if ((max_blocks || max_inodes) == !sbinfo)
> + if ((max_blocks || max_inodes) != shmem_have_quotas(sbinfo))
> return -EINVAL;
At this point helper_shell_command was already changed. Having a
possibility that a failed remount might still change some filesystem
parameters does not seem good.
> +
> /* But allow the pointless unlimited -> unlimited remount */
> - if (!sbinfo)
> + if (!max_blocks && !max_inodes)
> return 0;
> +
> return shmem_set_size(sbinfo, max_blocks, max_inodes);
> }
> -#endif
> +#endif /* CONFIG_TMPFS */
>
> static void shmem_put_super(struct super_block *sb)
> {
> - kfree(sb->s_fs_info);
> - sb->s_fs_info = NULL;
> +#ifdef CONFIG_TMPFS
> + struct shmem_sb_info *sb_info = SHMEM_SB(sb);
> +
> +# ifdef CONFIG_TMPFS_LOOKUP_TRAPS
> + kfree(sb_info->helper_shell_command);
> +# endif
> + kfree(sb_info);
> +#endif
#ifdef inside functions are generally frowned upon; you have two
nested ones here :(
> +
> + sb->s_fs_info = NULL; /* FIXME. Is this line necessary? */
> }
>
> #ifdef CONFIG_TMPFS_XATTR
> @@ -1954,14 +2030,17 @@
> int err = -ENOMEM;
>
> #ifdef CONFIG_TMPFS
> + substring_t helper_str;
> unsigned long blocks = 0;
> unsigned long inodes = 0;
> + struct shmem_sb_info *sbinfo;
>
> /*
> * Per default we only allow half of the physical ram per
> * tmpfs instance, limiting inodes to one per page of lowmem;
> * but the internal instance is left unlimited.
> */
> + helper_str.from = NULL;
This line divides apart the comment and the code it was attached to.
> if (!(sb->s_flags & MS_NOUSER)) {
> blocks = totalram_pages / 2;
> inodes = totalram_pages - totalhigh_pages;
> @@ -1969,24 +2048,32 @@
> inodes = blocks;
>
> if (shmem_parse_options(data, &mode,
> - &uid, &gid, &blocks, &inodes))
> + &uid, &gid, &blocks, &inodes,
> + &helper_str))
> return -EINVAL;
> }
>
> - if (blocks || inodes) {
> - struct shmem_sb_info *sbinfo;
> - sbinfo = kmalloc(sizeof(struct shmem_sb_info), GFP_KERNEL);
> - if (!sbinfo)
> - return -ENOMEM;
> - sb->s_fs_info = sbinfo;
> - spin_lock_init(&sbinfo->stat_lock);
> - sbinfo->max_blocks = blocks;
> - sbinfo->free_blocks = blocks;
> - sbinfo->max_inodes = inodes;
> - sbinfo->free_inodes = inodes;
> - }
> + sbinfo = kmalloc(sizeof(struct shmem_sb_info), GFP_KERNEL);
> + if (!sbinfo)
> + return -ENOMEM;
> +
> + sbinfo->limited = (blocks || inodes);
> + sb->s_fs_info = sbinfo;
> + spin_lock_init(&sbinfo->stat_lock);
> + sbinfo->max_blocks = blocks;
> + sbinfo->free_blocks = blocks;
> + sbinfo->max_inodes = inodes;
> + sbinfo->free_inodes = inodes;
> +
> +# ifdef CONFIG_TMPFS_LOOKUP_TRAPS
> + sbinfo->helper_shell_command = NULL;
> + if (maybe_replace_from_substr(&sbinfo->helper_shell_command,
> + &helper_str) != 0)
> + return -ENOMEM;
Leak of sbinfo.
> +# endif
> +
> sb->s_xattr = shmem_xattr_handlers;
> -#endif
> +#endif /* CONFIG_TMPFS */
>
> sb->s_maxbytes = SHMEM_MAX_BYTES;
> sb->s_blocksize = PAGE_CACHE_SIZE;
> @@ -2088,7 +2175,7 @@
> static struct inode_operations shmem_dir_inode_operations = {
> #ifdef CONFIG_TMPFS
> .create = shmem_create,
> - .lookup = simple_lookup,
> + .lookup = shmem_lookup,
> .link = shmem_link,
> .unlink = shmem_unlink,
> .symlink = shmem_symlink,
> @@ -2192,9 +2279,15 @@
> };
> static struct vfsmount *shm_mnt;
>
> -static int __init init_tmpfs(void)
> +/* init_tmpfs is exported so that devfs can get an earlier initialization
> + if necessary. */
> +int __init init_tmpfs(void)
Again, making this function non-static belongs to another patch where
it will actually be used.
> {
> int error;
> + static int initialized; /* = 0 */
__initdata (but as noted above, these additions should not be in this
patch anyway).
