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

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

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.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 include/linux/bpf.h |  2 ++
 kernel/bpf/stream.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index cc14ff8e0b88..bd3cde41795e 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -3618,10 +3618,12 @@ __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);
 
 bool bpf_prog_stream_error_limit(struct bpf_prog *prog);
 
 #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 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();
+		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