3 quick questions about stack alignment for powerpc (32-bit) signal handlers

[Justin Hibbits]

Does this occur with gcc-built world and/or kernel?  You could put  
some printf()s in sendsig(), and there are KTR tracepoints already  
present.  The code assumes a fully aligned user stack, which should be  
correct, but may not be.

- Justin
On Jan 31, 2016, at 6:41 PM, Mark Millard wrote:

> I have submitted Bug 206810 for this 11.0-CURRENT/clang380-import  
> stack alignment problem for TARGET_ARCH=powerpc signal delivery.
>
> ===
> Mark Millard
> markmi at dsl-only.net
>
> On 2016-Jan-31, at 6:08 AM, Roman Divacky <rdivacky at vlakno.cz>  
> wrote:
>
> Fwiw, LLVM expect 16B aligned stack on PowerPC.
>
> On Sun, Jan 31, 2016 at 05:55:20AM -0800, Mark Millard wrote:
>> 3 quick FreeBSD for powerpc (32-bit) questions:
>>
>>
>> A) For PowerPC (32-bit) what is the stack alignment requirement by  
>> the ABI(s) that FreeBSD targets?
>>
>> B) Are signal handlers supposed to be given that alignment?
>>
>>
>> I ask because signal handlers are at times begin given just 4-byte  
>> alignment but clang 3.8.0 powerpc's code generation can depend on  
>> the alignment being more than 4.
>>
>> clang 3.8.0 can calculate addresses by, for example, masking in a  
>> 0x4 relative to what would need to be an aligned address with  
>> alignment 8 or more instead of adding 0x4 to a more arbitrary  
>> address.
>>
>> So far I've only seen less than 8 byte stack alignment via signal  
>> handler activity.
>>
>>
>> C) Which should be blamed for problems here: clang's code  
>> generation, FreeBSD's stack alignment handling for signals, or both?
>>
>> ===
>> Mark Millard
>> markmi at dsl-only.net
>>
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>