netdev - Re: [PATCH v2 bpf-next] libbpf: provide more helpful message on uninitialized global var

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Message-ID: <08DD65ED-34B4-47C0-B5ED-9A354CF5BA35@fb.com>
Date:   Tue, 23 Jul 2019 22:03:35 +0000
From:   Song Liu <songliubraving@...com>
To:     Andrii Nakryiko <andriin@...com>
CC:     bpf <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii.nakryiko@...il.com" <andrii.nakryiko@...il.com>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next] libbpf: provide more helpful message on
 uninitialized global var



> On Jul 23, 2019, at 2:11 PM, Andrii Nakryiko <andriin@...com> wrote:
> 
> When BPF program defines uninitialized global variable, it's put into
> a special COMMON section. Libbpf will reject such programs, but will
> provide very unhelpful message with garbage-looking section index.
> 
> This patch detects special section cases and gives more explicit error
> message.
> 
> Signed-off-by: Andrii Nakryiko <andriin@...com>

Acked-by: Song Liu <songliubraving@...com>

> ---
> tools/lib/bpf/libbpf.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 794dd5064ae8..8741c39adb1c 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -1760,15 +1760,22 @@ bpf_program__collect_reloc(struct bpf_program *prog, GElf_Shdr *shdr,
> 			 (long long) sym.st_value, sym.st_name, name);
> 
> 		shdr_idx = sym.st_shndx;
> +		insn_idx = rel.r_offset / sizeof(struct bpf_insn);
> +		pr_debug("relocation: insn_idx=%u, shdr_idx=%u\n",
> +			 insn_idx, shdr_idx);
> +
> +		if (shdr_idx >= SHN_LORESERVE) {
> +			pr_warning("relocation: not yet supported relo for non-static global \'%s\' variable in special section (0x%x) found in insns[%d].code 0x%x\n",
> +				   name, shdr_idx, insn_idx,
> +				   insns[insn_idx].code);
> +			return -LIBBPF_ERRNO__RELOC;
> +		}
> 		if (!bpf_object__relo_in_known_section(obj, shdr_idx)) {
> 			pr_warning("Program '%s' contains unrecognized relo data pointing to section %u\n",
> 				   prog->section_name, shdr_idx);
> 			return -LIBBPF_ERRNO__RELOC;
> 		}
> 
> -		insn_idx = rel.r_offset / sizeof(struct bpf_insn);
> -		pr_debug("relocation: insn_idx=%u\n", insn_idx);
> -
> 		if (insns[insn_idx].code == (BPF_JMP | BPF_CALL)) {
> 			if (insns[insn_idx].src_reg != BPF_PSEUDO_CALL) {
> 				pr_warning("incorrect bpf_call opcode\n");
> -- 
> 2.17.1
>