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

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Message-ID: <CAKU6vyZ1LOUtJz-XeQz8i8f9nyvN+TZxOMfEf6JP1Tfuwneiqw@mail.gmail.com>
Date: Wed, 20 Mar 2024 09:15:48 -0700
From: Xi Wang <xi.wang@...il.com>
To: Artem Savkov <asavkov@...hat.com>
Cc: Catalin Marinas <catalin.marinas@....com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next] arm64: bpf: zero upper bits after rev32

On Wed, Mar 13, 2024 at 7:02 AM Artem Savkov <asavkov@...hat.com> 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")?

> Signed-off-by: Artem Savkov <asavkov@...hat.com>
> ---
>  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
>