Re: riscv gcc-13 allyesconfig error the frame size of 2064 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]

["Arnd Bergmann" <arnd@arndb.de>]

On Fri, May 23, 2025, at 19:11, Kent Overstreet wrote:
> On Fri, May 23, 2025 at 05:17:15PM +0200, Arnd Bergmann wrote:
>> 
>> - KASAN_STACK adds extra redzones for each variable
>> - KASAN_STACK further prevents stack slots from getting
>>   reused inside one function, in order to better pinpoint
>>   which instance caused problems like out-of-scope access
>> - passing structures by value causes them to be put on
>>   the stack on some architectures, even when the structure
>>   size is only one or two registers
>
> We mainly do this with bkey_s_c, which is just two words: on x86_64,
> that gets passed in registers. Is riscv different?

Not sure, I think it's mostly older ABIs that are limited,
either not passing structures in registers at all, or only
possibly one but not two of them.

>> - sanitizers turn off optimizations that lead to better
>>   stack usage
>> - in some cases, the missed optimization ends up causing
>>   local variables to get spilled to the stack many times
>>   because of a combination of all the above.
>
> Yeesh.
>
> I suspect we should be running with a larger stack when the sanitizers
> are running, and perhaps tweak the warnings accordingly. I did a bunch
> of stack usage work after I found a kmsan build was blowing out the
> stack, but then running with max stack usage tracing enabled showed it
> to be a largely non issue on non-sanitizer builds, IIRC.

Enabling KASAN does double the available stack space. However, I don't
think we should use that as an excuse to raise the per-function
warning limit, because

 - the majority of all function stacks do not grow that much when
   sanitizers are enabled
 - allmodconfig enables KASAN and should still catch mistakes
   where a driver accidentally puts a large structure on the stack
 - 2KB on 64-bit targes is a really large limit. At some point
   in the past I had a series that lowered the limit to 1536 byte
   for 64-bit targets, but I never managed to get all the changes
   merged.
  

     Arnd