On Wed, Sep 10, 2025 at 07:21:22PM +0200, Amir Goldstein wrote:
> On Wed, Sep 10, 2025 at 4:39 PM Christian Brauner <brau...@kernel.org> wrote:
> >
> > A while ago we added support for file handles to pidfs so pidfds can be
> > encoded and decoded as file handles. Userspace has adopted this quickly
> > and it's proven very useful.
>
> > Pidfd file handles are exhaustive meaning
> > they don't require a handle on another pidfd to pass to
> > open_by_handle_at() so it can derive the filesystem to decode in.
> >
> > Implement the exhaustive file handles for namespaces as well.
>
> I think you decide to split the "exhaustive" part to another patch,
> so better drop this paragraph?
Yes, good point. I've dont that.
> I am missing an explanation about the permissions for
> opening these file handles.
>
> My understanding of the code is that the opener needs to meet one of
> the conditions:
> 1. user has CAP_SYS_ADMIN in the userns owning the opened namespace
> 2. current task is in the opened namespace
Yes.
>
> But I do not fully understand the rationale behind the 2nd condition,
> that is, when is it useful?
A caller is always able to open a file descriptor to it's own set of
namespaces. File handles will behave the same way.
> And as far as I can tell, your selftest does not cover this condition
> (only both true or both false)?
I've added this now.
>
> I suggest to start with allowing only the useful and important
> cases, so if cond #1 is useful enough, drop cond #2 and we can add
> it later if needed and then your selftests already cover cond #1 true and
> false.
>
> >
> > Signed-off-by: Christian Brauner <brau...@kernel.org>
>
> After documenting the permissions, with ot without dropping cond #2
> feel free to add:
>
> Reviewed-by: Amir Goldstein <amir7...@gmail.com>
Thanks!
>
> > ---
> > fs/nsfs.c | 176
> > +++++++++++++++++++++++++++++++++++++++++++++++
> > include/linux/exportfs.h | 6 ++
> > 2 files changed, 182 insertions(+)
> >
> > diff --git a/fs/nsfs.c b/fs/nsfs.c
> > index 6f8008177133..a1585a2f4f03 100644
> > --- a/fs/nsfs.c
> > +++ b/fs/nsfs.c
> > @@ -13,6 +13,12 @@
> > #include <linux/nsfs.h>
> > #include <linux/uaccess.h>
> > #include <linux/mnt_namespace.h>
> > +#include <linux/ipc_namespace.h>
> > +#include <linux/time_namespace.h>
> > +#include <linux/utsname.h>
> > +#include <linux/exportfs.h>
> > +#include <linux/nstree.h>
> > +#include <net/net_namespace.h>
> >
> > #include "mount.h"
> > #include "internal.h"
> > @@ -417,12 +423,182 @@ static const struct stashed_operations
> > nsfs_stashed_ops = {
> > .put_data = nsfs_put_data,
> > };
> >
> > +struct nsfs_fid {
> > + u64 ns_id;
> > + u32 ns_type;
> > + u32 ns_inum;
> > +} __attribute__ ((packed));
> > +
> > +#define NSFS_FID_SIZE (sizeof(struct nsfs_fid) / sizeof(u32))
> > +
> > +static int nsfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
> > + struct inode *parent)
> > +{
> > + struct nsfs_fid *fid = (struct nsfs_fid *)fh;
> > + struct ns_common *ns = inode->i_private;
> > + int len = *max_len;
> > +
> > + /*
> > + * TODO:
> > + * For hierarchical namespaces we should start to encode the
> > + * parent namespace. Then userspace can walk a namespace
> > + * hierarchy purely based on file handles.
> > + */
> > + if (parent)
> > + return FILEID_INVALID;
> > +
> > + if (len < NSFS_FID_SIZE) {
> > + *max_len = NSFS_FID_SIZE;
> > + return FILEID_INVALID;
> > + }
> > +
> > + len = NSFS_FID_SIZE;
> > +
> > + fid->ns_id = ns->ns_id;
> > + fid->ns_type = ns->ops->type;
> > + fid->ns_inum = inode->i_ino;
> > + *max_len = len;
> > + return FILEID_NSFS;
> > +}
> > +
> > +static struct dentry *nsfs_fh_to_dentry(struct super_block *sb, struct fid
> > *fh,
> > + int fh_len, int fh_type)
> > +{
> > + struct path path __free(path_put) = {};
> > + struct nsfs_fid *fid = (struct nsfs_fid *)fh;
> > + struct user_namespace *owning_ns = NULL;
> > + struct ns_common *ns;
> > + int ret;
> > +
> > + if (fh_len < NSFS_FID_SIZE)
> > + return NULL;
> > +
> > + switch (fh_type) {
> > + case FILEID_NSFS:
> > + break;
> > + default:
> > + return NULL;
> > + }
> > +
> > + scoped_guard(rcu) {
