Re: [PATCH bpf-next v3 06/12] bpf: Add dump_stack() analogue to print to BPF stderr

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Jun 23, 2025 at 08:12:46PM -0700, Kumar Kartikeya Dwivedi wrote:

SNIP

> diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
> index 75ceb6379368..5fb11202ab9c 100644
> --- a/kernel/bpf/stream.c
> +++ b/kernel/bpf/stream.c
> @@ -2,6 +2,7 @@
>  /* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
>  
>  #include <linux/bpf.h>
> +#include <linux/filter.h>
>  #include <linux/bpf_mem_alloc.h>
>  #include <linux/percpu.h>
>  #include <linux/refcount.h>
> @@ -483,3 +484,46 @@ bool bpf_prog_stream_error_limit(struct bpf_prog *prog)
>  {
>  	return atomic_fetch_add(1, &prog->aux->stream_error_cnt) >= BPF_PROG_STREAM_ERROR_CNT;
>  }
> +
> +struct dump_stack_ctx {
> +	struct bpf_stream_stage *ss;
> +	int err;
> +};
> +
> +static bool dump_stack_cb(void *cookie, u64 ip, u64 sp, u64 bp)
> +{
> +	struct dump_stack_ctx *ctxp = cookie;
> +	const char *file = "", *line = "";
> +	struct bpf_prog *prog;
> +	int num, ret;
> +
> +	if (is_bpf_text_address(ip)) {
> +		rcu_read_lock();
> +		prog = bpf_prog_ksym_find(ip);
> +		rcu_read_unlock();

do you need to check prog != NULL ?

also is_bpf_text_address calls bpf_ksym_find and bpf_prog_ksym_find calls it again,
I think it'd be better just to call bpf_prog_ksym_find from here

jirka


> +		ret = bpf_prog_get_file_line(prog, ip, &file, &line, &num);
> +		if (ret < 0)
> +			goto end;
> +		ctxp->err = bpf_stream_stage_printk(ctxp->ss, "%pS\n  %s @ %s:%d\n",
> +						    (void *)ip, line, file, num);
> +		return !ctxp->err;
> +	}
> +end:
> +	ctxp->err = bpf_stream_stage_printk(ctxp->ss, "%pS\n", (void *)ip);
> +	return !ctxp->err;
> +}
> +
> +int bpf_stream_stage_dump_stack(struct bpf_stream_stage *ss)
> +{
> +	struct dump_stack_ctx ctx = { .ss = ss };
> +	int ret;
> +
> +	ret = bpf_stream_stage_printk(ss, "CPU: %d UID: %d PID: %d Comm: %s\n",
> +				      raw_smp_processor_id(), __kuid_val(current_real_cred()->euid),
> +				      current->pid, current->comm);
> +	ret = ret ?: bpf_stream_stage_printk(ss, "Call trace:\n");
> +	if (!ret)
> +		arch_bpf_stack_walk(dump_stack_cb, &ctx);
> +	ret = ret ?: ctx.err;
> +	return ret ?: bpf_stream_stage_printk(ss, "\n");
> +}
> -- 
> 2.47.1
> 
>