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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Oct 14, 2020 at 10:59 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> From: Alexei Starovoitov <ast@kernel.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@kernel.org>
> ---

Number of underscores is a bit subtle a difference, but this fixes the bug, so:

Acked-by: Andrii Nakryiko <andrii@kernel.org>


>  kernel/bpf/verifier.c                         | 38 ++++++++++++-------
>  .../testing/selftests/bpf/verifier/regalloc.c | 26 +++++++++++++
>  2 files changed, 51 insertions(+), 13 deletions(-)
>

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