selftests/seccomp: Compare bitmap vs filter overhead

As part of the seccomp benchmarking, include the expectations with
regard to the timing behavior of the constant action bitmaps, and report
inconsistencies better.

Example output with constant action bitmaps on x86:

$ sudo ./seccomp_benchmark 100000000
Current BPF sysctl settings:
net.core.bpf_jit_enable = 1
net.core.bpf_jit_harden = 0
Benchmarking 200000000 syscalls...
129.359381409 - 0.008724424 = 129350656985 (129.4s)
getpid native: 646 ns
264.385890006 - 129.360453229 = 135025436777 (135.0s)
getpid RET_ALLOW 1 filter (bitmap): 675 ns
399.400511893 - 264.387045901 = 135013465992 (135.0s)
getpid RET_ALLOW 2 filters (bitmap): 675 ns
545.872866260 - 399.401718327 = 146471147933 (146.5s)
getpid RET_ALLOW 3 filters (full): 732 ns
696.337101319 - 545.874097681 = 150463003638 (150.5s)
getpid RET_ALLOW 4 filters (full): 752 ns
Estimated total seccomp overhead for 1 bitmapped filter: 29 ns
Estimated total seccomp overhead for 2 bitmapped filters: 29 ns
Estimated total seccomp overhead for 3 full filters: 86 ns
Estimated total seccomp overhead for 4 full filters: 106 ns
Estimated seccomp entry overhead: 29 ns
Estimated seccomp per-filter overhead (last 2 diff): 20 ns
Estimated seccomp per-filter overhead (filters / 4): 19 ns
Expectations:
native ≤ 1 bitmap (646 ≤ 675): ✔️
native ≤ 1 filter (646 ≤ 732): ✔️
per-filter (last 2 diff) ≈ per-filter (filters / 4) (20 ≈ 19): ✔️
1 bitmapped ≈ 2 bitmapped (29 ≈ 29): ✔️
entry ≈ 1 bitmapped (29 ≈ 29): ✔️
entry ≈ 2 bitmapped (29 ≈ 29): ✔️
native + entry + (per filter * 4) ≈ 4 filters total (755 ≈ 752): ✔️

[YiFei: Changed commit message to show stats for this patch series]
Signed-off-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

tools/testing/selftests/seccomp/seccomp_benchmark.c

···
  */
 #define _GNU_SOURCE
 #include <assert.h>
+#include <limits.h>
+#include <stdbool.h>
+#include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>
 #include <unistd.h>
 #include <linux/filter.h>
 #include <linux/seccomp.h>
+#include <sys/param.h>
 #include <sys/prctl.h>
 #include <sys/syscall.h>
 #include <sys/types.h>
···
 	return samples * seconds;
 }
 
+bool approx(int i_one, int i_two)
+{
+	double one = i_one, one_bump = one * 0.01;
+	double two = i_two, two_bump = two * 0.01;
+
+	one_bump = one + MAX(one_bump, 2.0);
+	two_bump = two + MAX(two_bump, 2.0);
+
+	/* Equal to, or within 1% or 2 digits */
+	if (one == two ||
+	    (one > two && one <= two_bump) ||
+	    (two > one && two <= one_bump))
+		return true;
+	return false;
+}
+
+bool le(int i_one, int i_two)
+{
+	if (i_one <= i_two)
+		return true;
+	return false;
+}
+
+long compare(const char *name_one, const char *name_eval, const char *name_two,
+	     unsigned long long one, bool (*eval)(int, int), unsigned long long two)
+{
+	bool good;
+
+	printf("\t%s %s %s (%lld %s %lld): ", name_one, name_eval, name_two,
+	       (long long)one, name_eval, (long long)two);
+	if (one > INT_MAX) {
+		printf("Miscalculation! Measurement went negative: %lld\n", (long long)one);
+		return 1;
+	}
+	if (two > INT_MAX) {
+		printf("Miscalculation! Measurement went negative: %lld\n", (long long)two);
+		return 1;
+	}
+
+	good = eval(one, two);
+	printf("%s\n", good ? "✔️" : "❌");
+
+	return good ? 0 : 1;
+}
+
 int main(int argc, char *argv[])
 {
+	struct sock_filter bitmap_filter[] = {
+		BPF_STMT(BPF_LD|BPF_W|BPF_ABS, offsetof(struct seccomp_data, nr)),
+		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
+	};
+	struct sock_fprog bitmap_prog = {
+		.len = (unsigned short)ARRAY_SIZE(bitmap_filter),
+		.filter = bitmap_filter,
+	};
 	struct sock_filter filter[] = {
+		BPF_STMT(BPF_LD|BPF_W|BPF_ABS, offsetof(struct seccomp_data, args[0])),
 		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, filter1, filter2;
+
+	long ret, bits;
+	unsigned long long samples, calc;
+	unsigned long long native, filter1, filter2, bitmap1, bitmap2;
+	unsigned long long entry, per_filter1, per_filter2;
 
 	printf("Current BPF sysctl settings:\n");
 	system("sysctl net.core.bpf_jit_enable");
···
 	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
 	assert(ret == 0);
 
