--- Comment #9 from commit-h...@freebsd.org ---
A commit references this bug:
Date: Sun Dec 18 14:31:12 UTC 2016
New revision: 310228
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.
Security: The potential for variables to unexpectedly become zero
has worrying consequences for security in general, but
not so much in this particular context.
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
Note that I did not see a good reason to revert the addition of zero
checks introduced in r310013. Better safe than sorry.
Tested by: royger
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