[Xcb] [PATCH libxcb] Fix alignment issues in FD passing code

[Uli Schlachter]

On 11.11.2013 23:11, Mark Kettenis wrote:
> From: Mark Kettenis <kettenis at openbsd.org>
> 
> A char array on the stack is not guaranteed to have more than byte alignment.
> This means that casting it to a 'struct cmsghdr' and accessing its members
> may result in unaligned access.  This will generate SIGBUS on struct
> alignment architectures like OpenBSD/sparc64.  The canonical solution is to
> use a union to force proper alignment.

Since this mail had a response on xorg-devel which didn't go to the xcb list:

On 12.11.2013 01:52, Mouse wrote:
>> A char array on the stack is not guaranteed to have more than byte
>> alignment.  This means that casting it to a 'struct cmsghdr' and
>> accessing its members may result in unaligned access.  This will
>> generate SIGBUS on struct alignment architectures like
>> OpenBSD/sparc64.  The canonical solution is to use a union to force
>> proper alignment.
>
> This is good enough for access to the struct cmsghdr.  It is, however,
> not good enough to use CMSG_DATA (nor, I think, CMSG_NXTHDR) on the
> resulting cmsghdr; as far as I can tell there is no way to align the
> buffer correctly for that.  If you're willing to make the assumption
> (which I find no grounds for in 3542) that it's the alignment of some
> object type, you can malloc the buffer, but in at least some
> implementations, the alignment required is chosen by the kernel and can
> potentially vary, either between runs (eg, across a
> mostly-binary-compatible OS version change) or even at run time.  In
> any case, I see no grounds, except for an unwarranted familiarity with
> certain common implementations, for assuming that the alignment of
> struct cmsghdr is sufficient.
>
> This is one of the reasons I consider the CMSG_* interface
> fundamentally broken.
>
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(Just writing this to make sure it gets seen and whoever wants to handle this
patch knows this)

Uli
-- 
If you have to type the letters "A-E-S" into your source code, you're doing it
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