Commit f80f88f0 authored Apr 28, 2021 by Florent Revest Committed by Andrii Nakryiko Apr 30, 2021

selftests/bpf: Fix the snprintf test



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: c2e39c6b ("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>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210428152501.1024509-1-revest@chromium.org

parent d4eecfb2

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

+2 −0

Original line number	Diff line number	Diff line
		@@ -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;

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

+5 −0

Original line number	Diff line number	Diff line
		@@ -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",