[PATCH bpf-next] selftests/bpf: Fix the snprintf test

[Florent Revest <revest@chromium.org>]

The BPF program for the snprintf selftest runs on all syscall entries.
On busy multicore systems this can cause concurrency issues.

For example it was observed that sometimes the userspace part of the
test reads "    4 0000" instead of "    4 000" (extra '0' at the end)
which seems to happen just before snprintf on another core sets
end[-1] = '\0'.

This patch adds a pid filter to the test to ensure that no
bpf_snprintf() will write over the test's output buffers while the
userspace reads the values.

Fixes: c2e39c6bdc7e ("selftests/bpf: Add a series of tests for bpf_snprintf")
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Florent Revest <revest@chromium.org>
---
 tools/testing/selftests/bpf/prog_tests/snprintf.c | 2 ++
 tools/testing/selftests/bpf/progs/test_snprintf.c | 5 +++++
 2 files changed, 7 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/snprintf.c b/tools/testing/selftests/bpf/prog_tests/snprintf.c
index a958c22aec75..dffbcaa1ec98 100644
--- a/tools/testing/selftests/bpf/prog_tests/snprintf.c
+++ b/tools/testing/selftests/bpf/prog_tests/snprintf.c
@@ -43,6 +43,8 @@ void test_snprintf_positive(void)
 	if (!ASSERT_OK_PTR(skel, "skel_open"))
 		return;
 
+	skel->bss->pid = getpid();
+
 	if (!ASSERT_OK(test_snprintf__attach(skel), "skel_attach"))
 		goto cleanup;
 
diff --git a/tools/testing/selftests/bpf/progs/test_snprintf.c b/tools/testing/selftests/bpf/progs/test_snprintf.c
index 951a0301c553..e35129bea0a0 100644
--- a/tools/testing/selftests/bpf/progs/test_snprintf.c
+++ b/tools/testing/selftests/bpf/progs/test_snprintf.c
@@ -5,6 +5,8 @@
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
 
+__u32 pid = 0;
+
 char num_out[64] = {};
 long num_ret = 0;
 
@@ -42,6 +44,9 @@ int handler(const void *ctx)
 	static const char str1[] = "str1";
 	static const char longstr[] = "longstr";
 
+	if ((int)bpf_get_current_pid_tgid() != pid)
+		return 0;
+
 	/* Integer types */
 	num_ret  = BPF_SNPRINTF(num_out, sizeof(num_out),
 				"%d %u %x %li %llu %lX",
-- 
2.31.1.498.g6c1eba8ee3d-goog