Messages in this thread Patch in this message | | | Date | Sun, 10 Mar 2013 23:04:43 +0000 | From | Al Viro <> | Subject | Re: BUG_ON(nd->inode->i_op->follow_link); |
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On Thu, Mar 07, 2013 at 10:30:52AM -0500, Dave Jones wrote:
> Code: 44 24 08 48 89 43 08 48 8b 40 30 81 4b 38 00 10 00 00 48 89 43 30 48 8b 40 20 48 83 78 08 00 75 0a 48 8b 5d f0 4c 8b 65 f8 c9 c3 <0f> 0b 66 2e 0f 1f 84 00 00 00 00 00 66 66 66 66 90 55 ba 10 00 > RIP [<ffffffff811c1414>] nd_jump_link+0x54/0x60 > RSP <ffff880112e31c78>
Attempt to follow procfs symlink resolving in a symlink inode. Interesting... Oh, hell...
OK, I can easily reproduce that one: open a symlink with O_PATH|O_NOFOLLOW, then try to open /proc/self/fd/<fd>. Boom...
The interesting part is what to do with it; it's not enough to make that BUG_ON() to STFU, unfortunately. I'm not sure what's the right behaviour here - the thing is, following a symlink depends on which directory it's in and with procfs ones we really don't know that; don't even know if the bugger is still linked, for that matter.
Basically, the target of symlink is a function of symlink contents *and* the directory it's in. Consider somebody doing O_PATH open of a symlink (with O_NOFOLLOW). Then unlink it. What should following /proc/self/fd/<that descriptor> mean?
FWIW, I'm seriously tempted to turn that BUG_ON() into return -ELOOP, rather than attempt to invent a semantics for following those guys. I think we'd discussed that corner case when doing O_PATH patchset, and IIRC decided to go with "fail with -ELOOP"; looks like that had slipped through the cracks and never got done. IOW, how about the following?
make nd_jump_link() fail sanely when asked to jump into a symlink
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> --- diff --git a/fs/namei.c b/fs/namei.c index 57ae9c8..9029d60 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -682,13 +682,18 @@ static inline void path_to_nameidata(const struct path *path, * Helper to directly jump to a known parsed path from ->follow_link, * caller must have taken a reference to path beforehand. */ -void nd_jump_link(struct nameidata *nd, struct path *path) +void *nd_jump_link(struct nameidata *nd, struct path *path) { + if (unlikely(path->dentry->d_inode->i_op->follow_link)) { + path_put(path); + return ERR_PTR(-ELOOP); + } path_put(&nd->path); nd->path = *path; nd->inode = nd->path.dentry->d_inode; nd->flags |= LOOKUP_JUMPED; + return NULL; } static inline void put_link(struct nameidata *nd, struct path *link, void *cookie) diff --git a/fs/proc/base.c b/fs/proc/base.c index 69078c7..b323ee6 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -1437,8 +1437,7 @@ static void *proc_pid_follow_link(struct dentry *dentry, struct nameidata *nd) if (error) goto out; - nd_jump_link(nd, &path); - return NULL; + return nd_jump_link(nd, &path); out: return ERR_PTR(error); } diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c index 66b51c0..11c7b97 100644 --- a/fs/proc/namespaces.c +++ b/fs/proc/namespaces.c @@ -135,8 +135,7 @@ static void *proc_ns_follow_link(struct dentry *dentry, struct nameidata *nd) } ns_path.mnt = mntget(nd->path.mnt); - nd_jump_link(nd, &ns_path); - error = NULL; + error = nd_jump_link(nd, &ns_path); out_put_task: put_task_struct(task); diff --git a/include/linux/namei.h b/include/linux/namei.h index 5a5ff57..bf55ed9 100644 --- a/include/linux/namei.h +++ b/include/linux/namei.h @@ -82,7 +82,7 @@ extern int follow_up(struct path *); extern struct dentry *lock_rename(struct dentry *, struct dentry *); extern void unlock_rename(struct dentry *, struct dentry *); -extern void nd_jump_link(struct nameidata *nd, struct path *path); +extern void *nd_jump_link(struct nameidata *nd, struct path *path); static inline void nd_set_link(struct nameidata *nd, char *path) {
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