[Bug 215209] Kernel panic at boot under XEN: integer divide fault while in kernel mode

[bugzilla-noreply]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215209

--- Comment #10 from commit-h...@freebsd.org ---
A commit references this bug:

Author: dim
Date: Sun Dec 18 14:31:12 UTC 2016
New revision: 310228
URL: https://svnweb.freebsd.org/changeset/base/310228

Log:
  MFC r310013 (by cperciva):

  Check that blkfront devices have a non-zero number of sectors and a
  non-zero sector size.  Such a device would be a virtual disk of zero
  bytes; clearly not useful, and not something we should try to attach.

  As a fortuitous side effect, checking that these values are non-zero
  here results in them not *becoming* zero later on the function.  This
  odd behaviour began with r309124 (clang 3.9.0) but is challenging to
  debug; making any changes to this function whatsoever seems to affect
  the llvm optimizer behaviour enough to make the unexpected zeroing of
  the sector_size variable cease.

  PR:   215209
  Security: The potential for variables to unexpectedly become zero
has worrying consequences for security in general, but
not so much in this particular context.

  MFC r310086:

  In xbd_connect(), use correct scanf conversion specifiers for the
  feature_barrier and feature_flush variables.  Otherwise, adjacent
  variables on the stack, such as sector_size, may be overwritten, with
  disastrous results.

  Note that I did not see a good reason to revert the addition of zero
  checks introduced in r310013.  Better safe than sorry.

  PR:   215209
  Tested by:royger

Changes:
_U  stable/10/
  stable/10/sys/dev/xen/blkfront/blkfront.c
_U  stable/11/
  stable/11/sys/dev/xen/blkfront/blkfront.c
_U  stable/9/
_U  stable/9/sys/
  stable/9/sys/dev/xen/blkfront/blkfront.c

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[Bug 215209] Kernel panic at boot under XEN: integer divide fault while in kernel mode

[bugzilla-noreply]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215209

--- Comment #9 from commit-h...@freebsd.org ---
A commit references this bug:

Author: dim
Date: Sun Dec 18 14:31:12 UTC 2016
New revision: 310228
URL: https://svnweb.freebsd.org/changeset/base/310228

Log:
  MFC r310013 (by cperciva):

  Check that blkfront devices have a non-zero number of sectors and a
  non-zero sector size.  Such a device would be a virtual disk of zero
  bytes; clearly not useful, and not something we should try to attach.

  As a fortuitous side effect, checking that these values are non-zero
  here results in them not *becoming* zero later on the function.  This
  odd behaviour began with r309124 (clang 3.9.0) but is challenging to
  debug; making any changes to this function whatsoever seems to affect
  the llvm optimizer behaviour enough to make the unexpected zeroing of
  the sector_size variable cease.

  PR:   215209
  Security: The potential for variables to unexpectedly become zero
has worrying consequences for security in general, but
not so much in this particular context.

  MFC r310086:

  In xbd_connect(), use correct scanf conversion specifiers for the
  feature_barrier and feature_flush variables.  Otherwise, adjacent
  variables on the stack, such as sector_size, may be overwritten, with
  disastrous results.

  Note that I did not see a good reason to revert the addition of zero
  checks introduced in r310013.  Better safe than sorry.

  PR:   215209
  Tested by:royger

Changes:
_U  stable/10/
  stable/10/sys/dev/xen/blkfront/blkfront.c
_U  stable/11/
  stable/11/sys/dev/xen/blkfront/blkfront.c
_U  stable/9/
_U  stable/9/sys/
  stable/9/sys/dev/xen/blkfront/blkfront.c

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[Bug 215209] Kernel panic at boot under XEN: integer divide fault while in kernel mode

[bugzilla-noreply]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215209

--- Comment #7 from commit-h...@freebsd.org ---
A commit references this bug:

Author: cperciva
Date: Tue Dec 13 06:54:14 UTC 2016
New revision: 310013
URL: https://svnweb.freebsd.org/changeset/base/310013

Log:
  Check that blkfront devices have a non-zero number of sectors and a
  non-zero sector size.  Such a device would be a virtual disk of zero
  bytes; clearly not useful, and not something we should try to attach.

  As a fortuitous side effect, checking that these values are non-zero
  here results in them not *becoming* zero later on the function.  This
  odd behaviour began with r309124 (clang 3.9.0) but is challenging to
  debug; making any changes to this function whatsoever seems to affect
  the llvm optimizer behaviour enough to make the unexpected zeroing of
  the sector_size variable cease.

  PR:   215209
  Security: The potential for variables to unexpectedly become zero
has worrying consequences for security in general, but
not so much in this particular context.

Changes:
  head/sys/dev/xen/blkfront/blkfront.c

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[Bug 215209] Kernel panic at boot under XEN: integer divide fault while in kernel mode

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215209

--- Comment #6 from Sylvain Garrigues  ---
(In reply to Colin Percival from comment #5)

I had a system running CURRENT as of Nov. 3 so it was pre-r309124 and I
upgraded to CURRENT as of Dec. 10.

At first since the machine didn't survive a reboot I thought I did a bad
mergemaster on passwd files. So I installed a fresh EC2 AMI which also happened
to be from CURRENT-amd64-2016-12-10 and I had the same issue. I looked at the
console and saw the kernel panic.

Tried the EC2 AMI from CURRENT-amd64-2016-11-30, same problem.
Finally tried the EC2 AMI from CURRENT-amd64-2016-11-01, NO problem.

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[Bug 215209] Kernel panic at boot under XEN: integer divide fault while in kernel mode

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215209

--- Comment #4 from Roger Pau Monné  ---
I don't seem to be able to reproduce this with r309875, can you check if you
still get the panic with that or any later revision?

Thanks, Roger.

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[Bug 215209] Kernel panic at boot under XEN: integer divide fault while in kernel mode

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215209

Roger Pau Monné  changed:

   What|Removed |Added

 CC||roy...@freebsd.org

--- Comment #3 from Roger Pau Monné  ---
I'm aware of this, I've already realised last week but haven't been able to
debug it. Will get to it now. In the meantime, you can boot the the previous
kernel using:

> boot kernel.old

From the loader command line.

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[Bug 215209] Kernel panic at boot under XEN: integer divide fault while in kernel mode

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215209

Mark Linimon  changed:

   What|Removed |Added

 CC|freebsd-am...@freebsd.org   |
   Assignee|freebsd-b...@freebsd.org|freebsd-xen@FreeBSD.org

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[Bug 215209] Kernel panic at boot under XEN: integer divide fault while in kernel mode

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215209

--- Comment #2 from Colin Percival  ---
This seems to have been introduced by the import of clang 3.9.0 in r309124. 
I'll work on tracking this down further next week; I'm working on NFS right now
and it's best if I don't context-switch.

As you say, best to avoid HEAD right now on EC2.

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[Bug 215209] Kernel panic at boot under XEN: integer divide fault while in kernel mode

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215209

Sylvain Garrigues  changed:

   What|Removed |Added

 CC||freebsd-xen@FreeBSD.org,
   ||sylv...@sylvaingarrigues.co
   ||m

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