On Mon, 2006-09-04 at 12:52 +0100, David Howells wrote:
> Andrew Morton <[email protected]> wrote:
>
> > sony:/home/akpm> ls -l /net/bix/usr/src
> > total 0
> >
> > sony:/home/akpm> showmount -e bix
> > Export list for bix:
> > / *
> > /usr/src *
> > /mnt/export *
>
> Yes, but what's your /etc/exports now? Not all options appear to showmount.
>
> Can you add "nohide" to the /usr/src and /mnt/export lines and "fsid=0" to the
> / line if you don't currently have them and try again?
>
> > iirc, we decided this is related to the fs-cache infrastructure work which
> > went into git-nfs. I think David can reproduce this?
>
> I'd only reproduced it with SELinux in enforcing mode.
>
> Under such conditions, unless there's a readdir on the root directory, the
> subdirs under which exports exist will remain as incorrectly negative
> dentries.
>
> The problem is a conjunction of circumstances:
>
> (1) nfs_lookup() has a shortcut in it that skips contact with the server if
> we're doing a lookup with intent to create. This leaves an incorrectly
> negative dentry if there _is_ actually an object on the server.
>
> (2) The mkdir procedure is aborted between the lookup() op and the mkdir() op
> by SELinux (see vfs_mkdir()). Note that SELinux isn't the _only_ method
> by which the abort can occur.
>
> (3) One of my patches correctly assigns the security label to the automounted
> root dentry.
>
> (4) SELinux then aborts the automounter's mkdir() call because the automounter
> does _not_ carry the correct security label to write to the NFS directory.
>
> (5) The incorrectly set up dentry from (1) remains because the the mkdir() op
> is not invoked to set it right.
>
> The only bit I added was (3), but that's not the only circumstance in which
> this can occur.
>
>
> If, for example, I do "chmod a-w /" on the NFS server, I can see the same
> effects on the client without the need for SELinux to put its foot in the door.
> Automount does:
>
> [pid 3838] mkdir("/net", 0555) = -1 EEXIST (File exists)
> [pid 3838] stat64("/net", {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
> [pid 3838] mkdir("/net/trash", 0555) = -1 EEXIST (File exists)
> [pid 3838] stat64("/net/trash", {st_mode=S_IFDIR|0555, st_size=1024, ...}) = 0
> [pid 3838] mkdir("/net/trash/mnt", 0555) = -1 EACCES (Permission denied)
>
> And where I was listing the disputed directory, I see:
>
> [root@andromeda ~]# ls -lad /net/trash/usr/src
> drwxr-xr-x 4 root root 1024 Aug 30 10:35 /net/trash/usr/src/
> [root@andromeda ~]#
>
> which isn't what I'd expect. What I'd expect is:
>
> [root@andromeda ~]# ls -l /net/trash/usr/src
> total 15
> drwxr-xr-x 3 root root 1024 Aug 30 10:35 debug/
> -rw-r--r-- 1 root root 0 Aug 16 10:01 hello
> drwx------ 2 root root 12288 Aug 16 10:00 lost+found/
> [root@andromeda ~]#
One way to fix this is to simply not hash the dentry when we're doing
the O_EXCL intent optimisation, but rather to only hash it _after_ we've
successfully created the file on the server. Something like the attached
patch ought to do it.
Note, though, that this will not fix the autofs problem: autofs is
trying to perform a totally unnecessary mkdir(), and is giving up when
it is told that SELinux won't authorise that particular operation. This
is clearly an autofs bug...
Cheers,
Trond
--- Begin Message ---
- Subject: No Subject
- From: Trond Myklebust <[email protected]>
- Date:
- Nfs: nfs_lookup - don't hash dentry when optimising away the lookup
Signed-off-by: Trond Myklebust <[email protected]>
---
fs/nfs/dir.c | 13 ++++++++++---
1 files changed, 10 insertions(+), 3 deletions(-)
diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index 51328ae..e83a2ff 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -904,9 +904,14 @@ static struct dentry *nfs_lookup(struct
lock_kernel();
- /* If we're doing an exclusive create, optimize away the lookup */
- if (nfs_is_exclusive_create(dir, nd))
- goto no_entry;
+ /*
+ * If we're doing an exclusive create, optimize away the lookup
+ * but don't hash the dentry.
+ */
+ if (nfs_is_exclusive_create(dir, nd)) {
+ d_instantiate(dentry, NULL);
+ goto out_unlock;
+ }
error = NFS_PROTO(dir)->lookup(dir, &dentry->d_name, &fhandle, &fattr);
if (error == -ENOENT)
@@ -1161,6 +1166,8 @@ int nfs_instantiate(struct dentry *dentr
if (IS_ERR(inode))
return error;
d_instantiate(dentry, inode);
+ if (d_unhashed(dentry))
+ d_rehash(dentry);
return 0;
}
--- End Message ---
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