> +
> + if (initialized)
> + return 0;
>
> error = init_inodecache();
> if (error)
> @@ -2215,6 +2308,7 @@
> printk(KERN_ERR "Could not kern_mount tmpfs\n");
> goto out1;
> }
> + initialized = 1;
> return 0;
>
> out1:
> @@ -2310,3 +2404,5 @@
> vma->vm_ops = &shmem_vm_ops;
> return 0;
> }
> +EXPORT_SYMBOL(shmem_lock);
> +EXPORT_SYMBOL(shmem_nopage);
What do you need this for? These symbols are not used anythere in the
patch.
> --- linux-2.6.10-rc1-bk14/fs/Kconfig 2004-11-04 23:32:39.000000000 -0800
> +++ linux/fs/Kconfig 2004-11-05 23:24:24.000000000 -0800
> @@ -940,6 +940,23 @@
>
> If unsure, say N.
>
> +config TMPFS_LOOKUP_TRAPS
> + bool "tmpfs lookup trapping"
> + depends on TMPFS
> + help
> + This facility allows you to configure a tmpfs file system to
> + invoke a user level helper program whenever an attempt is made
> + to access a nonexistent file, enabling these helper programs
> + to define file systems are filled in on demand. There
> + is a version of devfs that requires it, and it can be
> + used to implement certain types of automatic mounting
> + of other file systems. It is a daemonless and, perhaps
> + consequently, smaller (under 2kB object on x86) alternative
> + to autofs for certain uses, although it cannot be used as a
> + replacement in all cases, and it is not available as a loadable
> + module (because tmpfs isn't). For examples and more information,
> + please see <file:Documentations/filesystems/lookup-trap.txt>.
> +
> config TMPFS_SECURITY
> bool "tmpfs Security Labels"
> depends on TMPFS_XATTR
> --- linux-2.6.10-rc1-bk14/fs/Makefile 2004-11-04 23:32:39.000000000 -0800
> +++ linux/fs/Makefile 2004-11-05 23:24:25.000000000 -0800
> @@ -44,6 +44,10 @@
> obj-y += devpts/
>
> obj-$(CONFIG_PROFILING) += dcookies.o
> +
> +ifdef CONFIG_TMPFS_LOOKUP_TRAPS
> + obj-$(CONFIG_TMPFS) += lookuptrap.o userhelper.o
> +endif
Just obj-$(CONFIG_TMPFS_LOOKUP_TRAPS) will do - it depends on
CONFIG_TMPFS anyway, no need to duplicate this logic in Makefile.
>
> # Do not add any filesystems before this line
> obj-$(CONFIG_REISERFS_FS) += reiserfs/
> --- linux-2.6.10-rc1-bk14/fs/lookuptrap.c 1969-12-31 16:00:00.000000000
> -0800
> +++ linux/fs/lookuptrap.c 2004-11-05 23:24:26.000000000 -0800
> @@ -0,0 +1,190 @@
> +/*
> + struct dentry *trapping_lookup(struct inode *dir,
> + struct dentry *dentry,
> + struct nameidata *nd,
> + char **shell_command_ptr)
> +
> + trapping_lookup() is an alternative to simple_lookup() (from libfs.c),
> + that adds the ability to invoke a user level helper program when
> + an attempt is made to access a nonexistant file. It invokes the
> + command via call_fs_helper() from fs/helper.c.
> +
> + trapping_lookup takes one parameter in addition to the parameters
> + that an inode_operations->lookup method requires. This paratmer,
> + shell_command_ptr is the address of a pointer to a string containing
> + the command to be executed trapping_lookup will always take a
> + read lock on superblock->s_umount, so it is multiprocessor-safe for your
> + file system's superblock mount and remount routines to modify that
> + address, since mount and remount will take a write lock on
> + superblock->s_umount.
> +
> + See tmpfs in mm/shmem.c for an example of use of trapping_lookup.
> +
> + trapping_lookup() is the only symbol exported from this file.
> +
> +
> + Written by Adam J. Richter
> + Copyright (C) 2004 Yggdrasil Computing, Inc.
> +
> + This program is free software; you can redistribute it and/or modify
> + it under the terms of the GNU General Public License as published by
> + the Free Software Foundation; either version 2 of the License, or
> + (at your option) any later version.
> +
> + This program is distributed in the hope that it will be useful,
> + but WITHOUT ANY WARRANTY; without even the implied warranty of
> + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> + GNU General Public License for more details.
> +
> + You should have received a copy of the GNU General Public License
> + along with this program; if not, write to the Free Software
> + Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
> + */
Most other files use this format for the initial comment:
- one-line description of the file
- copyright notice
- GPL boilerplate (if present at all)
- more verbose description if needed
> +
> +#include <linux/module.h>
> +#include <linux/fs.h>
> +#include <linux/dcache.h>
> +#include <linux/wait.h>
> +#include <linux/fsuserhelper.h>
> +
> +static inline int want_to_trap(struct nameidata *nd)
> +{
> + return (nd != NULL);
> +}
> +
> +
> +/*
> + * Retaining negative dentries for an in-memory filesystem just wastes
> + * memory and lookup time: arrange for them to be deleted immediately.
