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[PATCH] xfs: fix broken i_nlink accounting for whiteout tmpfile inode

To: xfs@xxxxxxxxxxx
Subject: [PATCH] xfs: fix broken i_nlink accounting for whiteout tmpfile inode
From: Brian Foster <bfoster@xxxxxxxxxx>
Date: Fri, 15 May 2015 15:19:28 -0400
Delivered-to: xfs@xxxxxxxxxxx
XFS uses the internal tmpfile() infrastructure for the whiteout inode
used for RENAME_WHITEOUT operations. For tmpfile inodes, XFS allocates
the inode, drops di_nlink, adds the inode to the agi unlinked list,
calls d_tmpfile() which correspondingly drops i_nlink of the vfs inode,
and then finishes the common inode setup (e.g., clear I_NEW and unlock).

The d_tmpfile() call was originally made inxfs_create_tmpfile(), but was
pulled up out of that function as part of the following commit to
resolve a deadlock issue:

        330033d6 xfs: fix tmpfile/selinux deadlock and initialize security

As a result, callers of xfs_create_tmpfile() are responsible for either
calling d_tmpfile() or fixing up i_nlink appropriately. The whiteout
tmpfile allocation helper does neither. As a result, the vfs ->i_nlink
becomes inconsistent with the on-disk ->di_nlink once xfs_rename() links
it back into the source dentry and calls xfs_bumplink().

Update the assert in xfs_rename() to help detect this problem in the
future and update xfs_rename_alloc_whiteout() to decrement the link
count as part of the manual tmpfile inode setup.

Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
 fs/xfs/xfs_inode.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
index d6ebc85..e54eb05 100644
--- a/fs/xfs/xfs_inode.c
+++ b/fs/xfs/xfs_inode.c
@@ -2883,7 +2883,13 @@ xfs_rename_alloc_whiteout(
        if (error)
                return error;
-       /* Satisfy xfs_bumplink that this is a real tmpfile */
+       /*
+        * Prepare the tmpfile inode as if it were created through the VFS.
+        * Otherwise, the link increment paths will complain about nlink 0->1.
+        * Drop the link count as done by d_tmpfile(), complete the inode setup
+        * and flag it as linkable.
+        */
+       drop_nlink(VFS_I(tmpfile));
        VFS_I(tmpfile)->i_state |= I_LINKABLE;
@@ -3151,7 +3157,7 @@ xfs_rename(
         * intermediate state on disk.
        if (wip) {
-               ASSERT(wip->i_d.di_nlink == 0);
+               ASSERT(VFS_I(wip)->i_nlink == 0 && wip->i_d.di_nlink == 0);
                error = xfs_bumplink(tp, wip);
                if (error)
                        goto out_trans_abort;

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