selftests/seccomp: Add simple seccomp overhead benchmark

This attempts to produce a comparison between native getpid() and a
RET_ALLOW-filtered getpid(), to measure the overhead cost of using
seccomp().
Signed-off-by: Kees Cook <keescook@chromium.org>

tools/testing/selftests/seccomp/Makefile

-TEST_GEN_PROGS := seccomp_bpf
-CFLAGS += -Wl,-no-as-needed -Wall
-LDFLAGS += -lpthread
+all:
 
 include ../lib.mk
 
-$(TEST_GEN_PROGS): seccomp_bpf.c ../kselftest_harness.h
-	$(CC) $(CFLAGS) $(LDFLAGS) $< -o $@
+.PHONY: all clean
+
+BINARIES := seccomp_bpf seccomp_benchmark
+CFLAGS += -Wl,-no-as-needed -Wall
+
+seccomp_bpf: seccomp_bpf.c ../kselftest_harness.h
+	$(CC) $(CFLAGS) $(LDFLAGS) -lpthread $< -o $@
+
+TEST_PROGS += $(BINARIES)
+EXTRA_CLEAN := $(BINARIES)
+
+all: $(BINARIES)

tools/testing/selftests/seccomp/seccomp_benchmark.c

+/*
+ * Strictly speaking, this is not a test. But it can report during test
+ * runs so relative performace can be measured.
+ */
+#define _GNU_SOURCE
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+#include <unistd.h>
+#include <linux/filter.h>
+#include <linux/seccomp.h>
+#include <sys/prctl.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+
+#define ARRAY_SIZE(a)    (sizeof(a) / sizeof(a[0]))
+
+unsigned long long timing(clockid_t clk_id, unsigned long long samples)
+{
+	pid_t pid, ret;
+	unsigned long long i;
+	struct timespec start, finish;
+
+	pid = getpid();
+	assert(clock_gettime(clk_id, &start) == 0);
+	for (i = 0; i < samples; i++) {
+		ret = syscall(__NR_getpid);
+		assert(pid == ret);
+	}
+	assert(clock_gettime(clk_id, &finish) == 0);
+
+	i = finish.tv_sec - start.tv_sec;
+	i *= 1000000000;
+	i += finish.tv_nsec - start.tv_nsec;
+
+	printf("%lu.%09lu - %lu.%09lu = %llu\n",
+		finish.tv_sec, finish.tv_nsec,
+		start.tv_sec, start.tv_nsec,
+		i);
+
+	return i;
+}
+
+unsigned long long calibrate(void)
+{
+	unsigned long long i;
+
+	printf("Calibrating reasonable sample size...\n");
+
+	for (i = 5; ; i++) {
+		unsigned long long samples = 1 << i;
+
+		/* Find something that takes more than 5 seconds to run. */
+		if (timing(CLOCK_REALTIME, samples) / 1000000000ULL > 5)
+			return samples;
+	}
+}
+
+int main(int argc, char *argv[])
+{
+	struct sock_filter filter[] = {
+		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
+	};
+	struct sock_fprog prog = {
+		.len = (unsigned short)ARRAY_SIZE(filter),
+		.filter = filter,
+	};
+	long ret;
+	unsigned long long samples;
+	unsigned long long native, filtered;
+
+	if (argc > 1)
+		samples = strtoull(argv[1], NULL, 0);
+	else
+		samples = calibrate();
+
+	printf("Benchmarking %llu samples...\n", samples);
+
+	native = timing(CLOCK_PROCESS_CPUTIME_ID, samples) / samples;
+	printf("getpid native: %llu ns\n", native);
+
+	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+	assert(ret == 0);
+
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog);
+	assert(ret == 0);
+
+	filtered = timing(CLOCK_PROCESS_CPUTIME_ID, samples) / samples;
+	printf("getpid RET_ALLOW: %llu ns\n", filtered);
+
+	printf("Estimated seccomp overhead per syscall: %llu ns\n",
+		filtered - native);
+
+	if (filtered == native)
+		printf("Trying running again with more samples.\n");
+
+	return 0;
+}