selftests/bpf: Add missing trampoline program type to trampoline_count test

Currently the trampoline_count test doesn't include any fmod_ret bpf
programs, fix it to make the test cover all possible trampoline program
types.

Since fmod_ret bpf programs can't be attached to __set_task_comm function,
as it's neither whitelisted for error injection nor a security hook, change
it to bpf_modify_return_test.

This patch also does some other cleanups such as removing duplicate code,
dropping inconsistent comments, etc.

Signed-off-by: Yuntao Wang <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

include/linux/bpf.h

···
 #define BPF_TRAMP_F_RET_FENTRY_RET	BIT(4)
 
 /* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
- * bytes on x86.  Pick a number to fit into BPF_IMAGE_SIZE / 2
+ * bytes on x86.
  */
 #define BPF_MAX_TRAMP_LINKS 38
 

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

···
 // SPDX-License-Identifier: GPL-2.0-only
 #define _GNU_SOURCE
-#include <sched.h>
-#include <sys/prctl.h>
 #include <test_progs.h>
 
 #define MAX_TRAMP_PROGS 38
 
 struct inst {
 	struct bpf_object *obj;
-	struct bpf_link   *link_fentry;
-	struct bpf_link   *link_fexit;
+	struct bpf_link   *link;
 };
 
-static int test_task_rename(void)
+static struct bpf_program *load_prog(char *file, char *name, struct inst *inst)
 {
-	int fd, duration = 0, err;
-	char buf[] = "test_overhead";
-
-	fd = open("/proc/self/comm", O_WRONLY|O_TRUNC);
-	if (CHECK(fd < 0, "open /proc", "err %d", errno))
-		return -1;
-	err = write(fd, buf, sizeof(buf));
-	if (err < 0) {
-		CHECK(err < 0, "task rename", "err %d", errno);
-		close(fd);
-		return -1;
-	}
-	close(fd);
-	return 0;
-}
-
-static struct bpf_link *load(struct bpf_object *obj, const char *name)
-{
+	struct bpf_object *obj;
 	struct bpf_program *prog;
-	int duration = 0;
+	int err;
+
+	obj = bpf_object__open_file(file, NULL);
+	if (!ASSERT_OK_PTR(obj, "obj_open_file"))
+		return NULL;
+
+	inst->obj = obj;
+
+	err = bpf_object__load(obj);
+	if (!ASSERT_OK(err, "obj_load"))
+		return NULL;
 
 	prog = bpf_object__find_program_by_name(obj, name);
-	if (CHECK(!prog, "find_probe", "prog '%s' not found\n", name))
-		return ERR_PTR(-EINVAL);
-	return bpf_program__attach_trace(prog);
+	if (!ASSERT_OK_PTR(prog, "obj_find_prog"))
+		return NULL;
+
+	return prog;
 }
 
 /* TODO: use different target function to run in concurrent mode */
 void serial_test_trampoline_count(void)
 {
-	const char *fentry_name = "prog1";
-	const char *fexit_name = "prog2";
-	const char *object = "test_trampoline_count.o";
-	struct inst inst[MAX_TRAMP_PROGS] = {};
-	int err, i = 0, duration = 0;
-	struct bpf_object *obj;
+	char *file = "test_trampoline_count.o";
+	char *const progs[] = { "fentry_test", "fmod_ret_test", "fexit_test" };
+	struct inst inst[MAX_TRAMP_PROGS + 1] = {};
+	struct bpf_program *prog;
 	struct bpf_link *link;
-	char comm[16] = {};
+	int prog_fd, err, i;
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
 
 	/* attach 'allowed' trampoline programs */
 	for (i = 0; i < MAX_TRAMP_PROGS; i++) {
-		obj = bpf_object__open_file(object, NULL);
-		if (!ASSERT_OK_PTR(obj, "obj_open_file")) {
-			obj = NULL;
+		prog = load_prog(file, progs[i % ARRAY_SIZE(progs)], &inst[i]);
+		if (!prog)
 			goto cleanup;
-		}
 
-		err = bpf_object__load(obj);
-		if (CHECK(err, "obj_load", "err %d\n", err))
+		link = bpf_program__attach(prog);
+		if (!ASSERT_OK_PTR(link, "attach_prog"))
 			goto cleanup;
-		inst[i].obj = obj;
-		obj = NULL;
 
