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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