> + */
> +static int always_delete_dentry(struct dentry *dentry)
> +{
> + return 1;
> +}
> +
> +/* Force revalidation of all negative dentries. Note that this routine
> + only gets called when some other routine has a reference to the dentry
> + or the dentry has an inode. Otherwise, unused negative dentries
> + are immediately dropped, because of always_delete_dentry. Apparently,
> + the only time blocking_dentry_valid ends up being called with an empty
> + inode is when a trapped reference is blocking on the user level helper.
> +*/
> +static int blocking_dentry_valid(struct dentry *dentry,
> + struct nameidata *nd)
> +{
> + struct wait_queue_head *queue_head;
> + struct semaphore *parent_inode_sem;
> + DEFINE_WAIT(my_wait);
> +
> + if (dentry->d_inode != NULL)
> + return 1;
> +
> + if (!want_to_trap(nd))
> + return 0;
> +
> + /* FIXME? I think there is no need to use dget_parent or
> + to take dcache_lock here, because dentry->d_parent never
> + changes and the dentry holds a reference to its parent,
> + and dentry->d_parent holds a reference to
> + dentry->d_d_parent->d_inode. Right??? -AJR 2004.11.05 */
> + parent_inode_sem = &dentry->d_parent->d_inode->i_sem;
> +
> + down(parent_inode_sem);
> +
> + queue_head = dentry->d_fsdata;
> + if (!queue_head) {
> + up(parent_inode_sem);
> + return (dentry->d_inode != NULL);
> + }
> +
> + prepare_to_wait(queue_head, &my_wait, TASK_INTERRUPTIBLE);
> + up(parent_inode_sem);
> + schedule();
> + finish_wait(queue_head, &my_wait);
> +
> + /* At this point, maybe the queue was woken up or maybe we got a
> + signal. There is no need to check, because we just return
> + in either case. */
> +
> + return (dentry->d_inode != NULL);
> +
> +}
> +
> +
> +static struct dentry_operations trapping_dentry_ops = {
> + .d_delete = always_delete_dentry,
> + .d_revalidate = blocking_dentry_valid,
> +};
> +
> +struct dentry *
> +trapping_lookup(struct inode *dir,
> + struct dentry *dentry,
> + struct nameidata *nd,
> + char **shell_command_ptr)
> +{
> + /*
> + We must do d_add before call_fs_helper to prevent
> + a duplicate dentry from being created if the helper
> + program attempts to access the same file name in /dev.
> + If simple_lookup returns non-NULL, then that is to an error
> + like a malformed file name, so we do not invoke trapfs_event.
> + If the file is not found but there was no other error,
> + simple_lookup returns NULL, and that is the only case
> + in which we want to generate a notification.
> +
> + We also filter out the final path element of mknod, mkdir
> + and symlink, because invoking the helper for mknod and mkdir
> + could lead to deadlock when trapfs loads a device driver
> + kernel module than. One would think that the way to filter
> + would be to look at nd->flags to check that LOOKUP_CREATE
> + is set and LOOKUP_OPEN is clear, but instead, the vfs
> + layers passed nd==NULL in these cases via a routine
> + called lookup_create (without any leading underscores),
> + so we filter out the case where nd == NULL.
> +
> + Filtering out nd==NULL has the unintented side-effect of
> + filtering out the final path component of arguments to
> + rmdir, unlink and rename (both source and destination).
> + For rmdir, unlink, and the source arguement to rename,
> + that's fine, since nobody cares about attempts to remove
> + nonexistant files. We're probably also OK skipping the
> + notifications with regard to the destination argument to
> + rename, although that is less clear.
> + */
> +
> + struct dentry *result;
> + wait_queue_head_t queue_head;
> + struct dentry *new;
> +
> + if (!want_to_trap(nd))
> + return simple_lookup(dir, dentry, nd);
> +
> + if (dentry->d_name.len > NAME_MAX)
> + return ERR_PTR(-ENAMETOOLONG);
> +
> + init_waitqueue_head(&queue_head);
> +
> + dentry->d_fsdata = &queue_head;
> + dentry->d_op = &trapping_dentry_ops;
> +
> + d_add(dentry, NULL);
> +
> + call_fs_helper(shell_command_ptr, &dir->i_sb->s_umount, "LOOKUP",
> + dentry);
> +
> + new = d_lookup(dentry->d_parent, &dentry->d_name);
> +
> + if (new != dentry) /* also handles new==NULL */
> + result = new;
> + else {
> + dput(new);
> + result = NULL;
> + }
> +
> + wake_up(&queue_head);
> + dentry->d_fsdata = NULL; /* Yes, we really can call wake_up() first.*/
> +
> + return result;
> +}
> +
> +EXPORT_SYMBOL_GPL(trapping_lookup);
Currently tmpfs cannot be modular, therefore the export is unneeded.
If more users of this function will appear, EXPORT_SYMBOL_GPL could be
easily added.
