opening a file on a stacked overlayfs is broken.

Hi,

Stacking an overlayfs on top of an overlayfs doens't work when it used to (tested on v4.9-rc5):

  #!/bin/bash -xeu
  tmpdir=$(mktemp -d)
  pushd ${tmpdir}

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

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

  mkdir -p {upper,work}
  mount -t overlay level_one upper -o lowerdir=${tmpdir}/upper,upperdir=upper,workdir=work
  stat upper/bar  # Works fine
  cat upper/bar   # open() returns ENXIO

I _think_ (I haven't bisected it) the guilty commit is 2d902671ce1c ("vfs:
merge .d_select_inode() into .d_real()"), where vfs_open() -> uses d_real() with a a NULL inode, which prevents any recursion from happening, when before d_select_inode() would do the right thing and recurse.

It should be noted this has already been broken and fixed in the past year in 1c8a47df36d7 ("ovl: fix open in stacked overlay").

Let me know if I can help,
Q

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.

---
 fs/overlayfs/super.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

index edd46a0e951d..ae0b36474a9a 100644
@@ -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

And indeed I've proven it here - this tentative patch doesn't work for the general case, it just fixes the simple test case embedded in the commit description.

Any help appreciated!
Q

Thanks for the report.

Following patch should fix it (it's there in your patch, so you weren't too far off). It needs more testing and review, but I think it fixes the basic problem.

Thanks,
Miklos

index edd46a0e951d..f4d2f63fa53a 100644
@@ -328,7 +328,7 @@ static struct dentry *ovl_d_real(struct dentry *dentry,
 	if (!real)
 		goto bug;
 
-	if (!inode || inode == d_inode(real))
+	if (inode && inode == d_inode(real))
 		return real;
 
 	/* Handle recursion */