> > + ns = ns_tree_lookup_rcu(fid->ns_id, fid->ns_type);
> > + if (!ns)
> > + return NULL;
> > +
> > + VFS_WARN_ON_ONCE(ns->ns_id != fid->ns_id);
> > + VFS_WARN_ON_ONCE(ns->ops->type != fid->ns_type);
> > + VFS_WARN_ON_ONCE(ns->inum != fid->ns_inum);
> > +
> > + if (!refcount_inc_not_zero(&ns->count))
> > + return NULL;
> > + }
> > +
> > + switch (ns->ops->type) {
> > +#ifdef CONFIG_CGROUPS
> > + case CLONE_NEWCGROUP:
> > + if (!current_in_namespace(to_cg_ns(ns)))
> > + owning_ns = to_cg_ns(ns)->user_ns;
> > + break;
> > +#endif
> > +#ifdef CONFIG_IPC_NS
> > + case CLONE_NEWIPC:
> > + if (!current_in_namespace(to_ipc_ns(ns)))
> > + owning_ns = to_ipc_ns(ns)->user_ns;
> > + break;
> > +#endif
> > + case CLONE_NEWNS:
> > + if (!current_in_namespace(to_mnt_ns(ns)))
> > + owning_ns = to_mnt_ns(ns)->user_ns;
> > + break;
> > +#ifdef CONFIG_NET_NS
> > + case CLONE_NEWNET:
> > + if (!current_in_namespace(to_net_ns(ns)))
> > + owning_ns = to_net_ns(ns)->user_ns;
> > + break;
> > +#endif
> > +#ifdef CONFIG_PID_NS
> > + case CLONE_NEWPID:
> > + if (!current_in_namespace(to_pid_ns(ns))) {
> > + owning_ns = to_pid_ns(ns)->user_ns;
> > + } else if (!READ_ONCE(to_pid_ns(ns)->child_reaper)) {
> > + ns->ops->put(ns);
> > + return ERR_PTR(-EPERM);
> > + }
> > + break;
> > +#endif
> > +#ifdef CONFIG_TIME_NS
> > + case CLONE_NEWTIME:
> > + if (!current_in_namespace(to_time_ns(ns)))
> > + owning_ns = to_time_ns(ns)->user_ns;
> > + break;
> > +#endif
> > +#ifdef CONFIG_USER_NS
> > + case CLONE_NEWUSER:
> > + if (!current_in_namespace(to_user_ns(ns)))
> > + owning_ns = to_user_ns(ns);
> > + break;
> > +#endif
> > +#ifdef CONFIG_UTS_NS
> > + case CLONE_NEWUTS:
> > + if (!current_in_namespace(to_uts_ns(ns)))
> > + owning_ns = to_uts_ns(ns)->user_ns;
> > + break;
> > +#endif
> > + default:
> > + return ERR_PTR(-EOPNOTSUPP);
> > + }
> > +
> > + if (owning_ns && !ns_capable(owning_ns, CAP_SYS_ADMIN)) {
> > + ns->ops->put(ns);
> > + return ERR_PTR(-EPERM);
> > + }
> > +
> > + /* path_from_stashed() unconditionally consumes the reference. */
> > + ret = path_from_stashed(&ns->stashed, nsfs_mnt, ns, &path);
> > + if (ret)
> > + return ERR_PTR(ret);
> > +
> > + return no_free_ptr(path.dentry);
> > +}
> > +
> > +/*
> > + * Make sure that we reject any nonsensical flags that users pass via
> > + * open_by_handle_at().
> > + */
> > +#define VALID_FILE_HANDLE_OPEN_FLAGS \
> > + (O_RDONLY | O_WRONLY | O_RDWR | O_NONBLOCK | O_CLOEXEC | O_EXCL)
> > +
> > +static int nsfs_export_permission(struct handle_to_path_ctx *ctx,
> > + unsigned int oflags)
> > +{
> > + if (oflags & ~(VALID_FILE_HANDLE_OPEN_FLAGS | O_LARGEFILE))
> > + return -EINVAL;
> > +
> > + /* nsfs_fh_to_dentry() is performs further permission checks. */
> > + return 0;
> > +}
> > +
> > +static struct file *nsfs_export_open(struct path *path, unsigned int
> > oflags)
> > +{
> > + /* Clear O_LARGEFILE as open_by_handle_at() forces it. */
> > + oflags &= ~O_LARGEFILE;
> > + return file_open_root(path, "", oflags, 0);
> > +}
> > +
> > +static const struct export_operations nsfs_export_operations = {
> > + .encode_fh = nsfs_encode_fh,
> > + .fh_to_dentry = nsfs_fh_to_dentry,
> > + .open = nsfs_export_open,
> > + .permission = nsfs_export_permission,
> > +};
> > +
> > static int nsfs_init_fs_context(struct fs_context *fc)
> > {
> > struct pseudo_fs_context *ctx = init_pseudo(fc, NSFS_MAGIC);
> > if (!ctx)
> > return -ENOMEM;
> > ctx->ops = &nsfs_ops;
> > + ctx->eops = &nsfs_export_operations;
> > ctx->dops = &ns_dentry_operations;
> > fc->s_fs_info = (void *)&nsfs_stashed_ops;
> > return 0;
> > diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
> > index cfb0dd1ea49c..3aac58a520c7 100644
> > --- a/include/linux/exportfs.h
> > +++ b/include/linux/exportfs.h
> > @@ -122,6 +122,12 @@ enum fid_type {
> > FILEID_BCACHEFS_WITHOUT_PARENT = 0xb1,
> > FILEID_BCACHEFS_WITH_PARENT = 0xb2,
> >
> > + /*
> > + *
> > + * 64 bit namespace identifier, 32 bit namespace type, 32 bit inode
> > number.
> > + */
> > + FILEID_NSFS = 0xf1,
> > +
> > /*
> > * 64 bit unique kernfs id
> > */
> >
> > --
> > 2.47.3
> >
