BPF_F_RB_OVERWRITE can advance overwrite_pos past consumer_pos. Callback traversal does not read overwrite_pos, so after the producer laps the consumer it can treat overwritten data as a record header.
An earlier proposal[0] copied the readable window before invoking callbacks. Review concluded that callbacks are a poor fit because copying penalizes zero-copy users and the API cannot report skipped records. Record the map flag and reject callback consumption with -EOPNOTSUPP. Link: https://lore.kernel.org/bpf/CAEf4Bzaq5drHWChXoRBnrmkb6reAsSVj8r=ubyfsup31fma...@mail.gmail.com/ [0] Fixes: feeaf1346f80 ("bpf: Add overwrite mode for BPF ring buffer") Assisted-by: Codex:gpt-5.5 Signed-off-by: Tamir Duberstein <[email protected]> --- tools/lib/bpf/libbpf.h | 1 + tools/lib/bpf/ringbuf.c | 4 +++ tools/testing/selftests/bpf/prog_tests/ringbuf.c | 39 ++++++++++++++++++++++++ 3 files changed, 44 insertions(+) diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h index 9ba6b9ad3498..ae46b17feaa6 100644 --- a/tools/lib/bpf/libbpf.h +++ b/tools/lib/bpf/libbpf.h @@ -1439,6 +1439,7 @@ struct ring_buffer; struct ring; struct user_ring_buffer; +/* Callback-based consumption is unsupported for BPF_F_RB_OVERWRITE maps. */ typedef int (*ring_buffer_sample_fn)(void *ctx, void *data, size_t size); struct ring_buffer_opts { diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c index 0dae4d95d309..5737e02c1670 100644 --- a/tools/lib/bpf/ringbuf.c +++ b/tools/lib/bpf/ringbuf.c @@ -30,6 +30,7 @@ struct ring { unsigned long *producer_pos; unsigned long mask; int map_fd; + bool overwrite; }; struct ring_buffer { @@ -118,6 +119,7 @@ int ring_buffer__add(struct ring_buffer *rb, int map_fd, r->sample_cb = sample_cb; r->ctx = ctx; r->mask = info.max_entries - 1; + r->overwrite = info.map_flags & BPF_F_RB_OVERWRITE; /* Map writable consumer page */ tmp = mmap(NULL, rb->page_size, PROT_READ | PROT_WRITE, MAP_SHARED, map_fd, 0); @@ -233,6 +235,8 @@ static inline int roundup_len(__u32 len) static int ringbuf_validate(const struct ring *r) { + if (unlikely(r->overwrite)) + return -EOPNOTSUPP; if (unlikely(!r->sample_cb)) return -EINVAL; return 0; diff --git a/tools/testing/selftests/bpf/prog_tests/ringbuf.c b/tools/testing/selftests/bpf/prog_tests/ringbuf.c index 9ce996bcea8c..29be2476c478 100644 --- a/tools/testing/selftests/bpf/prog_tests/ringbuf.c +++ b/tools/testing/selftests/bpf/prog_tests/ringbuf.c @@ -602,6 +602,43 @@ static void ringbuf_map_key_subtest(void) test_ringbuf_map_key_lskel__destroy(skel_map_key); } +static void ringbuf_overwrite_callback_subtest(void) +{ + LIBBPF_OPTS(bpf_map_create_opts, opts, .map_flags = BPF_F_RB_OVERWRITE); + struct ring_buffer *ringbuf; + struct ring *ring; + int map_fd, err; + + map_fd = bpf_map_create(BPF_MAP_TYPE_RINGBUF, NULL, 0, 0, getpagesize(), + &opts); + if (!ASSERT_OK_FD(map_fd, "bpf_map_create")) + return; + + ringbuf = ring_buffer__new(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__consume_n(ringbuf, 0); + ASSERT_EQ(err, -EOPNOTSUPP, "ringbuf_consume_zero"); + err = ring_buffer__consume(ringbuf); + ASSERT_EQ(err, -EOPNOTSUPP, "ringbuf_consume"); + err = ring_buffer__poll(ringbuf, 0); + ASSERT_EQ(err, -EOPNOTSUPP, "ringbuf_poll"); + err = ring__consume_n(ring, 0); + ASSERT_EQ(err, -EOPNOTSUPP, "ring_consume_zero"); + err = ring__consume(ring); + ASSERT_EQ(err, -EOPNOTSUPP, "ring_consume"); + +cleanup_ringbuf: + ring_buffer__free(ringbuf); +cleanup_fd: + close(map_fd); +} + static void ringbuf_overwrite_mode_subtest(void) { unsigned long size, len1, len2, len3, len4, len5; @@ -676,6 +713,8 @@ void test_ringbuf(void) ringbuf_map_key_subtest(); if (test__start_subtest("ringbuf_write")) ringbuf_write_subtest(); + if (test__start_subtest("ringbuf_overwrite_callback")) + ringbuf_overwrite_callback_subtest(); if (test__start_subtest("ringbuf_overwrite_mode")) ringbuf_overwrite_mode_subtest(); } -- 2.55.0.rc0.159.gbe5d7338c2