> --- linux-2.6.10-rc1-bk14/fs/userhelper.c 1969-12-31 16:00:00.000000000
> -0800
> +++ linux/fs/userhelper.c 2004-11-05 23:24:27.000000000 -0800
> @@ -0,0 +1,190 @@
> +/*
> + helper.c -- Invoke user level helper command for a struct dentry.
> +
> + Written by Adam J. Richter
> + Copyright (C) 2004 Yggdrasil Computing, Inc.
> +
> + This program is free software; you can redistribute it and/or modify
> + it under the terms of the GNU General Public License as published by
> + the Free Software Foundation; either version 2 of the License, or
> + (at your option) any later version.
> +
> + This program is distributed in the hope that it will be useful,
> + but WITHOUT ANY WARRANTY; without even the implied warranty of
> + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> + GNU General Public License for more details.
> +
> + You should have received a copy of the GNU General Public License
> + along with this program; if not, write to the Free Software
> + Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
> + */
> +
> +#include <linux/module.h>
> +#include <linux/fs.h>
> +#include <linux/ctype.h>
> +#include <linux/fsuserhelper.h>
> +
> +static int path_len (struct dentry *de, struct dentry *root)
> +{
> + int len = 0;
> + while (de != root) {
> + len += de->d_name.len + 1; /* count the '/' */
> + de = de->d_parent;
> + }
> + return len; /* -1 because we omit the leading '/',
> + +1 because we include trailing '\0' */
> +}
> +
> +static int write_path_from_mnt (struct dentry *de, char *path, int buflen)
> +{
> + struct dentry *mnt_root = de->d_parent->d_inode->i_sb->s_root;
> + int len;
> + char *path_orig = path;
> +
> + if (de == NULL || de == mnt_root)
> + return -EINVAL;
> +
> + spin_lock(&dcache_lock);
> + len = path_len(de, mnt_root);
> + if (len > buflen) {
> + spin_unlock(&dcache_lock);
> + return -ENAMETOOLONG;
> + }
> +
> + path += len - 1;
> + *path = '\0';
> +
> + for (;;) {
> + path -= de->d_name.len;
> + memcpy(path, de->d_name.name, de->d_name.len);
> + de = de->d_parent;
> + if (de == mnt_root)
> + break;
> + *(--path) = '/';
> + }
> +
> + spin_unlock(&dcache_lock);
> +
> + BUG_ON(path != path_orig);
> +
> + return 0;
> +}
> +
> +static inline int
> +calc_argc(const char *str_in, int *str_len)
> +{
> + const char *str = str_in;
> + int argc = 0;
> + while (*str) {
> + while (*str == ' ' || *str == '\t')
> + str++;
> + argc++;
> + while (*str != ' ' && *str != '\t' && *str)
> + str++;
> + }
> + *str_len = str - str_in;
> + return argc;
> +}
> +
> +static char **
> +gen_argv(char *str_in, int argc_extra, int *argc_out)
> +{
> + int argc;
> + char **argv;
> + char *str_out;
> + int str_len;
> +
> + if (!str_in)
> + return NULL;
> +
> + while (*str_in == ' ' || *str_in == '\t')
> + str_in++;
> +
> + if (*str_in == '\0')
> + return NULL;
> +
> + argc = calc_argc(str_in, &str_len);
> +
> + argv = kmalloc(((argc + argc_extra) * sizeof(char*)) + str_len + 1,
> + GFP_KERNEL);
> + if (!argv)
> + return NULL;
> +
> + str_out = (char*) (argv + argc + argc_extra);
> +
> + argc = 0;
> + while (*str_in) {
> + argv[argc++] = str_out;
> +
> + while (*str_in != ' ' && *str_in != '\t' && *str_in)
> + *(str_out++) = *(str_in++);
> +
> + *(str_out++) = '\0';
> +
> + while (*str_in == ' ' || *str_in == '\t')
> + str_in++;
> + }
> + *argc_out = argc;
> + return argv;
> +}
> +
> +/*
> + Warning: dentry_usermodehelper releases and retakes
> + dentry->d_parent->d_inode->i_sem. It must be called with this
> + semaphore already held.
> +
> + command_p is a pointer to a single string. It is *not* in argv format.
> + Instead, elements are separated by spaces.
> +*/
> +void call_fs_helper(char **command_ptr,
> + struct rw_semaphore *command_rwsem,
> + const char *event,
> + struct dentry *dentry)
> +{
> + char path[64];
Having such arbitrary limits on the path length is bad.
> + int argc;
> + char **argv;
> + struct semaphore *parent_inode_sem = &dentry->d_parent->d_inode->i_sem;
> +
> + if (write_path_from_mnt(dentry, path, sizeof(path)) == 0) {
> +
> + /*
> + FIXME. We would not need the extra memory allocation,
> + string copying, error branch and lines of source code
> + due to err_strdup(), and we could put gen_argv
What is err_strdup()?
