> On Jun 27, 2018, at 7:35 PM, Warner Losh <i...@bsdimp.com> wrote:
>
>
>
> On Wed, Jun 27, 2018 at 8:14 PM, Devin Teske <dte...@freebsd.org
> <mailto:dte...@freebsd.org>> wrote:
>
>> On Jun 27, 2018, at 6:58 PM, Warner Losh <i...@bsdimp.com
>> <mailto:i...@bsdimp.com>> wrote:
>>
>>
>>
>> On Wed, Jun 27, 2018 at 7:49 PM, Devin Teske <dte...@freebsd.org
>> <mailto:dte...@freebsd.org>> wrote:
>>
>>> On Jun 27, 2018, at 5:59 PM, Warner Losh <i...@bsdimp.com
>>> <mailto:i...@bsdimp.com>> wrote:
>>>
>>>
>>>
>>> On Wed, Jun 27, 2018 at 5:52 PM, Gleb Smirnoff <gleb...@freebsd.org
>>> <mailto:gleb...@freebsd.org>> wrote:
>>> On Wed, Jun 27, 2018 at 04:11:09AM +0000, Warner Losh wrote:
>>> W> Author: imp
>>> W> Date: Wed Jun 27 04:11:09 2018
>>> W> New Revision: 335690
>>> W> URL: https://svnweb.freebsd.org/changeset/base/335690
>>> <https://svnweb.freebsd.org/changeset/base/335690>
>>> W>
>>> W> Log:
>>> W> Fix devctl generation for core files.
>>> W>
>>> W> We have a problem with vn_fullpath_global when the file exists. Work
>>> W> around it by printing the full path if the core file name starts with
>>> /,
>>> W> or current working directory followed by the filename if not.
>>>
>>> Is this going to work when a core is dumped not at current working
>>> directory,
>>> but at absolute path? e.g. kern.corefile=/var/log/cores/%N.core
>>>
>>> Yes. That works.
>>>
>>> Looks like the vn_fullpath_global needs to be fixed rather than problem
>>> workarounded.
>>>
>>> It can't be fixed reliably. FreeBSD does not and cannot map a vnode to a
>>> name. The only reason we're able to at all is due to the name cache. And
>>> when we recreate a file, we invalidate the name cache. And even if we fixed
>>> that, there's no guarantee the name cache won't get flushed before we
>>> translate the name.... Linux can do this because it keeps the path name
>>> associated with the inode. FreeBSD simply doesn't.
>>>
>>
>> They said it couldn't be done, but I personally have done it ...
>>
>> I map vnodes to full paths in dwatch. It's not impossible, just implausibly
>> hard.
>>
>> I derived my formula by reading the C code which was very twisty-turny and
>> rather hard to read at times (and I have been using C since 1998 on
>> everything from AIX and OSF/1 to HP/UX, Cygwin, MinGW, FreeBSD, countless
>> Linux-like things, IRIX, and a God-awful remainder of many others; the vnode
>> code was an adventure to say the least).
>>
>> You're welcome to see how it's done in /usr/libexec/dwatch/vop_create
>>
>> I load up a clause-local variable named "path" with a path constructed from
>> a pointer to a vnode structure argument to VOP_CREATE(9).
>>
>> The D code could easily be rewritten back into C to walk the vnode to
>> completion (compared to the D code which is bounded by depth-limit since
>> DTrace doesn't provide loops; so you have to, as I have done, use a
>> higher-level language wrapper to repeat the code to some desired limit;
>> dwatch defaulting to 64 for directory depth limit).
>>
>> Compared this, say, to vfssnoop.d from Chapter 5 of the DTrace book which
>> utilizes the vfs:namei:lookup: probes. My approach in dwatch is far more
>> accurate, produces full paths, and I've benchmarked it at lower overhead
>> with better results.
>>
>> Maybe some of this can be useful? If not, just ignore me.
>>
>> IMHO, there's no benefit from the crazy hoops than the super simple
>> heuristic I did: if the path starts with / print it. If the path doesn't
>> print cwd / and then the path.
