[RFC - is this a bug?] powerpc/lib/sstep: Asking for some light on this, please. :)

[Naveen N Rao]

On Fri, Nov 17, 2023 at 12:36:01PM -0600, Gustavo A. R. Silva wrote:
> Hi all,
> 
> I'm trying to fix the following -Wstringop-overflow warnings, and I'd like
> to get your feedback on this, please:
> 
> In function 'do_byte_reverse',
>     inlined from 'do_vec_store' at /home/gus/linux/arch/powerpc/lib/sstep.c:722:3,
>     inlined from 'emulate_loadstore' at /home/gus/linux/arch/powerpc/lib/sstep.c:3510:9:
> arch/powerpc/lib/sstep.c:287:23: error: writing 8 bytes into a region of size 0 [-Werror=stringop-overflow=]
>   287 |                 up[3] = tmp;
>       |                 ~~~~~~^~~~~
> arch/powerpc/lib/sstep.c: In function 'emulate_loadstore':
> arch/powerpc/lib/sstep.c:708:11: note: at offset [24, 39] into destination object 'u' of size 16
>   708 |         } u;
>       |           ^
> In function 'do_byte_reverse',
>     inlined from 'do_vec_store' at /home/gus/linux/arch/powerpc/lib/sstep.c:722:3,
>     inlined from 'emulate_loadstore' at /home/gus/linux/arch/powerpc/lib/sstep.c:3510:9:
> arch/powerpc/lib/sstep.c:289:23: error: writing 8 bytes into a region of size 0 [-Werror=stringop-overflow=]
>   289 |                 up[2] = byterev_8(up[1]);
>       |                 ~~~~~~^~~~~~~~~~~~~~~~~~
> arch/powerpc/lib/sstep.c: In function 'emulate_loadstore':
> arch/powerpc/lib/sstep.c:708:11: note: at offset 16 into destination object 'u' of size 16
>   708 |         } u;
>       |           ^
> In function 'do_byte_reverse',
>     inlined from 'do_vec_load' at /home/gus/linux/arch/powerpc/lib/sstep.c:691:3,
>     inlined from 'emulate_loadstore' at /home/gus/linux/arch/powerpc/lib/sstep.c:3439:9:
> arch/powerpc/lib/sstep.c:287:23: error: writing 8 bytes into a region of size 0 [-Werror=stringop-overflow=]
>   287 |                 up[3] = tmp;
>       |                 ~~~~~~^~~~~
> arch/powerpc/lib/sstep.c: In function 'emulate_loadstore':
> arch/powerpc/lib/sstep.c:681:11: note: at offset [24, 39] into destination object 'u' of size 16
>   681 |         } u = {};
>       |           ^
> arch/powerpc/lib/sstep.c:681:11: note: at offset [24, 39] into destination object 'u' of size 16
> arch/powerpc/lib/sstep.c:681:11: note: at offset [24, 39] into destination object 'u' of size 16
> 
> The origing of the issue seems to be the following calls to function `do_vec_store()`, when
> `size > 16`, or more precisely when `size == 32`
> 
> arch/powerpc/lib/sstep.c:
> 3436         case LOAD_VMX:
> 3437                 if (!(regs->msr & MSR_PR) && !(regs->msr & MSR_VEC))
> 3438                         return 0;
> 3439                 err = do_vec_load(op->reg, ea, size, regs, cross_endian);
> 3440                 break;
> ...
> 3507         case STORE_VMX:
> 3508                 if (!(regs->msr & MSR_PR) && !(regs->msr & MSR_VEC))
> 3509                         return 0;
> 3510                 err = do_vec_store(op->reg, ea, size, regs, cross_endian);
> 3511                 break;

LOAD_VMX and STORE_VMX are set in analyse_instr() and size does not 
exceed 16. So, the warning looks incorrect to me.

- Naveen