> + into the set_fs_helper, if call_usermodehelper
set_fs_helper also does not seem to exist.
> + and execve had a callback to inform us when
> + execve was done copying argv and envp. With
> + such a facility, we could just hold helper->rw_sem
helper->rw_sem should be command_rwsem, right?
> + up to that point, without having to make a copy of the
> + argument (which we currently do) or hold the semaphore
> + until the helper process exits (which would cause a
> + deadlock if a helper process ever tried to change
> + the helper string of a file system, especially since
> + there is not such a thing as rw_down_read_interruptible
> + that would make the deadlock breakable).
> + */
> +
> + up(parent_inode_sem);
> +
> + down_read(command_rwsem);
> + argv = gen_argv(*command_ptr, 3, &argc);
> + up_read(command_rwsem);
> +
> + if (argv != NULL) {
> + static char *envp[] =
> + {"PATH=/bin:/sbin:/usr/bin:/usr/sbin",
> + "HOME=/", NULL };
> +
> + argv[argc++] = (char*) event;
> + argv[argc++] = path;
> + argv[argc] = NULL;
> +
> + call_usermodehelper(argv[0], argv, envp, 1);
> + kfree(argv);
> + }
> +
> + down(parent_inode_sem);
> + }
> +}
> +EXPORT_SYMBOL_GPL(call_fs_helper);
Again, this export is not needed now.
> --- linux-2.6.10-rc1-bk14/Documentation/filesystems/lookup-trap.txt
> 1969-12-31 16:00:00.000000000 -0800
> +++ linux/Documentation/filesystems/lookup-trap.txt 2004-11-05
> 23:24:28.000000000 -0800
> @@ -0,0 +1,436 @@
> +User's Guide To Trapping Directory Lookup Operations in Tmpfs
> +Version 0.2
> +
> +
> +1. INSTRUCTIONS FOR THE IMPATIENT
> +
> + % modprobe tmpfs
> + % mount -t tmpfs -o helper=/path/to/helper/program whatever /mnt
> + % ls /mnt/foo
> + # Notice that "/path/to/helper/program LOOKUP foo" was executed.
> +
> +
> +2. OVERVIEW (from the Kconfig help)
> +
> + Tmpfs now allows user level programs to implement file systems
> +that are filled in on demand. This feature works by invoking a
> +configurable helper program on attempts to open() or stat() a
> +nonexistent file. The access waits until the helper finishes, so the
> +helper can install the missing file if desired.
> +
> + Using this facility, a shell script or small C program can
> +implement a file system that automatically mounts remote file systems
> +or creates device files on demand, similar to autofs or devfs,
> +respectively. Tmpfs is, however, daemonless and, perhaps
> +consequently, smaller than either of these, and may avoid some
> +recursion problems.
> +
> + Tmpfs might also be useful for debugging programs where you
> +want to trap the first access a particular file or perhaps in
> +automatic installation of missing command or libraries by specifying a
> +tmpfs file system in certain search paths.
> +
> + This access trapping facility is designed to be easily ported
> +to other file systems. Kernel developers should examine the following
> +source files for more information:
> +
> + include/linux/fsuserhelper
> + include/linux/lookuptrap.h
> + fs/lookuptrap.c
> + fs/userhelper.c
> + mm/shmem.c (for an example of tmpfs using trapping_lookup)
> +
> +
> +3. GETTING STARTED
> +
> +3.1 MOUNTING THE FILE SYSTEM
> +
> + First, build and boot a kernel with tmpfs either compiled in
> +or built as a module. If you compile tmpfs as a module, you may have
> +to load it, although, since the module name ("tmpfs.ko") and the file
> +system name ("tmpfs") match, that may be unnecessary if you've
> +configured modprobe automatically.
It is impossible to compile tmpfs as a module (at least for now).
> +
> + % modprobe tmpfs
> +
> + Now let's mount a tmpfs file system on /mnt.
> +
> + % mount -t tmpfs blah /mnt
> +
> + The file system will behave exactly like a ramfs file system.
> +In fact, tmpfs is derived from ramfs. You can create files,
> +directories, symbolic links and device nodes in it, and they will
> +exist only in the computer's main memory. The contents of the file
> +system will disappear as soon as you unmount it.
> +
> + If you mount multiple instances of tmpfs, you will get
> +separate file systems.
> +
> +
> +3.2. THE HELPER PROGRAM
> +
> + What distinguishes tmpfs from ramfs is that it can invoke a
> +user level helper program when an attempt is made to open or stat a
> +nonexistent file for the first time (if the name is not already in the
> +dcache). The user level program is set with the "helper" mount
> +option. It is possible to set, clear or change the helper command at
> +any time, so let's go back to our example and put a helper command on
> +the tmpfs file system that we mounted on /mnt:
> +
> + % mount -o remount,helper=/tmp/helper /mnt
> +
> + If you use the file system in /mnt now, nothing appears to have
> +changed. Now let's put a simple shell script in /tmp/helper:
> +
> + % cat > /tmp/helper
> + #!/bin/sh
> + echo "$*" > /dev/console
> + ^D
> + % chmod a+x /tmp/helper
> +
> + Now you should see console messages like "LOOKUP foo" when you
> +try to access the file /mnt/foo for the first time.
