linux-kernel - Re: [PATCH bpf-next v2 2/2] selftests/bpf: check null propagation only neither reg is PTR_TO_BTF

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Message-ID: <7cfaaafa-0eda-a314-5b22-7e22c029f4ad@linux.dev>
Date:   Mon, 19 Dec 2022 14:01:58 -0800
From:   Martin KaFai Lau <martin.lau@...ux.dev>
To:     Hao Sun <sunhao.th@...il.com>
Cc:     ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com,
        andrii@...nel.org, song@...nel.org, yhs@...com, kpsingh@...nel.org,
        sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org,
        davem@...emloft.net, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 2/2] selftests/bpf: check null propagation
 only neither reg is PTR_TO_BTF_ID

On 12/12/22 7:04 PM, Hao Sun wrote:
> Verify that nullness information is not porpagated in the branches
> of register to register JEQ and JNE operations if one of them is
> PTR_TO_BTF_ID.

Thanks for the fix and test.

> 
> Signed-off-by: Hao Sun <sunhao.th@...il.com>
> Acked-by: Yonghong Song <yhs@...com>
> ---
>   .../bpf/verifier/jeq_infer_not_null.c         | 22 +++++++++++++++++++
>   1 file changed, 22 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/verifier/jeq_infer_not_null.c b/tools/testing/selftests/bpf/verifier/jeq_infer_not_null.c
> index 67a1c07ead34..b2b215227d97 100644
> --- a/tools/testing/selftests/bpf/verifier/jeq_infer_not_null.c
> +++ b/tools/testing/selftests/bpf/verifier/jeq_infer_not_null.c
> @@ -172,3 +172,25 @@
>   	.prog_type = BPF_PROG_TYPE_XDP,
>   	.result = ACCEPT,
>   },
> +{
> +	"jne/jeq infer not null, PTR_TO_MAP_OR_NULL unchanged with PTR_TO_BTF_ID reg",
> +	.insns = {
> +	BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
> +	BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
> +	BPF_ST_MEM(BPF_DW, BPF_REG_2, 0, 0),
> +	BPF_LD_MAP_FD(BPF_REG_1, 0),
> +	/* r6 = bpf_map->inner_map_meta; */
> +	BPF_LDX_MEM(BPF_DW, BPF_REG_6, BPF_REG_1, 8),

This bpf_map->inner_map_meta requires CO-RE. It works now but could be fragile 
in different platform and in the future bpf_map changes. Take a look at the 
map_ptr_kern.c which uses "__attribute__((preserve_access_index))" at the 
"struct bpf_map".

Please translate this verifer test into a proper bpf prog in C code such that it 
can use the CO-RE in libbpf.  It should run under test_progs instead of 
test_verifier. The bpf prog can include the "vmlinux.h" to get the 
"__attribute__((preserve_access_index))" for free.  Take a look at 
https://lore.kernel.org/all/20221207201648.2990661-2-andrii@kernel.org/ which 
has example on how to check verifier message in test_progs.


> +	/* r0 = map_lookup_elem(r1, r2); */
> +	BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),
> +	/* if (r0 == r6) read *r0; */
> +	BPF_JMP_REG(BPF_JEQ, BPF_REG_6, BPF_REG_0, 1),
> +	BPF_EXIT_INSN(),
> +	BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0),
> +	BPF_EXIT_INSN(),
> +	},
> +	.fixup_map_hash_8b = { 3 },
> +	.prog_type = BPF_PROG_TYPE_XDP,
> +	.result = REJECT,
> +	.errstr = "R0 invalid mem access 'map_value_or_null'",
> +},