Re: [PATCH AUTOSEL 5.4 08/10] selftests/bpf: Fix array access with signed variable test

Jean-Philippe Brucker
2020-12-20T12:05:51Z

Hi,

On Sat, Dec 19, 2020 at 10:34:55PM -0500, Sasha Levin wrote:
> From: Jean-Philippe Brucker <jean-philippe@linaro.org>
>
> [ Upstream commit 77ce220c0549dcc3db8226c61c60e83fc59dfafc ]
>
> The test fails because of a recent fix to the verifier, even though this

That fix is commit b02709587ea3 ("bpf: Fix propagation of 32-bit signed
bounds from 64-bit bounds.") upstream, which only needed backport to 5.9.
So although backporting this patch to 5.4 shouldn't break anything, I
wouldn't bother.

Thanks,
Jean

> program is valid. In details what happens is:
>
> 7: (61) r1 = *(u32 *)(r0 +0)
>
> Load a 32-bit value, with signed bounds [S32_MIN, S32_MAX]. The bounds
> of the 64-bit value are [0, U32_MAX]...
>
> 8: (65) if r1 s> 0xffffffff goto pc+1
>
> ... therefore this is always true (the operand is sign-extended).
>
> 10: (b4) w2 = 11
> 11: (6d) if r2 s> r1 goto pc+1
>
> When true, the 64-bit bounds become [0, 10]. The 32-bit bounds are still
> [S32_MIN, 10].
>
> 13: (64) w1 <<= 2
>
> Because this is a 32-bit operation, the verifier propagates the new
> 32-bit bounds to the 64-bit ones, and the knowledge gained from insn 11
> is lost.
>
> 14: (0f) r0 += r1
> 15: (7a) *(u64 *)(r0 +0) = 4
>
> Then the verifier considers r0 unbounded here, rejecting the test. To
> make the test work, change insn 8 to check the sign of the 32-bit value.
>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> Acked-by: John Fastabend <john.fastabend@gmail.com>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
> tools/testing/selftests/bpf/verifier/array_access.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/bpf/verifier/array_access.c b/tools/testing/selftests/bpf/verifier/array_access.c
> index f3c33e128709b..a80d806ead15f 100644
> --- a/tools/testing/selftests/bpf/verifier/array_access.c
> +++ b/tools/testing/selftests/bpf/verifier/array_access.c
> @@ -68,7 +68,7 @@
> BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
> BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 9),
> BPF_LDX_MEM(BPF_W, BPF_REG_1, BPF_REG_0, 0),
> - BPF_JMP_IMM(BPF_JSGT, BPF_REG_1, 0xffffffff, 1),
> + BPF_JMP32_IMM(BPF_JSGT, BPF_REG_1, 0xffffffff, 1),
> BPF_MOV32_IMM(BPF_REG_1, 0),
> BPF_MOV32_IMM(BPF_REG_2, MAX_ENTRIES),
> BPF_JMP_REG(BPF_JSGT, BPF_REG_2, BPF_REG_1, 1),
> --
> 2.27.0
>