[PATCH bpf-next v1 3/3] libbpf: Wire up verifier warning display logic

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

Wire up the flushing of all accumulated messages in a program's stderr
stream after verification is complete. An example is shown below of a
warning printed now about usage of deprecated kfuncs.

$ ./test_progs -t kfunc_implicit_args/test_kfunc_implicit_arg_legacy_impl -v
...
libbpf: prog 'test_kfunc_implicit_arg_legacy_impl': VERIFIER WARNINGS:
WARNING: kfunc_implicit_args.c:40 calls deprecated kfunc bpf_kfunc_implicit_arg_legacy_impl(), which will be removed.
WARNING: Switch to kfunc bpf_kfunc_implicit_arg_legacy() instead.
WARNING: For older kernels, choose the kfunc using bpf_ksym_exists(bpf_kfunc_implicit_arg_legacy).
...

Note that test_progs overrides the default logging function, hence -v is
necessary. By default these messages would be printed for the user.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 tools/lib/bpf/libbpf.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 9ea41f40dc82..f308bacd083f 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -7826,6 +7826,27 @@ static int libbpf_prepare_prog_load(struct bpf_program *prog,
 
 static void fixup_verifier_log(struct bpf_program *prog, char *buf, size_t buf_sz);
 
+static void bpf_object_load_prog_emit_stderr(struct bpf_program *prog, int prog_fd)
+{
+	char chunk[256];
+	bool emitted = false;
+	int ret;
+
+	while ((ret = bpf_prog_stream_read(prog_fd, BPF_STREAM_STDERR, chunk,
+					   sizeof(chunk), NULL)) > 0) {
+		if (!emitted) {
+			pr_warn("prog '%s': VERIFIER WARNINGS:\n", prog->name);
+			emitted = true;
+		}
+		libbpf_print(LIBBPF_WARN, "%.*s", ret, chunk);
+	}
+
+	if (ret < 0) {
+		pr_debug("prog '%s': failed to read BPF stderr stream: %s\n",
+			 prog->name, errstr(ret));
+	}
+}
+
 static int bpf_object_load_prog(struct bpf_object *obj, struct bpf_program *prog,
 				struct bpf_insn *insns, int insns_cnt,
 				const char *license, __u32 kern_version, int *prog_fd)
@@ -7950,6 +7971,8 @@ static int bpf_object_load_prog(struct bpf_object *obj, struct bpf_program *prog
 				 prog->name, log_buf);
 		}
 
+		bpf_object_load_prog_emit_stderr(prog, ret);
+
 		if (obj->has_rodata && kernel_supports(obj, FEAT_PROG_BIND_MAP)) {
 			struct bpf_map *map;
 			int i;
-- 
2.52.0