From: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Date: Thu, 3 Jul 2025 20:48:12 +0000 (-0700)
Subject: bpf: Add dump_stack() analogue to print to BPF stderr
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=d7c431cafcb4917b0d87b5cd10637cd47b6c8d79;p=thirdparty%2Flinux.git

bpf: Add dump_stack() analogue to print to BPF stderr

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 program counter 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.

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20250703204818.925464-7-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index b267c378d8845..34dd90ec7fad7 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -3616,8 +3616,10 @@ __printf(2, 3)
 int bpf_stream_stage_printk(struct bpf_stream_stage *ss, const char *fmt, ...);
 int bpf_stream_stage_commit(struct bpf_stream_stage *ss, struct bpf_prog *prog,
 			    enum bpf_stream_id stream_id);
+int bpf_stream_stage_dump_stack(struct bpf_stream_stage *ss);
 
 #define bpf_stream_printk(ss, ...) bpf_stream_stage_printk(&ss, __VA_ARGS__)
+#define bpf_stream_dump_stack(ss) bpf_stream_stage_dump_stack(&ss)
 
 #define bpf_stream_stage(ss, prog, stream_id, expr)            \
 	({                                                     \
diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
index e434541358db8..8c842f845245a 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>
@@ -476,3 +477,50 @@ int bpf_stream_stage_commit(struct bpf_stream_stage *ss, struct bpf_prog *prog,
 	llist_add_batch(head, tail, &stream->log);
 	return 0;
 }
+
+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;
+
+	rcu_read_lock();
+	prog = bpf_prog_ksym_find(ip);
+	rcu_read_unlock();
+	if (prog) {
+		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);
+	if (ret)
+		return ret;
+	ret = bpf_stream_stage_printk(ss, "Call trace:\n");
+	if (ret)
+		return ret;
+	arch_bpf_stack_walk(dump_stack_cb, &ctx);
+	if (ctx.err)
+		return ctx.err;
+	return bpf_stream_stage_printk(ss, "\n");
+}