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

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Date: Wed, 20 Mar 2024 14:38:49 +0100
From: Artem Savkov <asavkov@...hat.com>
To: Xu Kuohai <xukuohai@...weicloud.com>
Cc: Xi Wang <xi.wang@...il.com>, 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 20, 2024 at 07:34:46PM +0800, Xu Kuohai wrote:
> On 3/13/2024 10:02 PM, Artem Savkov 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")
> > 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);
> 
> I think the problem only occurs when is64 == 1. In this case, the generated rev32
> insn reverses byte order in both high and low 32-bit word. To fix it, we could just
> set the first arg to 0 for A64_REV32:
> 
> emit(A64_REV32(0, dst, dst), ctx);
> 
> No need to add an extra uxtw isnn.

I can confirm this approach fixes the test issue as well.

> 
> >   			break;
> >   		case 64:
> >   			emit(A64_REV64(dst, dst), ctx);
> 
> 

-- 
 Artem