[PATCH bpf-next v2] selftests/bpf: Fix htab_update/reenter_update selftest failure

[Saket Kumar Bhaskar <skb99@linux.ibm.com>]

Since commit 31158ad02ddb ("rqspinlock: Add deadlock detection
and recovery") the updated path on re-entrancy now reports deadlock
via -EDEADLK instead of the previous -EBUSY.

Also, the way reentrancy was exercised (via fentry/lookup_elem_raw)
has been fragile because lookup_elem_raw may be inlined
(find_kernel_btf_id() will return -ESRCH).

To fix this fentry is attached to bpf_obj_free_fields() instead of
lookup_elem_raw() and:

- The htab map is made to use a BTF-described struct val with a
  struct bpf_timer so that check_and_free_fields() reliably calls
  bpf_obj_free_fields() on element replacement.

- The selftest is updated to do two updates to the same key (insert +
  replace) in prog_test.

- The selftest is updated to align with expected errno with the
  kernel’s current behavior.

Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
---
Changes since v1:
Addressed comments from Alexei:
* Fixed the scenario where test may fail when lookup_elem_raw()
  is inlined.

v1: https://lore.kernel.org/all/20251106052628.349117-1-skb99@linux.ibm.com/

 .../selftests/bpf/prog_tests/htab_update.c    | 38 ++++++++++++++-----
 .../testing/selftests/bpf/progs/htab_update.c | 19 +++++++---
 2 files changed, 41 insertions(+), 16 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/htab_update.c b/tools/testing/selftests/bpf/prog_tests/htab_update.c
index 2bc85f4814f4..96b65c1a321a 100644
--- a/tools/testing/selftests/bpf/prog_tests/htab_update.c
+++ b/tools/testing/selftests/bpf/prog_tests/htab_update.c
@@ -15,17 +15,17 @@ struct htab_update_ctx {
 static void test_reenter_update(void)
 {
 	struct htab_update *skel;
-	unsigned int key, value;
+	void *value = NULL;
+	unsigned int key, value_size;
 	int err;
 
 	skel = htab_update__open();
 	if (!ASSERT_OK_PTR(skel, "htab_update__open"))
 		return;
 
-	/* lookup_elem_raw() may be inlined and find_kernel_btf_id() will return -ESRCH */
-	bpf_program__set_autoload(skel->progs.lookup_elem_raw, true);
+	bpf_program__set_autoload(skel->progs.bpf_obj_free_fields, true);
 	err = htab_update__load(skel);
-	if (!ASSERT_TRUE(!err || err == -ESRCH, "htab_update__load") || err)
+	if (!ASSERT_TRUE(!err, "htab_update__load") || err)
 		goto out;
 
 	skel->bss->pid = getpid();
@@ -33,14 +33,32 @@ static void test_reenter_update(void)
 	if (!ASSERT_OK(err, "htab_update__attach"))
 		goto out;
 
-	/* Will trigger the reentrancy of bpf_map_update_elem() */
-	key = 0;
-	value = 0;
-	err = bpf_map_update_elem(bpf_map__fd(skel->maps.htab), &key, &value, 0);
-	if (!ASSERT_OK(err, "add element"))
+	value_size = bpf_map__value_size(skel->maps.htab);
+
+	value = calloc(1, value_size);
+	if (!ASSERT_OK_PTR(value, "calloc value"))
+		goto out;
+	/*
+	 * First update: plain insert. This should NOT trigger the re-entrancy
+	 * path, because there is no old element to free yet.
+	 */
+	err = bpf_map_update_elem(bpf_map__fd(skel->maps.htab), &key, &value, BPF_ANY);
+	if (!ASSERT_OK(err, "first update (insert)"))
+		goto out;
+
+	/*
+	 * Second update: replace existing element with same key and trigger
+	 * the reentrancy of bpf_map_update_elem().
+	 * check_and_free_fields() calls bpf_obj_free_fields() on the old
+	 * value, which is where fentry program runs and performs a nested
+	 * bpf_map_update_elem(), triggering -EDEADLK.
+	 */
+	memset(&value, 0, sizeof(value));
+	err = bpf_map_update_elem(bpf_map__fd(skel->maps.htab), &key, &value, BPF_ANY);
+	if (!ASSERT_OK(err, "second update (replace)"))
 		goto out;
 
-	ASSERT_EQ(skel->bss->update_err, -EBUSY, "no reentrancy");
+	ASSERT_EQ(skel->bss->update_err, -EDEADLK, "no reentrancy");
 out:
 	htab_update__destroy(skel);
 }
diff --git a/tools/testing/selftests/bpf/progs/htab_update.c b/tools/testing/selftests/bpf/progs/htab_update.c
index 7481bb30b29b..195d3b2fba00 100644
--- a/tools/testing/selftests/bpf/progs/htab_update.c
+++ b/tools/testing/selftests/bpf/progs/htab_update.c
@@ -6,24 +6,31 @@
 
 char _license[] SEC("license") = "GPL";
 
+/* Map value type: has BTF-managed field (bpf_timer) */
+struct val {
+	struct bpf_timer t;
+	__u64 payload;
+};
+
 struct {
 	__uint(type, BPF_MAP_TYPE_HASH);
 	__uint(max_entries, 1);
-	__uint(key_size, sizeof(__u32));
-	__uint(value_size, sizeof(__u32));
+	__type(key, __u32);
+	__type(value, struct val);
 } htab SEC(".maps");
 
 int pid = 0;
 int update_err = 0;
 
-SEC("?fentry/lookup_elem_raw")
-int lookup_elem_raw(void *ctx)
+SEC("?fentry/bpf_obj_free_fields")
+int bpf_obj_free_fields(void *ctx)
 {
-	__u32 key = 0, value = 1;
+	__u32 key = 0;
+	struct val value = { .payload = 1 };
 
 	if ((bpf_get_current_pid_tgid() >> 32) != pid)
 		return 0;
 
-	update_err = bpf_map_update_elem(&htab, &key, &value, 0);
+	update_err = bpf_map_update_elem(&htab, &key, &value, BPF_ANY);
 	return 0;
 }
-- 
2.51.0