netdev - Re: [PATCH bpf] bpf: Fix clobbering of r2 in bpf_gen_ld

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Date:   Tue, 8 Sep 2020 09:20:21 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Alexei Starovoitov <ast@...nel.org>, bryce.kahle@...adoghq.com,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf] bpf: Fix clobbering of r2 in bpf_gen_ld_abs

On Mon, Sep 7, 2020 at 3:04 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> Bryce reported that he saw the following with:
>
>   0:  r6 = r1
>   1:  r1 = 12
>   2:  r0 = *(u16 *)skb[r1]
>
> The xlated sequence was incorrectly clobbering r2 with pointer
> value of r6 ...
>
>   0: (bf) r6 = r1
>   1: (b7) r1 = 12
>   2: (bf) r1 = r6
>   3: (bf) r2 = r1
>   4: (85) call bpf_skb_load_helper_16_no_cache#7692160
>
> ... and hence call to the load helper never succeeded given the
> offset was too high. Fix it by reordering the load of r6 to r1.
>
> Other than that the insn has similar calling convention than BPF
> helpers, that is, r0 - r5 are scratch regs, so nothing else
> affected after the insn.
>
> Fixes: e0cea7ce988c ("bpf: implement ld_abs/ld_ind in native bpf")
> Reported-by: Bryce Kahle <bryce.kahle@...adoghq.com>
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>

Applied. Thanks