Date: Sun, 7 May 2017 01:57:48 +0530 From: Shubham Bansal <illusionist.neo@...il.com> To: David Miller <davem@...emloft.net> Cc: Daniel Borkmann <daniel@...earbox.net>, Kees Cook <keescook@...omium.org>, Mircea Gherzan <mgherzan@...il.com>, Network Development <netdev@...r.kernel.org>, kernel-hardening@...ts.openwall.com, linux-arm-kernel@...ts.infradead.org, ast@...com Subject: Re: arch: arm: bpf: Converting cBPF to eBPF for arm 32 bit Thanks David. Hi all, I have two questions about the code at arch/arm64/net/bpf_jit_comp.c. 1. At line 708, " const u8 r1 = bpf2a64[BPF_REG_1]; /* r1: struct sk_buff *skb */ ". Why is this code using BPF_REG_1 before saving it? As far as I know, BPF_REG_1 has pointer to bpf program context and this code clearly is overwriting that pointer which makes that pointer useless for future usage. It clearly looks like a bug. 2. At line 256, " emit(A64_LDR64(prg, tmp, r3), ctx); ". This line of code is used to load an array( of pointers ) element, where r3 is used as an index of that array. Shouldn't it be be arithmetic left shifted by 3 or multiplied by 8 to get the right address in that array of pointers ? Apologies if any of the above question is stupid to ask. Best, Shubham Best, Shubham Bansal On Sun, May 7, 2017 at 12:08 AM, David Miller <davem@...emloft.net> wrote: > From: Shubham Bansal <illusionist.neo@...il.com> > Date: Sat, 6 May 2017 22:18:16 +0530 > >> Hi Daniel, >> >> Thanks for the last reply about the testing of eBPF JIT. >> >> I have one issue though, I am not able to find what BPF_ABS and >> BPF_IND instruction does exactly. > > They are not instructions, they are modifiers for the BPF_LD > instruction which indicate an SKB load is to be performed. > > You never need to ask what a BPF instruction does, it is clear > defined in the BPF interperter found in kernel/bpf/core.c > > Look for the case statement LD_ABS_W and friends in __bpf_prog_run().
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