>>
>> I look forward to seeing your conversion of the D to C that works, even when
>> there's namespace cache pressure.
>>
>
> I was looking at the output of "dwatch -dX vop_create" (the -d flag to dwatch
> causes the generated dwatch to be printed instead of executed) and thinking
> to myself:
>
> I could easily convert this, as I can recognize the flattened loop structure.
> However, not many people will be able to do it. I wasn't really volunteering,
> but now I'm curious what other people see when they run "dwatch -dX
> vop_create". If it's half as bad as I think it is, then I'll gladly do it --
> but will have to balance it against work priorities.
>
> We don't have the data you do in vop_create anymore, just the vp at the point
> in the code where we want to do the reverse translation... But we also have a
> name that's been filled in from the template and cwd, which gives us almost
> the same thing as we'd hoped to dig out of the name cache...
>
Given:
struct vnode *vp, const char *fi_name
Here's a rough (not optimized; unchecked) crack at a C equivalent:
#include <sys/mount.h>
#include <sys/vnode.h>
#include <limits.h>
int i, max_depth = 64;
char *d_name = NULL;
char *fi_fs = NULL;
char *fi_mount = NULL;
char *cp;
struct namecache *ncp = NULL;
struct mount *mount = NULL;
struct vnode *dvp = NULL;
char *nc[max_depth+1];
char path[PATH_MAX] = __DECONST(char *, "");
if (vp != NULL) {
ncp = vp->v_cache_dst.tqh_first;
mount = vp->v_mount; /* ptr to vfs we are in */
if (vp->v_cache_dd != NULL)
d_name = vp->v_cache_dd->nc_name;
}
if (mount != NULL) {
fi_fs = mount->mnt_stat.f_fstypename;
if (fi_fs != NULL && *fi_fs != '\0') {
if (!strcmp(fi_fs, "devfs")) ncp = NULL;
} else {
if (fi_name == NULL || *fi_name == '\0') ncp = NULL;
}
fi_mount = mount->mnt_stat.f_mntonname;
} else {
ncp = NULL;
}
for (i = 0; i <= max_depth; i++) {
nc[i] = NULL;
}
if (ncp != NULL) {
/* Reach into vnode of parent of name */
if (ncp->nc_dvp != NULL)
dvp = ncp->nc_dvp->v_cache_dst.tqh_first;
if (dvp != NULL) nc[0] = dvp->nc_name;
}
if (nc[0] == NULL || *nc[0] == '\0' || !strcmp(nc[0], "/"))
dvp = NULL;
/*
* BEGIN Pathname-depth loop
*/
for (i = 1; i < max_depth; i++) {
if (dvp == NULL) break;
if (dvp->nc_dvp != NULL)
dvp = dvp->nc_dvp->v_cache_dst.tqh_first;
else
dvp = NULL;
if (dvp != NULL) nc[i] = dvp->nc_name;
}
/*
* END Pathname-depth loop
*/
if (dvp != NULL) {
if (dvp->nc_dvp != NULL)
dvp = dvp->nc_dvp->v_cache_dst.tqh_first;
else
dvp = NULL;
if (dvp != NULL && dvp->nc_dvp != NULL)
nc[max_depth] = __DECONST(char *, "...");
}
if (fi_mount != NULL) {
/*
* Join full path
* NB: Up-to but not including the parent directory (joined
below)
*/
strcat(path, fi_mount);
if (strcmp(fi_mount, "/")) strcat(path, "/");
for (i = max_depth; i >= 0; i--) {
char *namei = nc[i];
strcat(path, namei == NULL ? "" : namei);
if (namei != NULL) strcat(path, "/");
}
/* Join the parent directory name */
if (d_name != NULL && *d_name != '\0') {
strcat(path, d_name);
strcat(path, "/");
}
/* Join the entry name */
if (fi_name != NULL && *fi_name != '\0')
strcat(path, fi_name);
}
--
Devin
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