[Devel] [PATCH RH7] ms/ovl: fix d_real() for stacked fs

[Pavel Tikhomirov]

From: Miklos Szeredi <mszeredi at redhat.com>

Handling of recursion in d_real() is completely broken.  Recursion is only
done in the 'inode != NULL' case.  But when opening the file we have
'inode == NULL' hence d_real() will return an overlay dentry.  This won't
work since overlayfs doesn't define its own file operations, so all file
ops will fail.

Fix by doing the recursion first and the check against the inode second.

Bash script to reproduce the issue written by Quentin:

 - 8< - - - - - 8< - - - - - 8< - - - - - 8< - - - -
tmpdir=$(mktemp -d)
pushd ${tmpdir}

mkdir -p {upper,lower,work}
echo -n 'rocks' > lower/ksplice
mount -t overlay level_zero upper -o lowerdir=lower,upperdir=upper,workdir=work
cat upper/ksplice

tmpdir2=$(mktemp -d)
pushd ${tmpdir2}

mkdir -p {upper,work}
mount -t overlay level_one upper -o lowerdir=${tmpdir}/upper,upperdir=upper,workdir=work
ls -l upper/ksplice
cat upper/ksplice
 - 8< - - - - - 8< - - - - - 8< - - - - - 8< - - - -

found while investigating https://jira.sw.ru/browse/PSBM-58480

Reported-by: Quentin Casasnovas <quentin.casasnovas at oracle.com>
Signed-off-by: Miklos Szeredi <mszeredi at redhat.com>
Fixes: 2d902671ce1c ("vfs: merge .d_select_inode() into .d_real()")
Cc: <stable at vger.kernel.org> # v4.8+
Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
---
 fs/overlayfs/super.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index edd46a0..0e10085 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -328,11 +328,11 @@ static struct dentry *ovl_d_real(struct dentry *dentry,
 	if (!real)
 		goto bug;
 
+	/* Handle recursion */
+	real = d_real(real, inode, open_flags);
+
 	if (!inode || inode == d_inode(real))
 		return real;
-
-	/* Handle recursion */
-	return d_real(real, inode, open_flags);
 bug:
 	WARN(1, "ovl_d_real(%pd4, %s:%lu): real dentry not found\n", dentry,
 	     inode ? inode->i_sb->s_id : "NULL", inode ? inode->i_ino : 0);
-- 
2.9.3