linux-kernel - Re: [PATCH bpf v2 2/2] selftests/bpf: Add test for immediate spilled to stack

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Date:   Wed, 01 Nov 2023 13:05:33 +0200
From:   Eduard Zingerman <eddyz87@...il.com>
To:     Hao Sun <sunhao.th@...il.com>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
        Shung-Hsi Yu <shung-hsi.yu@...e.com>
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf v2 2/2] selftests/bpf: Add test for immediate
 spilled to stack

On Wed, 2023-11-01 at 08:33 +0100, Hao Sun wrote:
> Add a test to check if the verifier correctly reason about the sign
> of an immediate spilled to stack by BPF_ST instruction.
> 
> Signed-off-by: Hao Sun <sunhao.th@...il.com>
> ---
>  tools/testing/selftests/bpf/verifier/bpf_st_mem.c | 32 +++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/verifier/bpf_st_mem.c b/tools/testing/selftests/bpf/verifier/bpf_st_mem.c
> index 3af2501082b2..0ba23807c46c 100644
> --- a/tools/testing/selftests/bpf/verifier/bpf_st_mem.c
> +++ b/tools/testing/selftests/bpf/verifier/bpf_st_mem.c
> @@ -65,3 +65,35 @@
>  	.expected_attach_type = BPF_SK_LOOKUP,
>  	.runs = -1,
>  },
> +{
> +	"BPF_ST_MEM stack imm sign",
> +	/* Check if verifier correctly reasons about sign of an
> +	 * immediate spilled to stack by BPF_ST instruction.
> +	 *
> +	 *   fp[-8] = -44;
> +	 *   r0 = fp[-8];
> +	 *   if r0 s< 0 goto ret0;
> +	 *   r0 = -1;
> +	 *   exit;
> +	 * ret0:
> +	 *   r0 = 0;
> +	 *   exit;
> +	 */
> +	.insns = {
> +	BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, -44),
> +	BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_10, -8),
> +	BPF_JMP_IMM(BPF_JSLT, BPF_REG_0, 0, 2),
> +	BPF_MOV64_IMM(BPF_REG_0, -1),
> +	BPF_EXIT_INSN(),
> +	BPF_MOV64_IMM(BPF_REG_0, 0),
> +	BPF_EXIT_INSN(),
> +	},
> +	/* Use prog type that requires return value in range [0, 1] */
> +	.prog_type = BPF_PROG_TYPE_SK_LOOKUP,
> +	.expected_attach_type = BPF_SK_LOOKUP,
> +	.result = VERBOSE_ACCEPT,
> +	.runs = -1,
> +	.errstr = "0: (7a) *(u64 *)(r10 -8) = -44        ; R10=fp0 fp-8_w=-44\
> +	2: (c5) if r0 s< 0x0 goto pc+2\
> +	2: R0_w=-44",
> +},
> 

Please note that this test case fails on CI [0], full log below:

2023-11-01T07:49:51.2841702Z #116/p BPF_ST_MEM stack imm sign FAIL
2023-11-01T07:49:51.2843456Z Unexpected verifier log!
2023-11-01T07:49:51.2844968Z EXP: 2: R0_w=-44
2023-11-01T07:49:51.2845583Z RES:
2023-11-01T07:49:51.2846693Z func#0 @0
2023-11-01T07:49:51.2848932Z 0: R1=ctx(off=0,imm=0) R10=fp0
2023-11-01T07:49:51.2853045Z 0: (7a) *(u64 *)(r10 -8) = -44        ; R10=fp0 fp-8_w=-44
2023-11-01T07:49:51.2857391Z 1: (79) r0 = *(u64 *)(r10 -8)         ; R0_w=-44 R10=fp0 fp-8_w=-44
2023-11-01T07:49:51.2859127Z 2: (c5) if r0 s< 0x0 goto pc+2
2023-11-01T07:49:51.2862943Z mark_precise: frame0: last_idx 2 first_idx 0 subseq_idx -1 
2023-11-01T07:49:51.2867511Z mark_precise: frame0: regs=r0 stack= before 1: (79) r0 = *(u64 *)(r10 -8)
2023-11-01T07:49:51.2872217Z mark_precise: frame0: regs= stack=-8 before 0: (7a) *(u64 *)(r10 -8) = -44
2023-11-01T07:49:51.2872816Z 5: R0_w=-44
2023-11-01T07:49:51.2875653Z 5: (b7) r0 = 0                        ; R0_w=0
2023-11-01T07:49:51.2876493Z 6: (95) exit

I suspect that after recent logging fixes instruction number printed
after jump changed and that's why test case no longer passes.

Note: you can check CI status for submitted patch-sets using link [1].

[0] https://github.com/kernel-patches/bpf/actions/runs/6717053909/job/18254330860
[1] https://patchwork.kernel.org/project/netdevbpf/list/