Re: [PATCH bpf 2/6] libbpf: ringbuf: Prevent NULL callback crash

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Sat Jun 13, 2026 at 9:48 PM EDT, Tamir Duberstein wrote:
> ring_buffer__new() and ring_buffer__add() allow a NULL sample
> callback. When callback-based consumption reaches such a ring, it calls
> through the NULL function pointer and crashes.
>
> Validate every ring in a manager before polling or consuming. Return
> -EINVAL without consuming records from an earlier valid ring or waiting
> for an event. Perform the same check before honoring a zero record bound
> so invalid callback consumption consistently reports the error.
>

Can we just prevent a ring from being added with a NULL sample_cb?
What's the use for permitting it? Even if we don't, rechecking the
callbacks every single time we consume the ringbuf seems overkill.

> Fixes: bf99c936f947 ("libbpf: Add BPF ring buffer support")
> Assisted-by: Codex:gpt-5.5
> Signed-off-by: Tamir Duberstein <tamird@kernel.org>
> ---
>  tools/lib/bpf/libbpf.h                           | 11 ++-
>  tools/lib/bpf/ringbuf.c                          | 41 +++++++++--
>  tools/testing/selftests/bpf/prog_tests/ringbuf.c | 93 ++++++++++++++++++++++++
>  3 files changed, 134 insertions(+), 11 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index bba4e8464396..9ba6b9ad3498 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -1526,18 +1526,17 @@ LIBBPF_API int ring__map_fd(const struct ring *r);
>   *
>   * @param r A ringbuffer object.
>   * @return The number of records consumed (or INT_MAX, whichever is less), or
> - * a negative number if any of the callbacks return an error.
> + * a negative error code on failure.
>   */
>  LIBBPF_API int ring__consume(struct ring *r);
>  
>  /**
> - * @brief **ring__consume_n()** consumes up to a requested amount of items from
> - * a ringbuffer without event polling.
> + * @brief **ring__consume_n()** consumes up to a requested number of records
> + * from a ringbuffer without event polling.
>   *
>   * @param r A ringbuffer object.
> - * @param n Maximum amount of items to consume.
> - * @return The number of items consumed, or a negative number if any of the
> - * callbacks return an error.
> + * @param n Maximum number of records to consume.
> + * @return The number of records consumed, or a negative error code on failure.
>   */
>  LIBBPF_API int ring__consume_n(struct ring *r, size_t n);
>  
> diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c
> index f2bb619d5a75..ae7fa79b6217 100644
> --- a/tools/lib/bpf/ringbuf.c
> +++ b/tools/lib/bpf/ringbuf.c
> @@ -231,6 +231,24 @@ static inline int roundup_len(__u32 len)
>  	return (len + 7) / 8 * 8;
>  }
>  
> +static int ringbuf_validate(const struct ring *r)
> +{
> +	return r->sample_cb ? 0 : -EINVAL;
> +}
> +
> +static int ringbuf_validate_callbacks(const struct ring_buffer *rb)
> +{
> +	int i, err;
> +
> +	for (i = 0; i < rb->ring_cnt; i++) {
> +		err = ringbuf_validate(rb->rings[i]);
> +		if (err)
> +			return err;
> +	}
> +
> +	return 0;
> +}
> +
>  static int64_t ringbuf_process_ring(struct ring *r, size_t n)
>  {
>  	int *len_ptr, len, err;
> @@ -240,6 +258,9 @@ static int64_t ringbuf_process_ring(struct ring *r, size_t n)
>  	bool got_new_data;
>  	void *sample;
>  
> +	err = ringbuf_validate(r);
> +	if (err)
> +		return err;
>  	if (n == 0)
>  		return 0;
>  
> @@ -284,14 +305,17 @@ static int64_t ringbuf_process_ring(struct ring *r, size_t n)
>   * records.
>   *
>   * Returns number of records consumed across all registered ring buffers (or
> - * n, whichever is less), or negative number if any of the callbacks return
> - * error.
