[PATCH] ovl: tentative fix for broken vfs_open() on stacked overlayfs.

[Quentin Casasnovas <quentin.casasnovas@oracle.com>]

If two overlayfs filesystems are stacked on top of each other, then we need
to recurse when opening a file.  This used to work and was first broken by:

  4bacc9c9234c ("overlayfs: Make f_path always point to the overlay...")

and fixed by:

  1c8a47df36d7 ("ovl: fix open in stacked overlay")

But it looks like it was re-introduced in:

  2d902671ce1c ("vfs: merge .d_select_inode() into .d_real()")

I know close to nothing about VFS/overlayfs so this might be completely
wrong for many reasons but it fixes the following test case for me:

  #!/bin/bash -xeu

  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

For sanity checking, I've run the test-suite mentionned in
Documentation/filesystems/overlayfs.txt:

  git://git.infradead.org/users/dhowells/unionmount-testsuite.git

Though with and without this patch it returned zero so I am assuming it
does not contain any tests with stacked overlayfs.

Fixes: 2d902671ce1c ("vfs: merge .d_select_inode() into .d_real()")
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
---
 fs/overlayfs/super.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index edd46a0e951d..ae0b36474a9a 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -321,16 +321,22 @@ static struct dentry *ovl_d_real(struct dentry *dentry,
 	}
 
 	real = ovl_dentry_upper(dentry);
-	if (real && (!inode || inode == d_inode(real)))
-		return real;
+	if (real && inode && inode == d_inode(real))
+	    return real;
+
+	if (real && !inode && d_inode(real))
+		return d_real(real, d_inode(real), open_flags);
 
 	real = ovl_dentry_lower(dentry);
 	if (!real)
 		goto bug;
 
-	if (!inode || inode == d_inode(real))
+	if (inode && inode == d_inode(real))
 		return real;
 
+	if (!inode && d_inode(real))
+		return d_real(real, d_inode(real), open_flags);
+
 	/* Handle recursion */
 	return d_real(real, inode, open_flags);
 bug:
-- 
2.11.0.rc2