> +
> + You can also pass arguments to the helper program by using
> +spaces in the helper mount option, like so:
> +
> + % mount -o remount,helper='/tmp/helper my_argument' /mnt
> +
> + If you do this, your console messages will start to look
> +something like "my_argument LOOKUP foo". The arguments that
> +you specify come before "LOOKUP foo" to facilitate the use of
> +command interpreters, like, say, helper='/usr/bin/perl handler.pl'.
> +Arguments also make it easy to pass things like the mount point or
> +configuration files, which should make it easier to write facilities
> +that work on multiple mount points.
> +
> + You can also deactivate the helper at any time, like so:
> +
> + mount -o remount,helper='' /mnt
> +
> +4. PRACTICAL EXAMPLES
> +
> +4.1 AN NFS AUTOMOUNTER
> +
> + % cat > /usr/sbin/tmpfs-automount
> + #!/bin/sh
> + topdir=$1
> + host=${3%/*}
> + dir=$topdir/$host
> + mkdir $dir
> + mount -t nfs $host:/ $dir
> + ^D
> + % chmod a+x /usr/sbin/tmpfs-automount
> + % mkdir /auto
> + % mount -t tmpfs -o helper="/usr/sbin/tmpfs-automount /auto" x /auto
> +
> + Notice how we pass the additional argument "/auto" to the
> +tmpfs-automount command.
> +
> + If you want automatic unmount after a timeout, you'll probably
> +want to do something a little more elaborate, perhaps with a script that
> +runs from cron.
> +
> +
> +4.2 DEMAND LOADING OF DEVICE DRIVERS
> +
> + A version of devfs that uses tmpfs is under development and
> +running on the system I am using to write this document, but I am
> +still cleaning it up. Here is how it should work, although I have
> +not yet actually tried devfs_helper on it.
> +
> + The devfs_helper program was originally written for a stripped down
> +rewrite of devfs, from which tmpfs is derived. It can read your
> +/etc/devfs.conf file (the file previously used to configured
> +devfsd) and load modules specified by "LOOKUP" commands. Other
> +devfs.conf command are ignored.
> +
> + % ftp ftp.yggdrasil.com
> + login: anonymous
> + password; guest
> + ftp> cd /pub/dist/device_control/devfs
> + ftp> get devfs_helper-0.2.tar.gz
> + .....
> + ftp> quit
> + % tar xfpvz devfs_helper-0.2.tar.gz
> + % cd devfs_helper-0.2
> + % make
> + % make install
> + % mkdir /tmp/tmpdev
> + % mount -t devfs /tmp/tmpdev
> + % cp -apRx /dev/* /tmpdev/
> + % mount -t devfs -o helper=/sbin/devfs_helper blah /dev
> + % mount -t msods /dev/floppy/0 /mnt
> +
> + The above example should load the floppy.ko kernel module
> +if you have a a line in your /etc/devfs.conf file like this:
> +
> + LOOKUP floppy EXECUTE modprobe floppy
> +
> +
> + You should also be able to use execfs in this fashion to get
And what is execfs?
> +automatic loading of kernel modules on non-devfs systems, although
> +you'll need something like udev the larger udev to create the
"udev the larger udev"?
> +device files once the device drivers are registered.
> +
> +
> +4.3 DEBUGGING A PROGRAM TRYING TO ACCESS A FILE
> +
> + % cat > /tmp/call-sleep
> + #!/bin/sh
> + sleep 30
> + ^D
> + % mount -t tmpfs -o helper=/tmp/call-sleep foo /mnt
> + % mv .bashrc .bashrc-
> + % ln -s /mnt/whatever .bashrc
> + % gdb /bin/sh
> + GNU gdb 5.2
> + [blah blah blah]
> + (gdb) run
> + [program eventually hangs. Switch to another terminal session. You
> + cannot control-C out of it, a tmpfs bug from call_usermodehelper.]
> + % ps axf
> + [Find the process under gdb. Let say it's pid 1152.]
> + % kill -SEGV 1152
> + % ps auxww | grep sleep
> + [Find the sleeping tmpfs helper. Let's say it's pid 1120.]
> + % kill -9 1120
> + [Now back at the first session, running gdb on /bin/sh.]
> + Program received signal SIGSEGV, Segmentation fault.
> + 0xb7f303d4 in __libc_open () at __libc_open:-1
> + -1 __libc_open: No such file or directory.