> + * n, whichever is less), or a negative error code on failure.
>   */
>  int ring_buffer__consume_n(struct ring_buffer *rb, size_t n)
>  {
>  	int64_t err, res = 0;
>  	int i;
>  
> +	err = ringbuf_validate_callbacks(rb);
> +	if (err)
> +		return libbpf_err(err);
> +
>  	for (i = 0; i < rb->ring_cnt; i++) {
>  		struct ring *ring = rb->rings[i];
>  
> @@ -309,14 +333,17 @@ int ring_buffer__consume_n(struct ring_buffer *rb, size_t n)
>  
>  /* Consume available ring buffer(s) data without event polling.
>   * Returns number of records consumed across all registered ring buffers (or
> - * INT_MAX, whichever is less), or negative number if any of the callbacks
> - * return error.
> + * INT_MAX, whichever is less), or a negative error code on failure.
>   */
>  int ring_buffer__consume(struct ring_buffer *rb)
>  {
>  	int64_t err, res = 0;
>  	int i;
>  
> +	err = ringbuf_validate_callbacks(rb);
> +	if (err)
> +		return libbpf_err(err);
> +
>  	for (i = 0; i < rb->ring_cnt; i++) {
>  		struct ring *ring = rb->rings[i];
>  
> @@ -334,13 +361,17 @@ int ring_buffer__consume(struct ring_buffer *rb)
>  
>  /* Poll for available data and consume records, if any are available.
>   * Returns number of records consumed (or INT_MAX, whichever is less), or
> - * negative number, if any of the registered callbacks returned error.
> + * a negative error code on failure.
>   */
>  int ring_buffer__poll(struct ring_buffer *rb, int timeout_ms)
>  {
>  	int i, cnt;
>  	int64_t err, res = 0;
>  
> +	err = ringbuf_validate_callbacks(rb);
> +	if (err)
> +		return libbpf_err(err);
> +
>  	cnt = epoll_wait(rb->epoll_fd, rb->events, rb->ring_cnt, timeout_ms);
>  	if (cnt < 0)
>  		return libbpf_err(-errno);
> diff --git a/tools/testing/selftests/bpf/prog_tests/ringbuf.c b/tools/testing/selftests/bpf/prog_tests/ringbuf.c
> index 4f0558f14847..9ce996bcea8c 100644
> --- a/tools/testing/selftests/bpf/prog_tests/ringbuf.c
> +++ b/tools/testing/selftests/bpf/prog_tests/ringbuf.c
> @@ -401,6 +401,97 @@ static int process_n_sample(void *ctx, void *data, size_t len)
>  	return 0;
>  }
>  
> +static int process_noop_sample(void *ctx, void *data, size_t len)
> +{
> +	return 0;
> +}
> +
> +static void ringbuf_null_cb_subtest(void)
> +{
> +	struct test_ringbuf_n_lskel *skel_n;
> +	struct ring_buffer *ringbuf = NULL;
> +	struct ring *ring;
> +	unsigned long consumer_pos;
> +	int no_cb_map_fd = -1;
> +	int err;
> +
> +	skel_n = test_ringbuf_n_lskel__open();
> +	if (!ASSERT_OK_PTR(skel_n, "test_ringbuf_n_lskel__open"))
> +		return;
> +
> +	skel_n->maps.ringbuf.max_entries = getpagesize();
> +	skel_n->bss->pid = getpid();
> +	skel_n->bss->value = SAMPLE_VALUE;
> +
> +	err = test_ringbuf_n_lskel__load(skel_n);
> +	if (!ASSERT_OK(err, "test_ringbuf_n_lskel__load"))
