[patch] vfs: fix vfs_rename_dir for FS_RENAME_DOES_D_MOVE filesystems

[patch] vfs: fix vfs_rename_dir for FS_RENAME_DOES_D_MOVE filesystems

[Miklos Szeredi]

From: Miklos Szeredi <mszeredi@suse.cz>

vfs_rename_dir() doesn't properly account for filesystems with
FS_RENAME_DOES_D_MOVE.  If new_dentry has a target inode attached, it
unhashes the new_dentry prior to the rename() iop and rehashes it
after, but doesn't account for the possibility that rename() may have
swapped {old,new}_dentry.  For FS_RENAME_DOES_D_MOVE filesystems, it
rehashes new_dentry (now the old renamed-from name, which d_move()
expected to go away), such that a subsequent lookup will find it.

This was caught by the recently posted POSIX fstest suite, rename/10.t 
test 62 (and others) on ceph.

Fix by not rehashing the new dentry.  Rehashing would only make sense
if the rename failed (which should happen extremely rarely), but we
cannot handle that case correctly 100% of the time anyway, so...

Reported-by: Sage Weil <sage@newdream.net>
CC: Zach Brown <zach.brown@oracle.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
---
 fs/namei.c |    2 --
 1 file changed, 2 deletions(-)

Index: linux-2.6/fs/namei.c
===================================================================
--- linux-2.6.orig/fs/namei.c	2008-07-21 09:46:07.000000000 +0200
+++ linux-2.6/fs/namei.c	2008-07-21 11:56:01.000000000 +0200
@@ -2574,8 +2574,6 @@ static int vfs_rename_dir(struct inode *
 		if (!error)
 			target->i_flags |= S_DEAD;
 		mutex_unlock(&target->i_mutex);
-		if (d_unhashed(new_dentry))
-			d_rehash(new_dentry);
 		dput(new_dentry);
 	}
 	if (!error)

Re: [patch] vfs: fix vfs_rename_dir for FS_RENAME_DOES_D_MOVE filesystems

[Al Viro]

On Mon, Jul 21, 2008 at 01:41:47PM +0200, Miklos Szeredi wrote:
> From: Miklos Szeredi <mszeredi@suse.cz>
> 
> vfs_rename_dir() doesn't properly account for filesystems with
> FS_RENAME_DOES_D_MOVE.  If new_dentry has a target inode attached, it
> unhashes the new_dentry prior to the rename() iop and rehashes it
> after, but doesn't account for the possibility that rename() may have
> swapped {old,new}_dentry.  For FS_RENAME_DOES_D_MOVE filesystems, it
> rehashes new_dentry (now the old renamed-from name, which d_move()
> expected to go away), such that a subsequent lookup will find it.
> 
> This was caught by the recently posted POSIX fstest suite, rename/10.t 
> test 62 (and others) on ceph.
> 
> Fix by not rehashing the new dentry.  Rehashing would only make sense
> if the rename failed (which should happen extremely rarely), but we
> cannot handle that case correctly 100% of the time anyway, so...

Lovely.  AFAICS, that's a fallout from
commit 349457ccf2592c14bdf13b6706170ae2e94931b1
Author: Mark Fasheh <mark.fasheh@oracle.com>
Date:   Fri Sep 8 14:22:21 2006 -0700

    [PATCH] Allow file systems to manually d_move() inside of ->rename()

that had allowed that crap for directories.  Note that d_rehash() used
to be needed (d_move() would unhash the source otherwise) and d_move()
used to be unconditional until the changeset above.

It's _probably_ OK now, but I'd really like to think about NFS behaviour.
There are subtle traps in that area.

BTW, failing rename() is trivial - just have a non-empty target...

[PATCH] vfs: fix vfs_rename_dir for FS_RENAME_DOES_D_MOVE filesystems

[Sage Weil]

Al, Christoph,

Zach just ran into this bug as well.  Does this fix look reasonable?

thanks-
sage


----
>From Sage Weil <sage@newdream.net>

d_move() is strangely implemented in that it swaps the position of 
new_dentry and old_dentry in the namespace.  This is admittedly weird (see 
comments for d_move_locked()), but normally harmless: even though 
new_dentry swaps places with old_dentry, it is unhashed, and won't be seen 
by a subsequent lookup.

However, vfs_rename_dir() doesn't properly account for filesystems with 
FS_RENAME_DOES_D_MOVE.  If new_dentry has a target inode attached, it 
unhashes the new_dentry prior to the rename() iop and rehashes it after, 
but doesn't account for the possibility that rename() may have swapped 
{old,new}_dentry.  For FS_RENAME_DOES_D_MOVE filesystems, it rehashes 
new_dentry (now the old renamed-from name, which d_move() expected to go 
away), such that a subsequent lookup will find it.

