Re: x86-64: Maintain 16-byte stack alignment

On Tue, Jan 10, 2017 at 10:33:40PM +0800, Herbert Xu wrote:

I recently applied the patch

	https://patchwork.kernel.org/patch/9468391/

and ended up with a boot crash when it tried to run the x86 chacha20
code.  It turned out that the patch changed a manually aligned
stack buffer to one that is aligned by gcc.  What was happening was
that gcc can stack align to any value on x86-64 except 16.  The
reason is that gcc assumes that the stack is always 16-byte aligned,
which is not actually the case in the kernel.

BTW this is with Debian gcc 4.7.2 which does not allow an 8-byte
stack alignment as attempted by the Makefile:

$ gcc -S -O2 -mno-sse -mpreferred-stack-boundary=3 a.c
a.c:1:0: error: -mpreferred-stack-boundary=3 is not between 4 and 12
$ 

Obviously this is not an issue if your compiler actually allows
the 8-byte alignment.

Cheers,
-- 
Email: Herbert Xu [off-list ref]
Home Page: http://gondor.apana.org.au/~herbert/
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