> + in __libc_open
> + (gdb) where
> + #0 0xb7f303d4 in __libc_open () at __libc_open:-1
> + #1 0xb7f8b4c0 in __DTOR_END__ () from /lib/libc.so.6
> + #2 0x080921ef in _evalfile (filename=0x80dc788 "/tmp/junk/.bashrc",
> flags=9)
> + at evalfile.c:85
> + #3 0x08092635 in maybe_execute_file (
> + fname=0xfffffffe <Address 0xfffffffe out of bounds>,
> + force_noninteractive=1) at evalfile.c:218
> + #4 0x08059fe8 in run_startup_files () at shell.c:1019
> + #5 0x08059849 in main (argc=1, argv=0xbfffebc4, env=0xbfffebcc) at
> shell.c:581
> + #6 0xb7e88e02 in __libc_start_main (main=0x8059380 <main>, argc=1,
> + ubp_av=0xbfffebc4, init=0x805897c <_init>,
> + fini=0xb80005ac <_dl_debug_mask>, rtld_fini=0x8000, stack_end=0x0)
> + at ../sysdeps/generic/libc-start.c:129
> +
> +
> +4.4 AUTOMATIC LOADING OF MISSING PROGRAMS
> +
> + % cat > /usr/sbin/missing-program
> + #!/bin/sh
> + my-automatic-network-downloader $2
> + ^D
> + % chmod a+x /usr/sbin/missing-program
> + % mount -t tmpfs -o helper=/usr/sbin/my-automatic-installer /mnt
> + % PATH=$PATH:/mnt:$PATH
> + # We include $PATH a second time so that the program can be
> + # found after it is installed.
> + % kdevelop # Or some other program you don't have...
> +
> + ...or maybe something like this...
> +
> + % cat > /usr/sbin/missing-program
> + #!/bin/sh
> + export DISPLAY=:0
> + konqueror http://www.google.com/search?q="download+$2"; &
> + ^D
> + % chmod a+x /usr/sbin/missing-program
> + % mount -t tmpfs -o helper=/usr/sbin/missing-program glorp /mnt
> + % xhost localhost
> + % PATH=$PATH:/mnt
> + % kdevelop
> +
> +4.4.1 AUTOMATIC LOADING OF MISSING LIBRARIES
> +
> + Same as above, but with this line:
> + % LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/mnt:$LD_LIBRARY_PATH
> +
> +4.5 ADVANCED EXAMPLE: Generating plain files
> +
> + Doing automatic generation of plain files (as opposed to
> +directories, device files and symbolic links) requires more care,
> +because the helper program's attempt to open a file for writing
> +will itself invoke another instance of the user level handler.
> +
> + One approach, perhaps the only one, is to define some temporary
> +file name pattern that your user handler knows to ignore, create the
> +file with a temporary name that matches that pattern, and then
> +rename the temporary file to the real file name, since rename
> +operations are not trapped (and if renames ever are trapped in future,
Misplaced comma.
> +releases you could filter out renames where the source matched the
> +tempary file pattern).
> +
> + [FIXME. This example is untested! --Adam Richter, 2004.11.03]
> +
> +
> + % cat > /usr/sbin/my-decrypter
> + #!/bin/sh
> + encrypted_dir=$2
> + decrypted_dir=$3
> + key=$4
> + # $5 is "LOOKUP"
> + filename=$6
> +
> + case $target in ( tmp/* ) ; exit ;; esac
> + tmpfile=$decrypted_dir/$$
> + decrypt --key $key < $encrypted_dir/$filename > $tmpfile
> + mv $tmpfile $decrypted_dir/$filename
> + ^D
> + % chmod a+x /usr/sbin/my-decrypter
> + % mount -t tmpfs -o \
> + helper='/usr/sbin/my-decrypter /cryptdir /mnt key' x /mnt
> + % cat /mnt/my-secret-file
> + ....
> +
> + If you want to make the temporary directory completely
> +inaccessible from the public directory, you can create two mount
> +points, where the public directory for the file system is actually
> +a subdirectory of a larger hidden directory, like so:
> +
> + % mount -t tmpfs /some/place/hidden
> + % chmod go-rwx /someplace/hidden
> + % mkdir /some/place/hidden/mirror
> + % mount --bind /some/place/hidden/mirror /public
> +
> + Now you can set up a helper program that operates in some
> +other directory of /some/place/hidden, and then renames the
> +resultant files into /some/place/hidden/mirror.
> +
> +
> +4.5.1 A WARNING ABOUT SYMBOLIC LINKS AND THE GNU "ln -s" COMMAND
> +
> + If your helper program invoke the shell command "ln" to create
> +symbolic links, you may need to use "filter and rename" technique that
> +was described above for plain files, or one of several other
> +workarounds listed below. If you helper program just uses the
> +symlink() system call directly, you don't have to worry about this.
> +
> + What is the problem, exactly? The problem is that although
> +symlink system calls are not trapped, the "ln" command from version
> +5.2.1 of the GNU coreutils package (latest version as of this writing)
> +does a stat system call on the target before doing the symlink,
> +because it is specified to behave differently if the destination path
> +exists and is a directory. stat, unlink symlink, is trapped, in order
> +to support automatic creation of files in case a program stats a file
> +before deciding whether to open it, and for things like "ls
> +/auto/fileserver1.mycompany.com/". Consequently, a user level helper
> +shell script that simply does "ln -s whatever $target_file_name" will
> +deadlock (which can be broken by interrupting the child helper
> +program).
