Re: [PATCH bpf v3 2/2] selftests/bpf: add test for negative CO-RE accessor index rejection

[Paul Chaignon <paul.chaignon@gmail.com>]

On Sun, Apr 05, 2026 at 12:12:21AM +0800, Weiming Shi wrote:
> Add a selftest to verify that the kernel rejects BPF programs containing
> CO-RE relocations with negative accessor indices.  The test constructs a
> minimal BTF blob with a struct type and a CO-RE relocation whose access
> string is "0:-1".  Without the fix in the previous patch, this triggers
> an out-of-bounds read in bpf_core_parse_spec(); with the fix, the
> program load fails cleanly with -EINVAL.
> 
> Signed-off-by: Weiming Shi <bestswngs@gmail.com>

I checked that the new selftest does fail, in the expected manner, when
applied without the corresponding fix.

Tested-by: Paul Chaignon <paul.chaignon@gmail.com>

> ---
>  .../selftests/bpf/prog_tests/core_reloc_raw.c | 94 +++++++++++++++++++
>  1 file changed, 94 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/core_reloc_raw.c b/tools/testing/selftests/bpf/prog_tests/core_reloc_raw.c
> index a18d3680fb16..f7bf6c237d59 100644
> --- a/tools/testing/selftests/bpf/prog_tests/core_reloc_raw.c
> +++ b/tools/testing/selftests/bpf/prog_tests/core_reloc_raw.c
> @@ -118,8 +118,102 @@ static void test_bad_local_id(void)
>  	close(btf_fd);
>  }
>  
> +/* Check that verifier rejects BPF program containing CO-RE relocation
> + * with a negative accessor index (e.g. "0:-1").
> + */
> +static void test_negative_accessor(void)
> +{
> +	struct test_btf {
> +		struct btf_header hdr;
> +		__u32 types[18];
> +		char strings[20];
> +	} raw_btf = {
> +		.hdr = {
> +			.magic = BTF_MAGIC,
> +			.version = BTF_VERSION,
> +			.hdr_len = sizeof(struct btf_header),
> +			.type_off = 0,
> +			.type_len = sizeof(raw_btf.types),
> +			.str_off = offsetof(struct test_btf, strings) -
> +				   offsetof(struct test_btf, types),
> +			.str_len = sizeof(raw_btf.strings),
> +		},
> +		.types = {
> +			/* [1] int */
> +			BTF_TYPE_INT_ENC(1, BTF_INT_SIGNED, 0, 32, 4),
> +			/* [2] struct s { int x; } */
> +			BTF_STRUCT_ENC(5, 1, 4),
> +			BTF_MEMBER_ENC(7, 1, 0),
> +			/* [3] int (*)(int a) */
> +			BTF_FUNC_PROTO_ENC(1, 1),
> +			BTF_FUNC_PROTO_ARG_ENC(13, 1),
> +			/* [4] FUNC 'foo' */
> +			BTF_FUNC_ENC(9, 3),
> +		},
> +		/* offsets: 0:NUL 1:"int" 5:"s" 7:"x" 9:"foo" 13:"a" 15:"0:-1" */
> +		.strings = "\0int\0s\0x\0foo\0a\0" "0:-1",
> +	};
> +	__u32 log_level = 1 | 2 | 4;
> +	LIBBPF_OPTS(bpf_btf_load_opts, opts,
> +		    .log_buf = log,
> +		    .log_size = sizeof(log),
> +		    .log_level = log_level,
> +	);
> +	struct bpf_insn insns[] = {
> +		BPF_ALU64_IMM(BPF_MOV, BPF_REG_0, 0),
> +		BPF_EXIT_INSN(),
> +	};
> +	struct bpf_func_info funcs[] = {
> +		{ .insn_off = 0, .type_id = 4 }
> +	};
> +	struct bpf_core_relo relos[] = {
> +		{
> +			.insn_off = 0,
> +			.type_id = 2,		/* struct s */
> +			.access_str_off = 15,	/* "0:-1" */
> +			.kind = BPF_CORE_FIELD_BYTE_OFFSET,
> +		}
> +	};
> +	union bpf_attr attr;
> +	int prog_fd = -1;
> +	int btf_fd = -1;
> +
> +	btf_fd = bpf_btf_load(&raw_btf, sizeof(raw_btf), &opts);
> +	if (!ASSERT_GE(btf_fd, 0, "btf_load"))
> +		return;
> +
> +	log[0] = 0;
> +	memset(&attr, 0, sizeof(attr));
> +	attr.prog_btf_fd = btf_fd;
> +	attr.prog_type = BPF_TRACE_RAW_TP;
> +	attr.license = (__u64)"GPL";
> +	attr.insns = (__u64)&insns;
> +	attr.insn_cnt = sizeof(insns) / sizeof(*insns);
> +	attr.log_buf = (__u64)log;
> +	attr.log_size = sizeof(log);
> +	attr.log_level = log_level;
> +	attr.func_info = (__u64)funcs;
> +	attr.func_info_cnt = sizeof(funcs) / sizeof(*funcs);
> +	attr.func_info_rec_size = sizeof(*funcs);
> +	attr.core_relos = (__u64)relos;
> +	attr.core_relo_cnt = sizeof(relos) / sizeof(*relos);
> +	attr.core_relo_rec_size = sizeof(*relos);
> +	prog_fd = sys_bpf_prog_load(&attr, sizeof(attr), 1);
> +	if (prog_fd >= 0) {
> +		PRINT_FAIL("sys_bpf_prog_load() expected to fail\n");
> +		goto out;
> +	}
> +	ASSERT_HAS_SUBSTR(log, "failed: -22", "prog_load_log");
> +
> +out:
> +	close(prog_fd);
> +	close(btf_fd);
> +}
> +
>  void test_core_reloc_raw(void)
>  {
>  	if (test__start_subtest("bad_local_id"))
>  		test_bad_local_id();
> +	if (test__start_subtest("negative_accessor"))
> +		test_negative_accessor();
>  }
> -- 
> 2.43.0
> 
>