selftests/bpf: Add test for bpf_get_branch_snapshot

This test uses bpf_get_branch_snapshot from a fexit program. The test uses
a target function (bpf_testmod_loop_test) and compares the record against
kallsyms. If there isn't enough record matching kallsyms, the test fails.

Signed-off-by: Song Liu <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>
Acked-by: John Fastabend <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

···
 
 DEFINE_PER_CPU(int, bpf_testmod_ksym_percpu) = 123;
 
+noinline int bpf_testmod_loop_test(int n)
+{
+	int i, sum = 0;
+
+	/* the primary goal of this test is to test LBR. Create a lot of
+	 * branches in the function, so we can catch it easily.
+	 */
+	for (i = 0; i < n; i++)
+		sum += i;
+	return sum;
+}
+
 noinline ssize_t
 bpf_testmod_test_read(struct file *file, struct kobject *kobj,
 		      struct bin_attribute *bin_attr,
···
 		.len = len,
 	};
 
-	trace_bpf_testmod_test_read(current, &ctx);
+	/* This is always true. Use the check to make sure the compiler
+	 * doesn't remove bpf_testmod_loop_test.
+	 */
+	if (bpf_testmod_loop_test(101) > 100)
+		trace_bpf_testmod_test_read(current, &ctx);
 
 	return -EIO; /* always fail */
 }
···
 MODULE_AUTHOR("Andrii Nakryiko");
 MODULE_DESCRIPTION("BPF selftests module");
 MODULE_LICENSE("Dual BSD/GPL");
-

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

···
 	.output_len = sizeof(struct core_reloc_module_output),		\
 	.prog_sec_name = sec_name,					\
 	.raw_tp_name = tp_name,						\
-	.trigger = trigger_module_test_read,				\
+	.trigger = __trigger_module_test_read,				\
 	.needs_testmod = true,						\
 }
 
···
 	return 0;
 }
 
-static int trigger_module_test_read(const struct core_reloc_test_case *test)
+static int __trigger_module_test_read(const struct core_reloc_test_case *test)
 {
 	struct core_reloc_module_output *exp = (void *)test->output;
-	int fd, err;
 
-	fd = open("/sys/kernel/bpf_testmod", O_RDONLY);
-	err = -errno;
-	if (CHECK(fd < 0, "testmod_file_open", "failed: %d\n", err))
-		return err;
-
-	read(fd, NULL, exp->len); /* request expected number of bytes */
-	close(fd);
-
+	trigger_module_test_read(exp->len);
 	return 0;
 }
 

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

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021 Facebook */
+#include <test_progs.h>
+#include "get_branch_snapshot.skel.h"
+
+static int *pfd_array;
+static int cpu_cnt;
+
+static int create_perf_events(void)
+{
+	struct perf_event_attr attr = {0};
+	int cpu;
+
+	/* create perf event */
+	attr.size = sizeof(attr);
+	attr.type = PERF_TYPE_RAW;
+	attr.config = 0x1b00;
+	attr.sample_type = PERF_SAMPLE_BRANCH_STACK;
+	attr.branch_sample_type = PERF_SAMPLE_BRANCH_KERNEL |
+		PERF_SAMPLE_BRANCH_USER | PERF_SAMPLE_BRANCH_ANY;
+
+	cpu_cnt = libbpf_num_possible_cpus();
+	pfd_array = malloc(sizeof(int) * cpu_cnt);
+	if (!pfd_array) {
+		cpu_cnt = 0;
+		return 1;
+	}
+
+	for (cpu = 0; cpu < cpu_cnt; cpu++) {
+		pfd_array[cpu] = syscall(__NR_perf_event_open, &attr,
+					 -1, cpu, -1, PERF_FLAG_FD_CLOEXEC);
+		if (pfd_array[cpu] < 0)
+			break;
+	}
+
+	return cpu == 0;
+}
+
+static void close_perf_events(void)
+{
+	int cpu = 0;
+	int fd;
+
+	while (cpu++ < cpu_cnt) {
+		fd = pfd_array[cpu];
+		if (fd < 0)
+			break;
+		close(fd);
+	}
+	free(pfd_array);
+}
+
+void test_get_branch_snapshot(void)
+{
+	struct get_branch_snapshot *skel = NULL;
+	int err;
+
+	if (create_perf_events()) {
+		test__skip();  /* system doesn't support LBR */
+		goto cleanup;
+	}
+
+	skel = get_branch_snapshot__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "get_branch_snapshot__open_and_load"))
+		goto cleanup;
+
+	err = kallsyms_find("bpf_testmod_loop_test", &skel->bss->address_low);
+	if (!ASSERT_OK(err, "kallsyms_find"))
+		goto cleanup;
+
+	err = kallsyms_find_next("bpf_testmod_loop_test", &skel->bss->address_high);
+	if (!ASSERT_OK(err, "kallsyms_find_next"))
+		goto cleanup;
+
+	err = get_branch_snapshot__attach(skel);
+	if (!ASSERT_OK(err, "get_branch_snapshot__attach"))
+		goto cleanup;
+
+	trigger_module_test_read(100);
+
+	if (skel->bss->total_entries < 16) {
+		/* too few entries for the hit/waste test */
+		test__skip();
+		goto cleanup;
+	}
+
+	ASSERT_GT(skel->bss->test1_hits, 6, "find_looptest_in_lbr");
+
+	/* Given we stop LBR in software, we will waste a few entries.
+	 * But we should try to waste as few as possible entries. We are at
+	 * about 7 on x86_64 systems.
+	 * Add a check for < 10 so that we get heads-up when something
+	 * changes and wastes too many entries.
+	 */
+	ASSERT_LT(skel->bss->wasted_entries, 10, "check_wasted_entries");
+
+cleanup:
+	get_branch_snapshot__destroy(skel);
+	close_perf_events();
+}

