Re: [RFC PATCH bpf-next/net v1 08/13] bpf: Add dump_stack() analogue to print to BPF stderr

[Eduard Zingerman <eddyz87@gmail.com>]

Kumar Kartikeya Dwivedi <memxor@gmail.com> writes:

> Introduce a kernel function which is the analogue of dump_stack()
> printing some useful information and the stack trace. This is not
> exposed to BPF programs yet, but can be made available in the future.
>
> When we have a PC for a BPF program in the stack trace, also
> additionally output the filename and line number to make the trace
> helpful. The rest of the trace can be passed into ./decode_stacktrace.sh
> to obtain the line numbers for kernel symbols.
>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---

Seems to work as advertised.

Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>

[...]

> +static int dump_stack_bpf_linfo(u64 ip, const char **filep)
> +{
> +	struct bpf_prog *prog = bpf_prog_ksym_find(ip);
> +	int idx = -1, insn_start, insn_end, len;
> +	struct bpf_line_info *linfo;
> +	void **jited_linfo;
> +	struct btf *btf;
> +
> +	btf = prog->aux->btf;
> +	linfo = prog->aux->linfo;
> +	jited_linfo = prog->aux->jited_linfo;
> +
> +	if (!btf || !linfo || !prog->aux->jited_linfo)
> +		return -1;
> +	len = prog->aux->func ? prog->aux->func[prog->aux->func_idx]->len : prog->len;
> +
> +	linfo = &prog->aux->linfo[prog->aux->linfo_idx];
> +	jited_linfo = &prog->aux->jited_linfo[prog->aux->linfo_idx];
> +
> +	insn_start = linfo[0].insn_off;
> +	insn_end = insn_start + len;
> +
> +	for (int i = 0; linfo[i].insn_off >= insn_start && linfo[i].insn_off < insn_end; i++) {
> +		if (jited_linfo[i] >= (void *)ip)
> +			break;
> +		idx = i;
> +	}
> +
> +	if (idx == -1)
> +		return -1;
> +
> +	*filep = btf_name_by_offset(btf, linfo[idx].file_name_off);
> +	*filep = strrchr(*filep, '/') ?: *filep;
> +	*filep += 1;

Nit: `+= 1` not needed if '/' was not found?

> +
> +	return BPF_LINE_INFO_LINE_NUM(linfo[idx].line_col);
> +}

[...]