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

[Naveen N Rao]

On Mon, Nov 20, 2023 at 08:33:45AM -0600, Gustavo A. R. Silva wrote:
> 
> 
> On 11/20/23 08:25, Naveen N Rao wrote:
> > 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.
> 
> Does that mean that this piece of code is never actually executed:
> 
>  281         case 32: {
>  282                 unsigned long *up = (unsigned long *)ptr;
>  283                 unsigned long tmp;
>  284
>  285                 tmp = byterev_8(up[0]);
>  286                 up[0] = byterev_8(up[3]);
>  287                 up[3] = tmp;
>  288                 tmp = byterev_8(up[2]);
>  289                 up[2] = byterev_8(up[1]);
>  290                 up[1] = tmp;
>  291                 break;
>  292         }
> 
> or rather, under which conditions the above code is executed, if any?

Please see git history to understand the commit that introduced that 
code. You can do that by starting with a 'git blame' on the file. You'll 
find that the commit that introduced the above hunk was af99da74333b 
("powerpc/sstep: Support VSX vector paired storage access 
instructions").

The above code path is hit via 
do_vsx_load()->emulate_vsx_load()->do_byte_reverse()


- Naveen