Re: [PATCH] powerpc/lib: Avoid array bounds warnings in vec ops

[Gustavo A. R. Silva]

On 11/20/23 17:54, Michael Ellerman wrote:

Building with GCC 13 (which has -array-bounds enabled) there are several
warnings in sstep.c along the lines of:

   In function ‘do_byte_reverse’,
       inlined from ‘do_vec_load’ at arch/powerpc/lib/sstep.c:691:3,
       inlined from ‘emulate_loadstore’ at arch/powerpc/lib/sstep.c:3439:9:
   arch/powerpc/lib/sstep.c:289:23: error: array subscript 2 is outside array 
bounds of ‘u8[16]’ {aka ‘unsigned char[16]’} [-Werror=array-bounds=]
     289 |                 up[2] = byterev_8(up[1]);
         |                 ~~~~~~^~~~~~~~~~~~~~~~~~
   arch/powerpc/lib/sstep.c: In function ‘emulate_loadstore’:
   arch/powerpc/lib/sstep.c:681:11: note: at offset 16 into object ‘u’ of size 
16
     681 |         } u = {};
         |           ^

do_byte_reverse() supports a size up to 32 bytes, but in these cases the
caller is only passing a 16 byte buffer. In practice there is no bug,
do_vec_load() is only called from the LOAD_VMX case in emulate_loadstore().
That in turn is only reached when analyse_instr() recognises VMX ops,
and in all cases the size is no greater than 16:

   $ git grep -w LOAD_VMX arch/powerpc/lib/sstep.c
   arch/powerpc/lib/sstep.c:                        op->type = MKOP(LOAD_VMX, 
0, 1);
   arch/powerpc/lib/sstep.c:                        op->type = MKOP(LOAD_VMX, 
0, 2);
   arch/powerpc/lib/sstep.c:                        op->type = MKOP(LOAD_VMX, 
0, 4);
   arch/powerpc/lib/sstep.c:                        op->type = MKOP(LOAD_VMX, 
0, 16);

Similarly for do_vec_store().

Although the warning is incorrect, the code would be safer if it clamped
the size from the caller to the known size of the buffer. Do that using
min_t().

Reported-by: Bagas Sanjaya <bagasdo...@gmail.com>
Reported-by: Jan-Benedict Glaw <jbg...@lug-owl.de>
Reported-by: Gustavo A. R. Silva <gust...@embeddedor.com>
Signed-off-by: Michael Ellerman <m...@ellerman.id.au>

Reviewed-by: Gustavo A. R. Silva <gustavo...@kernel.org>
Build-tested-by: Gustavo A. R. Silva <gustavo...@kernel.org>

This indeed makes all those warnings go away. :)

Thanks, Michael!
--
Gustavo

---
  arch/powerpc/lib/sstep.c | 4 ++--
  1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
index a4ab8625061a..a13f05cfc7db 100644
--- a/arch/powerpc/lib/sstep.c
+++ b/arch/powerpc/lib/sstep.c
@@ -688,7 +688,7 @@ static nokprobe_inline int do_vec_load(int rn, unsigned 
long ea,
        if (err)
                return err;
        if (unlikely(cross_endian))
-               do_byte_reverse(&u.b[ea & 0xf], size);
+               do_byte_reverse(&u.b[ea & 0xf], min_t(size_t, size, sizeof(u)));
        preempt_disable();
        if (regs->msr & MSR_VEC)
                put_vr(rn, &u.v);
@@ -719,7 +719,7 @@ static nokprobe_inline int do_vec_store(int rn, unsigned 
long ea,
                u.v = current->thread.vr_state.vr[rn];
        preempt_enable();
        if (unlikely(cross_endian))
-               do_byte_reverse(&u.b[ea & 0xf], size);
+               do_byte_reverse(&u.b[ea & 0xf], min_t(size_t, size, sizeof(u)));
        return copy_mem_out(&u.b[ea & 0xf], ea, size, regs);
  }
  #endif /* CONFIG_ALTIVEC */