netdev - Re: [PATCH bpf-next] bpf: Fix register equivalence tracking.

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Date:   Wed, 14 Oct 2020 22:46:47 -0700
From:   Yonghong Song <yhs@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        <davem@...emloft.net>
CC:     <daniel@...earbox.net>, <john.fastabend@...il.com>,
        <netdev@...r.kernel.org>, <bpf@...r.kernel.org>,
        <kernel-team@...com>
Subject: Re: [PATCH bpf-next] bpf: Fix register equivalence tracking.



On 10/14/20 10:56 AM, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
> 
> The 64-bit JEQ/JNE handling in reg_set_min_max() was clearing reg->id in either
> true or false branch. In the case 'if (reg->id)' check was done on the other
> branch the counter part register would have reg->id == 0 when called into
> find_equal_scalars(). In such case the helper would incorrectly identify other
> registers with id == 0 as equivalent and propagate the state incorrectly.
> Fix it by preserving ID across reg_set_min_max().
> In other words any kind of comparison operator on the scalar register
> should preserve its ID to recognize:
> r1 = r2
> if (r1 == 20) {
>    #1 here both r1 and r2 == 20
> } else if (r2 < 20) {
>    #2 here both r1 and r2 < 20
> }
> 
> The patch is addressing #1 case. The #2 was working correctly already.
> 
> Fixes: 75748837b7e5 ("bpf: Propagate scalar ranges through register assignments.")
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>

This fixed an issue appeared in our production system where packets may
be incorrectly dropped.

Test-by: Yonghong Song <yhs@...com>