> +		goto cleanup;
> +
> +	err = test_ringbuf_n_lskel__attach(skel_n);
> +	if (!ASSERT_OK(err, "test_ringbuf_n_lskel__attach"))
> +		goto cleanup;
> +
> +	syscall(__NR_getpgid);
> +
> +	no_cb_map_fd = bpf_map_create(BPF_MAP_TYPE_RINGBUF, NULL, 0, 0,
> +				      getpagesize(), NULL);
> +	if (!ASSERT_OK_FD(no_cb_map_fd, "bpf_map_create"))
> +		goto cleanup;
> +
> +	/* Manager APIs must validate all rings before consuming any of them. */
> +	ringbuf = ring_buffer__new(skel_n->maps.ringbuf.map_fd,
> +				   process_noop_sample, NULL, NULL);
> +	if (!ASSERT_OK_PTR(ringbuf, "ring_buffer__new"))
> +		goto cleanup_fd;
> +
> +	ring = ring_buffer__ring(ringbuf, 0);
> +	if (!ASSERT_OK_PTR(ring, "ring_buffer__ring"))
> +		goto cleanup_ringbuf;
> +
> +	err = ring_buffer__add(ringbuf, no_cb_map_fd, NULL, NULL);
> +	if (!ASSERT_OK(err, "ring_buffer__add_no_cb"))
> +		goto cleanup_ringbuf;
> +
> +	consumer_pos = ring__consumer_pos(ring);
> +	ASSERT_GT(ring__producer_pos(ring), consumer_pos,
> +		  "producer_pos_mixed_cb");
> +
> +	err = ring_buffer__consume_n(ringbuf, 0);
> +	ASSERT_EQ(err, -EINVAL, "ringbuf_consume_zero_mixed_cb");
> +	err = ring_buffer__consume(ringbuf);
> +	ASSERT_EQ(err, -EINVAL, "ringbuf_consume_mixed_cb");
> +	err = ring_buffer__poll(ringbuf, 0);
> +	ASSERT_EQ(err, -EINVAL, "ringbuf_poll_mixed_cb");
> +	ASSERT_EQ(ring__consumer_pos(ring), consumer_pos,
> +		  "consumer_pos_mixed_cb");
> +
> +	ring_buffer__free(ringbuf);
> +	ringbuf =
> +		ring_buffer__new(skel_n->maps.ringbuf.map_fd, NULL, NULL, NULL);
> +	if (!ASSERT_OK_PTR(ringbuf, "ring_buffer__new_no_cb"))
> +		goto cleanup_fd;
> +
> +	ring = ring_buffer__ring(ringbuf, 0);
> +	if (!ASSERT_OK_PTR(ring, "ring_buffer__ring_no_cb"))
> +		goto cleanup_ringbuf;
> +	consumer_pos = ring__consumer_pos(ring);
> +
> +	err = ring_buffer__consume_n(ringbuf, 0);
> +	ASSERT_EQ(err, -EINVAL, "ringbuf_consume_zero_no_cb");
> +	err = ring__consume_n(ring, 0);
> +	ASSERT_EQ(err, -EINVAL, "ring_consume_zero_no_cb");
> +	err = ring__consume(ring);
> +	ASSERT_EQ(err, -EINVAL, "ring_consume_no_cb");
> +	ASSERT_EQ(ring__consumer_pos(ring), consumer_pos, "consumer_pos_no_cb");
> +
> +cleanup_ringbuf:
> +	ring_buffer__free(ringbuf);
> +cleanup_fd:
> +	close(no_cb_map_fd);
> +cleanup:
> +	test_ringbuf_n_lskel__destroy(skel_n);
> +}
> +
>  static void ringbuf_n_subtest(void)
>  {
>  	struct test_ringbuf_n_lskel *skel_n;
> @@ -579,6 +670,8 @@ void test_ringbuf(void)
>  		ringbuf_subtest();
>  	if (test__start_subtest("ringbuf_n"))
>  		ringbuf_n_subtest();
> +	if (test__start_subtest("ringbuf_null_cb"))
> +		ringbuf_null_cb_subtest();
>  	if (test__start_subtest("ringbuf_map_key"))
>  		ringbuf_map_key_subtest();
>  	if (test__start_subtest("ringbuf_write"))