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

···
 
 static int duration;
 
-static int trigger_module_test_read(int read_sz)
-{
-	int fd, err;
-
-	fd = open("/sys/kernel/bpf_testmod", O_RDONLY);
-	err = -errno;
-	if (CHECK(fd < 0, "testmod_file_open", "failed: %d\n", err))
-		return err;
-
-	read(fd, NULL, read_sz);
-	close(fd);
-
-	return 0;
-}
-
-static int trigger_module_test_write(int write_sz)
-{
-	int fd, err;
-	char *buf = malloc(write_sz);
-
-	if (!buf)
-		return -ENOMEM;
-
-	memset(buf, 'a', write_sz);
-	buf[write_sz-1] = '\0';
-
-	fd = open("/sys/kernel/bpf_testmod", O_WRONLY);
-	err = -errno;
-	if (CHECK(fd < 0, "testmod_file_open", "failed: %d\n", err)) {
-		free(buf);
-		return err;
-	}
-
-	write(fd, buf, write_sz);
-	close(fd);
-	free(buf);
-	return 0;
-}
-
 static int delete_module(const char *name, int flags)
 {
 	return syscall(__NR_delete_module, name, flags);

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

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021 Facebook */
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+__u64 test1_hits = 0;
+__u64 address_low = 0;
+__u64 address_high = 0;
+int wasted_entries = 0;
+long total_entries = 0;
+
+#define ENTRY_CNT 32
+struct perf_branch_entry entries[ENTRY_CNT] = {};
+
+static inline bool in_range(__u64 val)
+{
+	return (val >= address_low) && (val < address_high);
+}
+
+SEC("fexit/bpf_testmod_loop_test")
+int BPF_PROG(test1, int n, int ret)
+{
+	long i;
+
+	total_entries = bpf_get_branch_snapshot(entries, sizeof(entries), 0);
+	total_entries /= sizeof(struct perf_branch_entry);
+
+	for (i = 0; i < ENTRY_CNT; i++) {
+		if (i >= total_entries)
+			break;
+		if (in_range(entries[i].from) && in_range(entries[i].to))
+			test1_hits++;
+		else if (!test1_hits)
+			wasted_entries++;
+	}
+	return 0;
+}

tools/testing/selftests/bpf/test_progs.c

···
 	return chdir(flavor);
 }
 
+int trigger_module_test_read(int read_sz)
+{
+	int fd, err;
+
+	fd = open("/sys/kernel/bpf_testmod", O_RDONLY);
+	err = -errno;
+	if (!ASSERT_GE(fd, 0, "testmod_file_open"))
+		return err;
+
+	read(fd, NULL, read_sz);
+	close(fd);
+
+	return 0;
+}
+
+int trigger_module_test_write(int write_sz)
+{
+	int fd, err;
+	char *buf = malloc(write_sz);
+
+	if (!buf)
+		return -ENOMEM;
+
+	memset(buf, 'a', write_sz);
+	buf[write_sz-1] = '\0';
+
+	fd = open("/sys/kernel/bpf_testmod", O_WRONLY);
+	err = -errno;
+	if (!ASSERT_GE(fd, 0, "testmod_file_open")) {
+		free(buf);
+		return err;
+	}
+
+	write(fd, buf, write_sz);
+	close(fd);
+	free(buf);
+	return 0;
+}
+
 #define MAX_BACKTRACE_SZ 128
 void crash_handler(int signum)
 {

tools/testing/selftests/bpf/test_progs.h

···
 int compare_stack_ips(int smap_fd, int amap_fd, int stack_trace_len);
 int extract_build_id(char *build_id, size_t size);
 int kern_sync_rcu(void);
+int trigger_module_test_read(int read_sz);
+int trigger_module_test_write(int write_sz);
 
 #ifdef __x86_64__
 #define SYS_NANOSLEEP_KPROBE_NAME "__x64_sys_nanosleep"

tools/testing/selftests/bpf/trace_helpers.c

···
 // SPDX-License-Identifier: GPL-2.0
+#include <ctype.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
···
 		if (strcmp(name, sym) == 0) {
 			*addr = value;
 			goto out;
+		}
+	}
+	err = -ENOENT;
+
+out:
+	fclose(f);
+	return err;
+}
+
+/* find the address of the next symbol of the same type, this can be used
+ * to determine the end of a function.
+ */
+int kallsyms_find_next(const char *sym, unsigned long long *addr)
+{
+	char type, found_type, name[500];
+	unsigned long long value;
+	bool found = false;
+	int err = 0;
+	FILE *f;
+
+	f = fopen("/proc/kallsyms", "r");
+	if (!f)
+		return -EINVAL;
+
+	while (fscanf(f, "%llx %c %499s%*[^\n]\n", &value, &type, name) > 0) {
+		/* Different types of symbols in kernel modules are mixed
+		 * in /proc/kallsyms. Only return the next matching type.
+		 * Use tolower() for type so that 'T' matches 't'.
+		 */
+		if (found && found_type == tolower(type)) {
+			*addr = value;
+			goto out;
+		}
+		if (strcmp(name, sym) == 0) {
+			found = true;
+			found_type = tolower(type);
 		}
 	}
 	err = -ENOENT;

tools/testing/selftests/bpf/trace_helpers.h

···
 /* open kallsyms and find addresses on the fly, faster than load + search. */
 int kallsyms_find(const char *sym, unsigned long long *addr);
 
+/* find the address of the next symbol, this can be used to determine the
+ * end of a function
+ */
+int kallsyms_find_next(const char *sym, unsigned long long *addr);
+
 void read_trace_pipe(void);
 
 ssize_t get_uprobe_offset(const void *addr, ssize_t base);