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Message-ID: <2cb24299-5322-6482-024a-427024f03b7d@meta.com> Date: Thu, 4 May 2023 13:18:04 -0700 From: Yonghong Song <yhs@...a.com> To: Will Deacon <will@...nel.org>, bpf@...r.kernel.org Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, Krzesimir Nowak <krzesimir@...volk.io>, Yonghong Song <yhs@...com>, Andrey Ignatov <rdna@...com> Subject: Re: [PATCH] bpf: Fix mask generation for 32-bit narrow loads of 64-bit fields On 5/2/23 9:57 AM, Will Deacon wrote: > A narrow load from a 64-bit context field results in a 64-bit load > followed potentially by a 64-bit right-shift and then a bitwise AND > operation to extract the relevant data. > > In the case of a 32-bit access, an immediate mask of 0xffffffff is used > to construct a 64-bit BPP_AND operation which then sign-extends the mask > value and effectively acts as a glorified no-op. > > Fix the mask generation so that narrow loads always perform a 32-bit AND > operation. > > Cc: Alexei Starovoitov <ast@...nel.org> > Cc: Daniel Borkmann <daniel@...earbox.net> > Cc: John Fastabend <john.fastabend@...il.com> > Cc: Krzesimir Nowak <krzesimir@...volk.io> > Cc: Yonghong Song <yhs@...com> > Cc: Andrey Ignatov <rdna@...com> > Fixes: 31fd85816dbe ("bpf: permits narrower load from bpf program context fields") > Signed-off-by: Will Deacon <will@...nel.org> Thanks for the fix! You didn't miss anything. It is a bug and we did not find it probably because user always use 'u64 val = ctx->u64_field' in their bpf code... But I think the commit message can be improved. An example to show the difference without and with this patch can explain the issue much better. Acked-by: Yonghong Song <yhs@...com> > --- > > I spotted this while playing around with the JIT on arm64. I can't > figure out why 31fd85816dbe special-cases 8-byte ctx fields in the > first place, so I fear I may be missing something... > > kernel/bpf/verifier.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index fbcf5a4e2fcd..5871aa78d01a 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -17033,7 +17033,7 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env) > insn_buf[cnt++] = BPF_ALU64_IMM(BPF_RSH, > insn->dst_reg, > shift); > - insn_buf[cnt++] = BPF_ALU64_IMM(BPF_AND, insn->dst_reg, > + insn_buf[cnt++] = BPF_ALU32_IMM(BPF_AND, insn->dst_reg, > (1ULL << size * 8) - 1); > } > }
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