On 11/20/23 09:21, Naveen N Rao wrote:
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()

It seems there is another path (at least the one that triggers the
-Wstringop-overflow warnings):

emulate_step()->emulate_loadstore()->do_vec_load()->do_byte_reverse()

emulate_step() {
...
if (OP_IS_LOAD_STORE(type)) {
                err = emulate_loadstore(regs, &op);
                if (err)
                        return 0;
                goto instr_done;
        }
...
}

----> emulate_loadstore() {
...
#ifdef CONFIG_ALTIVEC
        case LOAD_VMX:
                if (!(regs->msr & MSR_PR) && !(regs->msr & MSR_VEC))
                        return 0;
                err = do_vec_load(op->reg, ea, size, regs, cross_endian); // 
with size == 32
                break;
#endif
...
#ifdef CONFIG_ALTIVEC
        case STORE_VMX:
                if (!(regs->msr & MSR_PR) && !(regs->msr & MSR_VEC))
                        return 0;
                err = do_vec_store(op->reg, ea, size, regs, cross_endian); // 
with size == 32
                break;
#endif
...
}

----> do_vec_load() // Here is where we have the union of size 16 bytes:
{
...
        union {
                __vector128 v;
                u8 b[sizeof(__vector128)];
        } u = {};
...
        if (unlikely(cross_endian))
                do_byte_reverse(&u.b[ea & 0xf], size);
...
}

----> do_byte_reverse()
{
...
        case 32: {
                unsigned long *up = (unsigned long *)ptr;  // this is a pointer 
to u.b
                unsigned long tmp;

                tmp = byterev_8(up[0]);
                up[0] = byterev_8(up[3]);
                up[3] = tmp;                // and here...
                tmp = byterev_8(up[2]);
                up[2] = byterev_8(up[1]);   // and here we are accessing ptr 
out-of-bounds
                up[1] = tmp;
                break;
        }
...
}

This is where I'm trying to determine if there is a bug. If this path is 
actually
executed, then it seems we have an issue.

Is it possible that commit af99da74333b overlooked the path described above?

--
Gustavo

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