> +
> + There are several possible solutions to this problem.
> +
> + 1. You can use a simpler symlink program, such as ssln,
> + instead of "ln -s".
> +
> + 2. If the final path element of the symlink's contents is
> + the same as the final path element of the symlink's name,
> + then you can just specify the directory name as the second
> + argument to "ln -s". In other words, change
> +
> + ln -s foo/bar /the/target/bar
> + ...to...
> + ln -s foo/bar /the/target
> +
> + 3. You can use the same filtering techniques for symlink
> + as discussed for plain files in the previous section
> + (create them with speical temporary names that your helper
> + program knows to ignore and then rename them into place).
> +
> + 4. Invoke perl to do the symlink, if you know you have it available:
> +
> + perl -e 'symlink("contents", "target");'
> +
> + 5. You can port shell script to C or perl or some other
> + language that gives you direct access to the symlink()
> + system call.
> +
> +
> + Perhaps, in the future, the GNU ln command could be changed so
> +that, when it is called with exactly two file names, it would try to
> +do the symlink() and then check if the target is a directory only if
> +the symlink attempt failed.
> +
> +
> +4.6. OTHER USES?
> +
> + I would be interested in hearing about any other uses that you up
> +with for tmpfs, especially if I can include them in this document.
> +
> +5. SERIALIZATION
> +
> + Note that many instances of the user level helper program
> +can potentially be running at the same time. It is up to "you",
> +the implementor of the helper program to determine what sort of
> +serialization you need and implement it. A simple solution to
> +enforce complete serialization would be to have every instance
> +of the helper program take an exclusive flock on some common
> +file.
> +
> +6. KERNEL DEVELOPER ANSWERS ABOUT IMPLEMENTATION DECISIONS
> +
> +6.1 Q: Why doesn't tmpfs provide REGISTER and UNREGISTER events when
> + new nodes are created or deleted from the file system, as the
> + mini-devfs implementation from which is derived did?
> +
> + A: {,UN}REGISTER in Richard Gooch's implementation of devfs enabled
> + things like automatically setting permissions and sound settings
> + on your sound device when the driver was loaded, even if
> + the loading of the driver had not been caused by devfs.
> + For tmpfs-based devfs, I expect to implement that in
> + a more complex way by shadowing the real devfs file system
> + and creating {,UN}REGISTER events as updates are propagated
> + from the real devfs to /dev. The advantages of this would be
> + that module initialization would not be blockable by a user
> + level program, and events like a device quickly appearing and
> + disappearing could be coalesced (i.e., ignored in this case).
> +
> + I'm not convinced that {,UN}REGISTER has to go, but I haven't
> + seen any compelling uses for it, and I know it's politically
> + easier to add a feature than to remove one, especially if anyone
> + has developed a dependence on it and does not want to port. So,
> + I'm starting out with tmpfs not providing {,UN}REGISTER events.
> +
> +6.2 Q: Why isn't this facility implemented as an overlay file system?
> I'd
> + like to be able to apply it to, say, /dev, without having
> + to start out with /dev being a devfs file system. I could
> + also demand load certain facilities based on accesses to /proc.
> +
> + A: There are about two dozen routines in inode_operations and
> + file_operations that would require pass-through versions, and
> + they are not as trivial as you might think because of
> + locking issues involved in going through the vfs layer again.
> + Also, currently, tmpfs, like ramfs and sysfs, use a struct
> + inode and a struct dentry in kernel low memory for every
> + existing node in the file system, about half a kilobyte
> + per entry. So, tmpfs would need to be converted to allow
> + inode structures to be released if it were to overlay
> + potentially large directory trees. Also there are issues
> + related to the underlying file system changing "out from
> + under" tmpfs. Perhaps in the future this an be implemented.
> +
> +
> +6.3. Q: Why isn't tmpfs a built-in kernel facility that can be
> + applied to any file, like dnotify? That would also have
> + the above advantages and could eliminate the mount complexity
> + of overlays.
> +
> + A: I thought about defining a separating
> inode->directory_operations
> + from inode->inode_operations. Since all of those operations that
> + I want to intercept are called with inode->i_sem held, it
> + follows that it would be SMP-safe to change an inode->dir_ops
> + pointer dynamically, which would allow stacking of inode
> + operations. This approach could be used to remove the special
> + case code that is used to implement dnotify. But what happens
> + when the inode is freed? It would be necessary to intercept
> + the victim superblock's superblock_operations.drop_inode
> + routine, which could get pretty messy, especially, if, for
> + example, more than one trap was being used on the same file
> + system. Perhaps if drop_inode were moved to struct
> + inode_operations this would be easier.
> +
> +
> +Adam J. Richter ([EMAIL PROTECTED])
> +2004.11.02
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