-		if (rand() % 2) {
-			link = load(inst[i].obj, fentry_name);
-			if (!ASSERT_OK_PTR(link, "attach_prog")) {
-				link = NULL;
-				goto cleanup;
-			}
-			inst[i].link_fentry = link;
-		} else {
-			link = load(inst[i].obj, fexit_name);
-			if (!ASSERT_OK_PTR(link, "attach_prog")) {
-				link = NULL;
-				goto cleanup;
-			}
-			inst[i].link_fexit = link;
-		}
+		inst[i].link = link;
 	}
 
 	/* and try 1 extra.. */
-	obj = bpf_object__open_file(object, NULL);
-	if (!ASSERT_OK_PTR(obj, "obj_open_file")) {
-		obj = NULL;
+	prog = load_prog(file, "fmod_ret_test", &inst[i]);
+	if (!prog)
+		goto cleanup;
+
+	/* ..that needs to fail */
+	link = bpf_program__attach(prog);
+	if (!ASSERT_ERR_PTR(link, "attach_prog")) {
+		inst[i].link = link;
 		goto cleanup;
 	}
 
-	err = bpf_object__load(obj);
-	if (CHECK(err, "obj_load", "err %d\n", err))
-		goto cleanup_extra;
-
-	/* ..that needs to fail */
-	link = load(obj, fentry_name);
-	err = libbpf_get_error(link);
-	if (!ASSERT_ERR_PTR(link, "cannot attach over the limit")) {
-		bpf_link__destroy(link);
-		goto cleanup_extra;
-	}
-
 	/* with E2BIG error */
-	ASSERT_EQ(err, -E2BIG, "proper error check");
-	ASSERT_EQ(link, NULL, "ptr_is_null");
+	if (!ASSERT_EQ(libbpf_get_error(link), -E2BIG, "E2BIG"))
+		goto cleanup;
+	if (!ASSERT_EQ(link, NULL, "ptr_is_null"))
+		goto cleanup;
 
 	/* and finaly execute the probe */
-	if (CHECK_FAIL(prctl(PR_GET_NAME, comm, 0L, 0L, 0L)))
-		goto cleanup_extra;
-	CHECK_FAIL(test_task_rename());
-	CHECK_FAIL(prctl(PR_SET_NAME, comm, 0L, 0L, 0L));
+	prog_fd = bpf_program__fd(prog);
+	if (!ASSERT_GE(prog_fd, 0, "bpf_program__fd"))
+		goto cleanup;
 
-cleanup_extra:
-	bpf_object__close(obj);
+	err = bpf_prog_test_run_opts(prog_fd, &opts);
+	if (!ASSERT_OK(err, "bpf_prog_test_run_opts"))
+		goto cleanup;
+
+	ASSERT_EQ(opts.retval & 0xffff, 4, "bpf_modify_return_test.result");
+	ASSERT_EQ(opts.retval >> 16, 1, "bpf_modify_return_test.side_effect");
+
 cleanup:
-	if (i >= MAX_TRAMP_PROGS)
-		i = MAX_TRAMP_PROGS - 1;
 	for (; i >= 0; i--) {
-		bpf_link__destroy(inst[i].link_fentry);
-		bpf_link__destroy(inst[i].link_fexit);
+		bpf_link__destroy(inst[i].link);
 		bpf_object__close(inst[i].obj);
 	}
 }

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

···
 // SPDX-License-Identifier: GPL-2.0
-#include <stdbool.h>
-#include <stddef.h>
 #include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
 
-struct task_struct;
-
-SEC("fentry/__set_task_comm")
-int BPF_PROG(prog1, struct task_struct *tsk, const char *buf, bool exec)
+SEC("fentry/bpf_modify_return_test")
+int BPF_PROG(fentry_test, int a, int *b)
 {
 	return 0;
 }
 
-SEC("fexit/__set_task_comm")
-int BPF_PROG(prog2, struct task_struct *tsk, const char *buf, bool exec)
+SEC("fmod_ret/bpf_modify_return_test")
+int BPF_PROG(fmod_ret_test, int a, int *b, int ret)
+{
+	return 0;
+}
+
+SEC("fexit/bpf_modify_return_test")
+int BPF_PROG(fexit_test, int a, int *b, int ret)
 {
 	return 0;
 }