selftests/bpf: Add a test for open coded kmem_cache iter

The new subtest is attached to sleepable fentry of syncfs() syscall.
It iterates the kmem_cache using bpf_for_each loop and count the number
of entries.  Finally it checks it with the number of entries from the
regular iterator.

  $ ./vmtest.sh -- ./test_progs -t kmem_cache_iter
  ...
  #130/1   kmem_cache_iter/check_task_struct:OK
  #130/2   kmem_cache_iter/check_slabinfo:OK
  #130/3   kmem_cache_iter/open_coded_iter:OK
  #130     kmem_cache_iter:OK
  Summary: 1/3 PASSED, 0 SKIPPED, 0 FAILED

Also simplify the code by using attach routine of the skeleton.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>

Author: namhyung

tools/testing/selftests/bpf/bpf_experimental.h

@@ -582,4 +582,10 @@ extern int bpf_wq_set_callback_impl(struct bpf_wq *wq,
 		unsigned int flags__k, void *aux__ign) __ksym;
 #define bpf_wq_set_callback(timer, cb, flags) \
 	bpf_wq_set_callback_impl(timer, cb, flags, NULL)
+
+struct bpf_iter_kmem_cache;
+extern int bpf_iter_kmem_cache_new(struct bpf_iter_kmem_cache *it) __weak __ksym;
+extern struct kmem_cache *bpf_iter_kmem_cache_next(struct bpf_iter_kmem_cache *it) __weak __ksym;
+extern void bpf_iter_kmem_cache_destroy(struct bpf_iter_kmem_cache *it) __weak __ksym;
+
 #endif

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

@@ -68,29 +68,31 @@ static void subtest_kmem_cache_iter_check_slabinfo(struct kmem_cache_iter *skel)
 	fclose(fp);
 }
 
+static void subtest_kmem_cache_iter_open_coded(struct kmem_cache_iter *skel)
+{
+	/* To trigger the open coded iterator attached to the syscall */
+	syncfs(0);
+
+	/* It should be same as we've seen from the explicit iterator */
+	ASSERT_EQ(skel->bss->open_coded_seen, skel->bss->kmem_cache_seen, "open_code_seen_eq");
+}
+
 void test_kmem_cache_iter(void)
 {
-	DECLARE_LIBBPF_OPTS(bpf_iter_attach_opts, opts);
 	struct kmem_cache_iter *skel = NULL;
-	union bpf_iter_link_info linfo = {};
-	struct bpf_link *link;
 	char buf[256];
 	int iter_fd;
 
 	skel = kmem_cache_iter__open_and_load();
 	if (!ASSERT_OK_PTR(skel, "kmem_cache_iter__open_and_load"))
 		return;
 
-	opts.link_info = &linfo;
-	opts.link_info_len = sizeof(linfo);
-
-	link = bpf_program__attach_iter(skel->progs.slab_info_collector, &opts);
-	if (!ASSERT_OK_PTR(link, "attach_iter"))
+	if (!ASSERT_OK(kmem_cache_iter__attach(skel), "skel_attach"))
 		goto destroy;
 
-	iter_fd = bpf_iter_create(bpf_link__fd(link));
+	iter_fd = bpf_iter_create(bpf_link__fd(skel->links.slab_info_collector));
 	if (!ASSERT_GE(iter_fd, 0, "iter_create"))
-		goto free_link;
+		goto detach;
 
 	memset(buf, 0, sizeof(buf));
 	while (read(iter_fd, buf, sizeof(buf) > 0)) {
@@ -105,11 +107,13 @@ void test_kmem_cache_iter(void)
 		subtest_kmem_cache_iter_check_task_struct(skel);
 	if (test__start_subtest("check_slabinfo"))
 		subtest_kmem_cache_iter_check_slabinfo(skel);
+	if (test__start_subtest("open_coded_iter"))
+		subtest_kmem_cache_iter_open_coded(skel);
 
 	close(iter_fd);
 
-free_link:
-	bpf_link__destroy(link);
+detach:
+	kmem_cache_iter__detach(skel);
 destroy:
 	kmem_cache_iter__destroy(skel);
 }

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

@@ -2,6 +2,8 @@
 /* Copyright (c) 2024 Google */
 
 #include "bpf_iter.h"
+#include "bpf_experimental.h"
+#include "bpf_misc.h"
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
 
@@ -33,6 +35,7 @@ extern struct kmem_cache *bpf_get_kmem_cache(u64 addr) __ksym;
 /* Result, will be checked by userspace */
 int task_struct_found;
 int kmem_cache_seen;
+int open_coded_seen;
 
 SEC("iter/kmem_cache")
 int slab_info_collector(struct bpf_iter__kmem_cache *ctx)
@@ -85,3 +88,24 @@ int BPF_PROG(check_task_struct)
 		task_struct_found = -2;
 	return 0;
 }
+
+SEC("fentry.s/" SYS_PREFIX "sys_syncfs")
+int open_coded_iter(const void *ctx)
+{
+	struct kmem_cache *s;
+
+	bpf_for_each(kmem_cache, s) {
+		struct kmem_cache_result *r;
+		int idx = open_coded_seen;
+
+		r = bpf_map_lookup_elem(&slab_result, &idx);
+		if (r == NULL)
+			break;
+
+		open_coded_seen++;
+
+		if (r->obj_size != s->size)
+			break;
+	}
+	return 0;
+}