To correct this, move vfs_rename_dir()'s call to d_move() _before_ the 
target inode mutex is dealt with.  Since d_move() will have been called 
for all filesystems at this point, there is no need to rehash new_dentry 
unless the rename failed.  (If the rename succeeded, old_dentry should 
already be rehashed in the new location.)

The only in-tree filesystems with FS_RENAME_DOES_D_MOVE are ocfs2 and nfs.  
My suspicion is that they are not bitten by this particular bug because 
the incorrectly rehashed new_dentry gets rejected by d_revalidate().

This was caught by the recently posted POSIX fstest suite, rename/10.t 
test 62 (and others) on ceph.  With this patch, all tests succeed.


Signed-off-by: Sage Weil <sage@newdream.net>
---
 fs/namei.c |    9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

--- linux-2.6.25-orig/fs/namei.c	2008-04-16 19:49:44.000000000 -0700
+++ linux/fs/namei.c	2008-04-18 13:59:30.000000000 -0700
@@ -2488,17 +2488,18 @@
 		error = -EBUSY;
 	else 
 		error = old_dir->i_op->rename(old_dir, old_dentry, new_dir, new_dentry);
+	if (!error)
+		if (!(old_dir->i_sb->s_type->fs_flags & FS_RENAME_DOES_D_MOVE))
+			d_move(old_dentry, new_dentry);
 	if (target) {
 		if (!error)
 			target->i_flags |= S_DEAD;
 		mutex_unlock(&target->i_mutex);
-		if (d_unhashed(new_dentry))
+		/* only rehash new_dentry if the rename failed */
+		if (error && d_unhashed(new_dentry))
 			d_rehash(new_dentry);
 		dput(new_dentry);
 	}
-	if (!error)
-		if (!(old_dir->i_sb->s_type->fs_flags & FS_RENAME_DOES_D_MOVE))
-			d_move(old_dentry,new_dentry);
 	return error;
 }
 
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Re: [PATCH] vfs: fix vfs_rename_dir for FS_RENAME_DOES_D_MOVE filesystems

[Miklos Szeredi]

On Fri, 11 Jul 2008, Sage Weil wrote:
> However, vfs_rename_dir() doesn't properly account for filesystems with 
> FS_RENAME_DOES_D_MOVE.  If new_dentry has a target inode attached, it 
> unhashes the new_dentry prior to the rename() iop and rehashes it after, 
> but doesn't account for the possibility that rename() may have swapped 
> {old,new}_dentry.  For FS_RENAME_DOES_D_MOVE filesystems, it rehashes 
> new_dentry (now the old renamed-from name, which d_move() expected to go 
> away), such that a subsequent lookup will find it.
> 
> To correct this, move vfs_rename_dir()'s call to d_move() _before_ the 
> target inode mutex is dealt with.  Since d_move() will have been called 
> for all filesystems at this point, there is no need to rehash new_dentry 
> unless the rename failed.  (If the rename succeeded, old_dentry should 
> already be rehashed in the new location.)

I think rehashing the new dentry is bogus, even on error.  And it
looks racy with lookup as well.

I wonder what the original reason for that was?  Git history doesn't
tell...

So a better fix would be just to remove the rehashing completely.
Does the below patch work for you?

Thanks,
Miklos

---
 fs/namei.c |    2 --
 1 file changed, 2 deletions(-)

Index: linux-2.6/fs/namei.c
===================================================================
--- linux-2.6.orig/fs/namei.c	2008-07-11 22:09:32.000000000 +0200
+++ linux-2.6/fs/namei.c	2008-07-11 22:40:16.000000000 +0200
@@ -2643,8 +2643,6 @@ static int vfs_rename_dir(struct inode *
 		if (!error)
 			target->i_flags |= S_DEAD;
 		mutex_unlock(&target->i_mutex);
-		if (d_unhashed(new_dentry))
-			d_rehash(new_dentry);
 		dput(new_dentry);
 	}
 	if (!error)

Re: [PATCH] vfs: fix vfs_rename_dir for FS_RENAME_DOES_D_MOVE filesystems

[Zach Brown]

Miklos Szeredi wrote:
> On Fri, 11 Jul 2008, Sage Weil wrote:
>> However, vfs_rename_dir() doesn't properly account for filesystems with 
>> FS_RENAME_DOES_D_MOVE.  If new_dentry has a target inode attached, it 
>> unhashes the new_dentry prior to the rename() iop and rehashes it after, 
>> but doesn't account for the possibility that rename() may have swapped 
>> {old,new}_dentry.  For FS_RENAME_DOES_D_MOVE filesystems, it rehashes 
>> new_dentry (now the old renamed-from name, which d_move() expected to go 
>> away), such that a subsequent lookup will find it.
>>
>> To correct this, move vfs_rename_dir()'s call to d_move() _before_ the 
>> target inode mutex is dealt with.  Since d_move() will have been called 
>> for all filesystems at this point, there is no need to rehash new_dentry 
>> unless the rename failed.  (If the rename succeeded, old_dentry should 
>> already be rehashed in the new location.)
> 
> I think rehashing the new dentry is bogus, even on error.

