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 v2: Addressed CI failures: * Initialize key to 0 before the first update. * Used pointer value to pass for update and memset rather than &value.

v2: https://lore.kernel.org/all/20251114152653.356782-1-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 | 37 ++++++++++++++----- .../testing/selftests/bpf/progs/htab_update.c | 19 +++++++--- 2 files changed, 41 insertions(+), 15 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..d0b405eb2966 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 +33,33 @@ static void test_reenter_update(void) if (!ASSERT_OK(err, "htab_update__attach")) goto out;

- /* Will trigger the reentrancy of bpf_map_update_elem() */ + 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. + */ key = 0; - value = 0; - err = bpf_map_update_elem(bpf_map__fd(skel->maps.htab), &key, &value, 0); - if (!ASSERT_OK(err, "add element")) + 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, value_size); + 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; }