selftests/bpf: filter by pid to avoid cross-test interference

The test installs a kprobe on __sys_connect and checks that
bpf_probe_write_user() can modify the syscall argument. However, any
concurrent thread in any other test that calls connect() will also
trigger the kprobe and have its sockaddr silently overwritten, causing
flaky failures in unrelated tests.

Constrain the hook to the current test process by filtering on a PID
stored as a global variable in .bss. Initialize the .bss value from
user space before bpf_object__load() using bpf_map__set_initial_value(),
and validate the bss map value size to catch layout mismatches.

No new map is introduced and the test keeps the existing non-skeleton
flow.

Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

Author: ahjf-07

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

@@ -20,6 +20,11 @@ void serial_test_probe_user(void)
 	struct bpf_program *kprobe_progs[prog_count];
 	struct bpf_object *obj;
 	static const int zero = 0;
+	struct test_pro_bss {
+		struct sockaddr_in old;
+		__u32 test_pid;
+	};
+	struct test_pro_bss results = {};
 	size_t i;
 
 	obj = bpf_object__open_file(obj_file, &opts);
@@ -34,6 +39,23 @@ void serial_test_probe_user(void)
 			goto cleanup;
 	}
 
+	{
+		struct bpf_map *bss_map;
+		struct test_pro_bss bss_init = {};
+
+		bss_init.test_pid = getpid();
+		bss_map = bpf_object__find_map_by_name(obj, "test_pro.bss");
+		if (!ASSERT_OK_PTR(bss_map, "find_bss_map"))
+			goto cleanup;
+		if (!ASSERT_EQ(bpf_map__value_size(bss_map), sizeof(bss_init),
+			       "bss_size"))
+			goto cleanup;
+		err = bpf_map__set_initial_value(bss_map, &bss_init,
+						 sizeof(bss_init));
+		if (!ASSERT_OK(err, "set_bss_init"))
+			goto cleanup;
+	}
+
 	err = bpf_object__load(obj);
 	if (CHECK(err, "obj_load", "err %d\n", err))
 		goto cleanup;
@@ -62,11 +84,13 @@ void serial_test_probe_user(void)
 	connect(sock_fd, &curr, sizeof(curr));
 	close(sock_fd);
 
-	err = bpf_map_lookup_elem(results_map_fd, &zero, &tmp);
+	err = bpf_map_lookup_elem(results_map_fd, &zero, &results);
 	if (CHECK(err, "get_kprobe_res",
 		  "failed to get kprobe res: %d\n", err))
 		goto cleanup;
 
+	memcpy(&tmp, &results.old, sizeof(tmp));
+
 	in = (struct sockaddr_in *)&tmp;
 	if (CHECK(memcmp(&tmp, &orig, sizeof(orig)), "check_kprobe_res",
 		  "wrong kprobe res from probe read: %s:%u\n",

tools/testing/selftests/bpf/progs/test_probe_user.c

@@ -5,13 +5,22 @@
 #include <bpf/bpf_core_read.h>
 #include "bpf_misc.h"
 
-static struct sockaddr_in old;
+struct test_pro_bss {
+	struct sockaddr_in old;
+	__u32 test_pid;
+};
+
+struct test_pro_bss bss;
 
 static int handle_sys_connect_common(struct sockaddr_in *uservaddr)
 {
 	struct sockaddr_in new;
+	__u32 cur = bpf_get_current_pid_tgid() >> 32;
+
+	if (bss.test_pid && cur != bss.test_pid)
+		return 0;
 
-	bpf_probe_read_user(&old, sizeof(old), uservaddr);
+	bpf_probe_read_user(&bss.old, sizeof(bss.old), uservaddr);
 	__builtin_memset(&new, 0xab, sizeof(new));
 	bpf_probe_write_user(uservaddr, &new, sizeof(new));