[PATCH 0/6] Sort out overlay layers and fs arrays

Amir Goldstein
2019-11-17T15:43:43Z

Miklos,

When I started generalizing the lower_layers/lower_fs arrays
I noticed a bug that was introduced in v4.17 with xino.

In the case of lower layer on upper fs, we do not have a pseudo_dev
assigned to lower layer and we expose the real lower st_dev;st_ino.
This happens on non-samefs when xino is disabled (default).
This is a very real bug, not really a corner case and I have an
an xfstest [1] for it that I will post later.

In the mean while, I also pushed a fix to unionmount-testsuite devel
branch [2] to demonstrate the issue.

With upstream kernel, this test ends up with a copied up file
from middle layer, whose on same fs as upper and its exposed
st_dev;st_ino are invalid:

./run --ov=1 --verify hard-link
...
/mnt/a/no_foo110: File unexpectedly on upper layer

Patch 1 in the series is a small fix for stable that fixes the
v4.17 regression in favor of a different, less severe regression.
The new regression can be demonstrated with:

./run --ov=1 --verify --xino hard-link
...
/mnt/a/no_foo110: inode number/layer changed on copy up
(got 39:24707, was 39:24700)

Patches 2-4 generalize the lower_{layer/fs} arrays to layer/fs arrays
and get rid of some special casing of upper layer.

Patches 5-6 use the cleanup to solve the corner case that you pointed
out with bas_uuid [3] and to fix the regression introduced by patch 1.

After patch 6, both unionmount-testsuite configurations
above pass the test st_dev;st_ino verifications.

I doubt if patches 2-6 are stable material, because not sure the
corner cases they fix are worth the trouble.

The series depends on the bad_uuid patch v5 that I posted on Thursday.

I was also considering setting xino=on by default if xino_auto
is enabled, because what have we got to loose?

The inodes whose st_ino fit in lower bits (by far more common) will
use overlay st_dev and the inodes whose st_ino overflow the lower bits
will use pseudo_dev. Seems like a win-win situation, but I wanted to
get your feedback on this before sending out a patch.

Thanks,
Amir.

[1] https://github.com/amir73il/xfstests/commit/c667f26839ae487c509b95abae670fdca1c535c8
[2] https://github.com/amir73il/unionmount-testsuite/commit/1724ef2245c5e56f73e436b37407d00ef498f9bc
[3] https://lore.kernel.org/lkml/CAJfpegufS=OGcvFbWEVumNSCPO_JXyEuJNAbmO5ubscSarVtRQ@mail.gmail.com/

Amir Goldstein (6):
ovl: fix corner case of non-unique st_dev;st_ino
ovl: generalize the lower_layers[] array
ovl: simplify ovl_same_sb() helper
ovl: generalize the lower_fs[] array
ovl: fix corner case of conflicting lower layer uuid
ovl: fix corner case of non-constant st_dev;st_ino

fs/overlayfs/export.c | 6 +-
fs/overlayfs/inode.c | 35 +++++------
fs/overlayfs/namei.c | 10 ++--
fs/overlayfs/overlayfs.h | 23 ++++++-
fs/overlayfs/ovl_entry.h | 14 +++--
fs/overlayfs/readdir.c | 11 ++--
fs/overlayfs/super.c | 125 ++++++++++++++++++++++-----------------
fs/overlayfs/util.c | 18 ++----
8 files changed, 132 insertions(+), 110 deletions(-)

--
2.17.1