GFS2: pull request for high-priority bug

GFS2: pull request for high-priority bug

[Bob Peterson]

Hi Linus,

Please consider pulling the following additional patch for the GFS2 file system.

Regards,

Bob Peterson

----------------------------------------------------------------
The following changes since commit ae50dfd61665086e617cc9e554a1285d52765670:

  Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net (2017-03-14 21:31:23 -0700)

are available in the git repository at:

  git://git.kernel.org/pub/scm/linux/kernel/git/gfs2/linux-gfs2.git tags/gfs2-4.11-rc3.fixes

for you to fetch changes up to 28ea06c46fbcab63fd9a55531387b7928a18a590:

  gfs2: Avoid alignment hole in struct lm_lockname (2017-03-15 10:06:07 -0400)

----------------------------------------------------------------
This is an emergency patch for 4.11-rc3

The GFS2 developers uncovered a really nasty problem that
can lead to random corruption and kernel panic, much like
the last one. Andreas Gruenbacher wrote this simple one-line
patch to fix the problem:

28ea06c gfs2: Avoid alignment hole in struct lm_lockname

----------------------------------------------------------------
Andreas Gruenbacher (1):
      gfs2: Avoid alignment hole in struct lm_lockname

 fs/gfs2/incore.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Re: GFS2: pull request for high-priority bug

[Linus Torvalds]

On Wed, Mar 15, 2017 at 7:32 AM, Bob Peterson <rpeterso@redhat.com> wrote:
>
> Andreas Gruenbacher (1):
>       gfs2: Avoid alignment hole in struct lm_lockname

So I've pulled this because I think it fixes a real bug, but honestly
I think it's the wrong fix.

Marking that lm_lockname structure "packed, aligned(4)" means that the
compiler will now think that the 64-bit fields in it may be unaligned
- including on architectures where that can be very expensive and the
compiler now might generate stupid unaligned instruction sequences to
load those values.

So the *correct* fix, I think, would have been:

 - add a comment about not having holes in the struct due to the hashing

 - sort the fields by size (so "ln_number" first, then "ln_sbd", then "ln_type")

 - use offsetofend(struct lm_lockname, ln_type) instead of sizeof() when hashing

which avoids the "possibly generate garbage code" issue due to the
quick-and-dirty one-liner approach.

Hmm?

                 Linus

Re: GFS2: pull request for high-priority bug

[Bob Peterson]

----- Original Message -----
| On Wed, Mar 15, 2017 at 7:32 AM, Bob Peterson <rpeterso@redhat.com> wrote:
| >
| > Andreas Gruenbacher (1):
| >       gfs2: Avoid alignment hole in struct lm_lockname
| 
| So I've pulled this because I think it fixes a real bug, but honestly
| I think it's the wrong fix.
| 
| Marking that lm_lockname structure "packed, aligned(4)" means that the
| compiler will now think that the 64-bit fields in it may be unaligned
| - including on architectures where that can be very expensive and the
| compiler now might generate stupid unaligned instruction sequences to
| load those values.
| 
| So the *correct* fix, I think, would have been:
| 
|  - add a comment about not having holes in the struct due to the hashing
| 
|  - sort the fields by size (so "ln_number" first, then "ln_sbd", then
|  "ln_type")
| 
|  - use offsetofend(struct lm_lockname, ln_type) instead of sizeof() when
|  hashing
| 
| which avoids the "possibly generate garbage code" issue due to the
| quick-and-dirty one-liner approach.
| 
| Hmm?
| 
|                  Linus

Hi Linus,

Thanks. Yes, good ideas.
I see your point and I'll see if we can get that fixed up for the next merge window.

Bob Peterson