Re: [PATCH bpf-next] arm64: bpf: zero upper bits after rev32

[Xu Kuohai]

On 3/21/2024 12:15 AM, Xi Wang wrote:
> On Wed, Mar 13, 2024 at 7:02 AM Artem Savkov <asavkov@xxxxxxxxxx> wrote:
> > Commit d63903bbc30c7 ("arm64: bpf: fix endianness conversion bugs")
> > added upper bits zeroing to byteswap operations, but it assumes they
> > will be already zeroed after rev32, which is not the case on some
> > systems at least:
> >
> > [ 9757.262607] test_bpf: #312 BSWAP 16: 0x0123456789abcdef -> 0xefcd jited:1 8 PASS
> > [ 9757.264435] test_bpf: #313 BSWAP 32: 0x0123456789abcdef -> 0xefcdab89 jited:1 ret 1460850314 != -271733879 (0x5712ce8a != 0xefcdab89)FAIL (1 times)
> > [ 9757.266260] test_bpf: #314 BSWAP 64: 0x0123456789abcdef -> 0x67452301 jited:1 8 PASS
> > [ 9757.268000] test_bpf: #315 BSWAP 64: 0x0123456789abcdef >> 32 -> 0xefcdab89 jited:1 8 PASS
> > [ 9757.269686] test_bpf: #316 BSWAP 16: 0xfedcba9876543210 -> 0x1032 jited:1 8 PASS
> > [ 9757.271380] test_bpf: #317 BSWAP 32: 0xfedcba9876543210 -> 0x10325476 jited:1 ret -1460850316 != 271733878 (0xa8ed3174 != 0x10325476)FAIL (1 times)
> > [ 9757.273022] test_bpf: #318 BSWAP 64: 0xfedcba9876543210 -> 0x98badcfe jited:1 7 PASS
> > [ 9757.274721] test_bpf: #319 BSWAP 64: 0xfedcba9876543210 >> 32 -> 0x10325476 jited:1 9 PASS
> >
> > Fixes: d63903bbc30c7 ("arm64: bpf: fix endianness conversion bugs")
>
> This tag is not right.  It's unlikely that the bug has been around for 9 years.
>
> Maybe you meant 1104247f3f979 ("bpf, arm64: Support unconditional bswap")?

Agree, thanks for pointing it out.

> > Signed-off-by: Artem Savkov <asavkov@xxxxxxxxxx>
> > ---
> >   arch/arm64/net/bpf_jit_comp.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> > index c5b461dda4385..e86e5ba74dca2 100644
> > --- a/arch/arm64/net/bpf_jit_comp.c
> > +++ b/arch/arm64/net/bpf_jit_comp.c
> > @@ -944,7 +944,8 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
> >                          break;
> >                  case 32:
> >                          emit(A64_REV32(is64, dst, dst), ctx);
> > -                       /* upper 32 bits already cleared */
> > +                       /* zero-extend 32 bits into 64 bits */
> > +                       emit(A64_UXTW(is64, dst, dst), ctx);
>
> The fix can pass the tests, but emitting an extra instruction is
> unnecessary as the bug applies only to unconditional bswap.
>
> >                          break;
> >                  case 64:
> >                          emit(A64_REV64(dst, dst), ctx);
> > --
> > 2.44.0