-	/* One filter */
+	/* One filter resulting in a bitmap */
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &bitmap_prog);
+	assert(ret == 0);
+
+	bitmap1 = timing(CLOCK_PROCESS_CPUTIME_ID, samples) / samples;
+	printf("getpid RET_ALLOW 1 filter (bitmap): %llu ns\n", bitmap1);
+
+	/* Second filter resulting in a bitmap */
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &bitmap_prog);
+	assert(ret == 0);
+
+	bitmap2 = timing(CLOCK_PROCESS_CPUTIME_ID, samples) / samples;
+	printf("getpid RET_ALLOW 2 filters (bitmap): %llu ns\n", bitmap2);
+
+	/* Third filter, can no longer be converted to bitmap */
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog);
 	assert(ret == 0);
 
 	filter1 = timing(CLOCK_PROCESS_CPUTIME_ID, samples) / samples;
-	printf("getpid RET_ALLOW 1 filter: %llu ns\n", filter1);
+	printf("getpid RET_ALLOW 3 filters (full): %llu ns\n", filter1);
 
-	if (filter1 == native)
-		printf("No overhead measured!? Try running again with more samples.\n");
-
-	/* Two filters */
-	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog);
+	/* Fourth filter, can not be converted to bitmap because of filter 3 */
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &bitmap_prog);
 	assert(ret == 0);
 
 	filter2 = timing(CLOCK_PROCESS_CPUTIME_ID, samples) / samples;
-	printf("getpid RET_ALLOW 2 filters: %llu ns\n", filter2);
+	printf("getpid RET_ALLOW 4 filters (full): %llu ns\n", filter2);
 
-	/* Calculations */
-	printf("Estimated total seccomp overhead for 1 filter: %llu ns\n",
-		filter1 - native);
+	/* Estimations */
+#define ESTIMATE(fmt, var, what)	do {			\
+		var = (what);					\
+		printf("Estimated " fmt ": %llu ns\n", var);	\
+		if (var > INT_MAX)				\
+			goto more_samples;			\
+	} while (0)
 
-	printf("Estimated total seccomp overhead for 2 filters: %llu ns\n",
-		filter2 - native);
+	ESTIMATE("total seccomp overhead for 1 bitmapped filter", calc,
+		 bitmap1 - native);
+	ESTIMATE("total seccomp overhead for 2 bitmapped filters", calc,
+		 bitmap2 - native);
+	ESTIMATE("total seccomp overhead for 3 full filters", calc,
+		 filter1 - native);
+	ESTIMATE("total seccomp overhead for 4 full filters", calc,
+		 filter2 - native);
+	ESTIMATE("seccomp entry overhead", entry,
+		 bitmap1 - native - (bitmap2 - bitmap1));
+	ESTIMATE("seccomp per-filter overhead (last 2 diff)", per_filter1,
+		 filter2 - filter1);
+	ESTIMATE("seccomp per-filter overhead (filters / 4)", per_filter2,
+		 (filter2 - native - entry) / 4);
 
-	printf("Estimated seccomp per-filter overhead: %llu ns\n",
-		filter2 - filter1);
+	printf("Expectations:\n");
+	ret |= compare("native", "≤", "1 bitmap", native, le, bitmap1);
+	bits = compare("native", "≤", "1 filter", native, le, filter1);
+	if (bits)
+		goto more_samples;
 
-	printf("Estimated seccomp entry overhead: %llu ns\n",
-		filter1 - native - (filter2 - filter1));
+	ret |= compare("per-filter (last 2 diff)", "≈", "per-filter (filters / 4)",
+			per_filter1, approx, per_filter2);
 
+	bits = compare("1 bitmapped", "≈", "2 bitmapped",
+			bitmap1 - native, approx, bitmap2 - native);
+	if (bits) {
+		printf("Skipping constant action bitmap expectations: they appear unsupported.\n");
+		goto out;
+	}
+
+	ret |= compare("entry", "≈", "1 bitmapped", entry, approx, bitmap1 - native);
+	ret |= compare("entry", "≈", "2 bitmapped", entry, approx, bitmap2 - native);
+	ret |= compare("native + entry + (per filter * 4)", "≈", "4 filters total",
+			entry + (per_filter1 * 4) + native, approx, filter2);
+	if (ret == 0)
+		goto out;
+
+more_samples:
+	printf("Saw unexpected benchmark result. Try running again with more samples?\n");
+out:
 	return 0;
 }

tools/testing/selftests/seccomp/settings

···
-timeout=90
+timeout=120