So we'd just come back through lookup to repopulate the existing
destination name that vfs_rename_dir() unhashed before calling
->rename() in the case that the rename fails?  That seems gross, but
relatively harmless.

> So a better fix would be just to remove the rehashing completely.
> Does the below patch work for you?

It'd work for my case, yeah.

- z

Re: [PATCH] vfs: fix vfs_rename_dir for FS_RENAME_DOES_D_MOVE filesystems

[Miklos Szeredi]

On Fri, 11 Jul 2008, Zach Brown wrote:
> Miklos Szeredi wrote:
> > I think rehashing the new dentry is bogus, even on error.
> 
> So we'd just come back through lookup to repopulate the existing
> destination name that vfs_rename_dir() unhashed before calling
> ->rename() in the case that the rename fails?  That seems gross, but
> relatively harmless.

We are talking about an _extremely_ rare event.  Even the
"vfs_rename_dir() with positive target" is very rare, let alone a
failing one.

If we are going to worry about directory removal failure cases, we
should start with rmdir(), which is a wee bit more common, than the
above case here.

Miklos

Re: [PATCH] vfs: fix vfs_rename_dir for FS_RENAME_DOES_D_MOVE filesystems

[Zach Brown]

Miklos Szeredi wrote:
> On Fri, 11 Jul 2008, Zach Brown wrote:
>> Miklos Szeredi wrote:
>>> I think rehashing the new dentry is bogus, even on error.
>> So we'd just come back through lookup to repopulate the existing
>> destination name that vfs_rename_dir() unhashed before calling
>> ->rename() in the case that the rename fails?  That seems gross, but
>> relatively harmless.
> 
> We are talking about an _extremely_ rare event.

Agreed, which is why I called it relatively harmless.

- z

Re: [PATCH] vfs: fix vfs_rename_dir for FS_RENAME_DOES_D_MOVE filesystems

[Sage Weil]

On Fri, 11 Jul 2008, Miklos Szeredi wrote:
> On Fri, 11 Jul 2008, Sage Weil wrote:
> > However, vfs_rename_dir() doesn't properly account for filesystems with 
> > FS_RENAME_DOES_D_MOVE.  If new_dentry has a target inode attached, it 
> > unhashes the new_dentry prior to the rename() iop and rehashes it after, 
> > but doesn't account for the possibility that rename() may have swapped 
> > {old,new}_dentry.  For FS_RENAME_DOES_D_MOVE filesystems, it rehashes 
> > new_dentry (now the old renamed-from name, which d_move() expected to go 
> > away), such that a subsequent lookup will find it.
> > 
> > To correct this, move vfs_rename_dir()'s call to d_move() _before_ the 
> > target inode mutex is dealt with.  Since d_move() will have been called 
> > for all filesystems at this point, there is no need to rehash new_dentry 
> > unless the rename failed.  (If the rename succeeded, old_dentry should 
> > already be rehashed in the new location.)
> 
> I think rehashing the new dentry is bogus, even on error.  And it
> looks racy with lookup as well.
> 
> I wonder what the original reason for that was?  Git history doesn't
> tell...

I assume just to leave the dentry in the same stat we originally found it 
in?  

> So a better fix would be just to remove the rehashing completely.
> Does the below patch work for you?

This would work as well, yeah.  I've no real preference, here...

thanks-
sage


> 
> Thanks,
> Miklos
> 
> ---
>  fs/namei.c |    2 --
>  1 file changed, 2 deletions(-)
> 
> Index: linux-2.6/fs/namei.c
> ===================================================================
> --- linux-2.6.orig/fs/namei.c	2008-07-11 22:09:32.000000000 +0200
> +++ linux-2.6/fs/namei.c	2008-07-11 22:40:16.000000000 +0200
> @@ -2643,8 +2643,6 @@ static int vfs_rename_dir(struct inode *
>  		if (!error)
>  			target->i_flags |= S_DEAD;
>  		mutex_unlock(&target->i_mutex);
> -		if (d_unhashed(new_dentry))
> -			d_rehash(new_dentry);
>  		dput(new_dentry);
>  	}
>  	if (!error)
> --
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