selftests/bpf: Reuse stderr parsing for libarena ASAN tests

Add code to directly test the output of libarena ASAN tests.
The code reuses testing infrastructure originally for BPF streams
to verify that ASAN emits call stacks when the selftests trigger
a memory error.

Since stderr() testing uses logic from test_progs, it is only
available on the test_progs-based selftest runner. The standalone
runner still uses internal ASAN state to verify access errors are
triaged as expected.

Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260426190338.4615-9-emil@etsalapatis.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/libarena/selftests/st_asan_buddy.bpf.c

@@ -5,6 +5,9 @@
 #include <libarena/asan.h>
 #include <libarena/buddy.h>
 
+/* Required for parsing the ASAN call stacks. */
+#include "test_progs_compat.h"
+
 extern buddy_t buddy;
 
 #ifdef BPF_ARENA_ASAN
@@ -141,6 +144,11 @@ static __always_inline int asan_test_buddy_blob_single(void)
 }
 
 SEC("syscall")
+__stderr("Memory violation for address {{.*}} for write of size 1")
+__stderr("CPU: {{[0-9]+}} UID: 0 PID: {{[0-9]+}} Comm: {{.*}}")
+__stderr("Call trace:\n"
+"{{([a-zA-Z_][a-zA-Z0-9_]*\\+0x[0-9a-fA-F]+/0x[0-9a-fA-F]+\n"
+"|[ \t]+[^\n]+\n)*}}")
 __weak int asan_test_buddy_oob(void)
 {
 	size_t sizes[] = {
@@ -174,6 +182,11 @@ __weak int asan_test_buddy_oob(void)
 }
 
 SEC("syscall")
+__stderr("Memory violation for address {{.*}} for write of size 1")
+__stderr("CPU: {{[0-9]+}} UID: 0 PID: {{[0-9]+}} Comm: {{.*}}")
+__stderr("Call trace:\n"
+"{{([a-zA-Z_][a-zA-Z0-9_]*\\+0x[0-9a-fA-F]+/0x[0-9a-fA-F]+\n"
+"|[ \t]+[^\n]+\n)*}}")
 __weak int asan_test_buddy_uaf(void)
 {
 	size_t sizes[] = { 16, 32, 64, 128, 256, 512, 1024, 16384 };
@@ -205,6 +218,11 @@ __weak int asan_test_buddy_uaf(void)
 }
 
 SEC("syscall")
+__stderr("Memory violation for address {{.*}} for write of size 1")
+__stderr("CPU: {{[0-9]+}} UID: 0 PID: {{[0-9]+}} Comm: {{.*}}")
+__stderr("Call trace:\n"
+"{{([a-zA-Z_][a-zA-Z0-9_]*\\+0x[0-9a-fA-F]+/0x[0-9a-fA-F]+\n"
+"|[ \t]+[^\n]+\n)*}}")
 __weak int asan_test_buddy_blob(void)
 {
 	const int iters = 10;

tools/testing/selftests/bpf/libarena/selftests/test_progs_compat.h

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: LGPL-2.1 OR BSD-2-Clause
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+#pragma once
+
+#ifdef __BPF__
+
+/* Selftests use these tags for compatibility with test_progs. */
+#define __test_tag(tag)		__attribute__((btf_decl_tag("comment:" XSTR(__COUNTER__) ":" tag)))
+#define __stderr(msg)		__test_tag("test_expect_stderr=" msg)
+#define __stderr_unpriv(msg)	__test_tag("test_expect_stderr_unpriv=" msg)
+
+#define XSTR(s) STR(s)
+#define STR(s) #s
+
+#endif

tools/testing/selftests/bpf/prog_tests/libarena_asan.c

@@ -25,6 +25,8 @@ static void run_libarena_asan_test(struct libarena_asan *skel,
 
 	ret = libarena_run_prog(bpf_program__fd(prog));
 	ASSERT_OK(ret, name);
+
+	verify_test_stderr(skel->obj, prog);
 }
 
 static void run_test(void)

tools/testing/selftests/bpf/test_loader.c

@@ -93,7 +93,7 @@ void test_loader_fini(struct test_loader *tester)
 	free(tester->log_buf);
 }
 
-static void free_msgs(struct expected_msgs *msgs)
+void free_msgs(struct expected_msgs *msgs)
 {
 	int i;
 
@@ -789,6 +789,43 @@ static void emit_stderr(const char *stderr, bool force)
 	fprintf(stdout, "STDERR:\n=============\n%s=============\n", stderr);
 }
 
+static void verify_stderr(int prog_fd, struct expected_msgs *msgs)
+{
+	LIBBPF_OPTS(bpf_prog_stream_read_opts, ropts);
+	char *buf;
+	int ret;
+
+	if (!msgs->cnt)
+		return;
+
+	buf = malloc(TEST_LOADER_LOG_BUF_SZ);
+	if (!ASSERT_OK_PTR(buf, "malloc"))
+		return;
+
+	ret = bpf_prog_stream_read(prog_fd, 2, buf, TEST_LOADER_LOG_BUF_SZ - 1,
+				    &ropts);
+	if (ret > 0) {
+		buf[ret] = '\0';
+		emit_stderr(buf, false);
+		validate_msgs(buf, msgs, emit_stderr);
+	} else {
+		ASSERT_GT(ret, 0, "stderr stream read");
+	}
+
+	free(buf);
+}
+
+void verify_test_stderr(struct bpf_object *obj, struct bpf_program *prog)
+{
+	struct test_spec spec = {};
+
+	if (parse_test_spec(NULL, obj, prog, &spec))
+		return;
+
+	verify_stderr(bpf_program__fd(prog), &spec.priv.stderr);
+	free_test_spec(&spec);
+}
+
 static void emit_stdout(const char *bpf_stdout, bool force)
 {
 	if (!force && env.verbosity == VERBOSE_NONE)
@@ -1314,17 +1351,7 @@ void run_subtest(struct test_loader *tester,
 			goto tobj_cleanup;
 		}
 
-		if (subspec->stderr.cnt) {
-			err = get_stream(2, bpf_program__fd(tprog),
-					 tester->log_buf, tester->log_buf_sz);
-			if (err <= 0) {
-				PRINT_FAIL("Unexpected retval from get_stream(): %d, errno = %d\n",
-					   err, errno);
-				goto tobj_cleanup;
-			}
-			emit_stderr(tester->log_buf, false /*force*/);
-			validate_msgs(tester->log_buf, &subspec->stderr, emit_stderr);
-		}
+		verify_stderr(bpf_program__fd(tprog), &subspec->stderr);
 
 		if (subspec->stdout.cnt) {
 			err = get_stream(1, bpf_program__fd(tprog),

tools/testing/selftests/bpf/test_progs.h

@@ -563,5 +563,7 @@ struct expected_msgs {
 
 void validate_msgs(const char *log_buf, struct expected_msgs *msgs,
 		   void (*emit_fn)(const char *buf, bool force));
+void free_msgs(struct expected_msgs *msgs);
+void verify_test_stderr(struct bpf_object *obj, struct bpf_program *prog);
 
 #endif /* __TEST_PROGS_H */