[PATCH v5 3/4] bpf: Add libbpf logic for user-space ring buffer

David Vernet
2022-09-02T23:43:56Z

Now that all of the logic is in place in the kernel to support user-space
produced ring buffers, we can add the user-space logic to libbpf. This
patch therefore adds the following public symbols to libbpf:

struct user_ring_buffer *
user_ring_buffer__new(int map_fd,
const struct user_ring_buffer_opts *opts);
void *user_ring_buffer__reserve(struct user_ring_buffer *rb, __u32 size);
void *user_ring_buffer__reserve_blocking(struct user_ring_buffer *rb,
__u32 size, int timeout_ms);
void user_ring_buffer__submit(struct user_ring_buffer *rb, void *sample);
void user_ring_buffer__discard(struct user_ring_buffer *rb,
void user_ring_buffer__free(struct user_ring_buffer *rb);

A user-space producer must first create a struct user_ring_buffer * object
with user_ring_buffer__new(), and can then reserve samples in the
ring buffer using one of the following two symbols:

void *user_ring_buffer__reserve(struct user_ring_buffer *rb, __u32 size);
void *user_ring_buffer__reserve_blocking(struct user_ring_buffer *rb,
__u32 size, int timeout_ms);

With user_ring_buffer__reserve(), a pointer to a 'size' region of the ring
buffer will be returned if sufficient space is available in the buffer.
user_ring_buffer__reserve_blocking() provides similar semantics, but will
block for up to 'timeout_ms' in epoll_wait if there is insufficient space
in the buffer. This function has the guarantee from the kernel that it will
receive at least one event-notification per invocation to
bpf_ringbuf_drain(), provided that at least one sample is drained, and the
BPF program did not pass the BPF_RB_NO_WAKEUP flag to bpf_ringbuf_drain().

Once a sample is reserved, it must either be committed to the ring buffer
with user_ring_buffer__submit(), or discarded with
user_ring_buffer__discard().

Signed-off-by: David Vernet <void@manifault.com>
---
tools/lib/bpf/libbpf.c | 10 +-
tools/lib/bpf/libbpf.h | 105 +++++++++++++
tools/lib/bpf/libbpf.map | 10 ++
tools/lib/bpf/libbpf_probes.c | 1 +
tools/lib/bpf/libbpf_version.h | 2 +-
tools/lib/bpf/ringbuf.c | 270 +++++++++++++++++++++++++++++++++
6 files changed, 395 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 6b580ba027ba..588cf0474743 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -2373,6 +2373,12 @@ static size_t adjust_ringbuf_sz(size_t sz)
return sz;
}

+static bool map_is_ringbuf(const struct bpf_map *map)
+{
+ return map->def.type == BPF_MAP_TYPE_RINGBUF ||
+ map->def.type == BPF_MAP_TYPE_USER_RINGBUF;
+}
+
static void fill_map_from_def(struct bpf_map *map, const struct btf_map_def *def)
{
map->def.type = def->map_type;
@@ -2387,7 +2393,7 @@ static void fill_map_from_def(struct bpf_map *map, const struct btf_map_def *def
map->btf_value_type_id = def->value_type_id;

/* auto-adjust BPF ringbuf map max_entries to be a multiple of page size */
- if (map->def.type == BPF_MAP_TYPE_RINGBUF)
+ if (map_is_ringbuf(map))
map->def.max_entries = adjust_ringbuf_sz(map->def.max_entries);

if (def->parts & MAP_DEF_MAP_TYPE)
@@ -4370,7 +4376,7 @@ int bpf_map__set_max_entries(struct bpf_map *map, __u32 max_entries)
map->def.max_entries = max_entries;

/* auto-adjust BPF ringbuf map max_entries to be a multiple of page size */
- if (map->def.type == BPF_MAP_TYPE_RINGBUF)
+ if (map_is_ringbuf(map))
map->def.max_entries = adjust_ringbuf_sz(map->def.max_entries);

return 0;
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 88a1ac34b12a..92fdbe484482 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -1011,6 +1011,7 @@ LIBBPF_API int bpf_tc_query(const struct bpf_tc_hook *hook,

/* Ring buffer APIs */
struct ring_buffer;
+struct user_ring_buffer;

typedef int (*ring_buffer_sample_fn)(void *ctx, void *data, size_t size);

@@ -1030,6 +1031,110 @@ LIBBPF_API int ring_buffer__poll(struct ring_buffer *rb, int timeout_ms);
LIBBPF_API int ring_buffer__consume(struct ring_buffer *rb);
LIBBPF_API int ring_buffer__epoll_fd(const struct ring_buffer *rb);

+struct user_ring_buffer_opts {
+ size_t sz; /* size of this struct, for forward/backward compatibility */
+};
+
+#define user_ring_buffer_opts__last_field sz
+
+/* @brief **user_ring_buffer__new()** creates a new instance of a user ring
+ * buffer.
+ *
+ * @param map_fd A file descriptor to a BPF_MAP_TYPE_RINGBUF map.
+ * @param opts Options for how the ring buffer should be created.
+ * @return A user ring buffer on success; NULL and errno being set on a
+ * failure.
+ */
+LIBBPF_API struct user_ring_buffer *
+user_ring_buffer__new(int map_fd, const struct user_ring_buffer_opts *opts);
+
+/* @brief **user_ring_buffer__reserve()** reserves a pointer to a sample in the
+ * user ring buffer.
+ * @param rb A pointer to a user ring buffer.
+ * @param size The size of the sample, in bytes.
+ * @return A pointer to a reserved region of the user ring buffer; NULL, and
+ * errno being set if a sample could not be reserved.
+ *
+ * This function is *not* thread safe, and callers must synchronize accessing
+ * this function if there are multiple producers. If a size is requested that
+ * is larger than the size of the entire ring buffer, errno will be set to
+ * E2BIG and NULL is returned. If the ring buffer could accommodate the size,
+ * but currently does not have enough space, errno is set to ENOSPC and NULL is
+ * returned.
+ *
+ * After initializing the sample, callers must invoke
+ * **user_ring_buffer__submit()** to post the sample to the kernel. Otherwise,
+ * the sample must be freed with **user_ring_buffer__discard()**.
+ */
+LIBBPF_API void *user_ring_buffer__reserve(struct user_ring_buffer *rb, __u32 size);
+
+/* @brief **user_ring_buffer__reserve_blocking()** reserves a record in the
+ * ring buffer, possibly blocking for up to @timeout_ms until a sample becomes
+ * available.
+ * @param rb The user ring buffer.
+ * @param size The size of the sample, in bytes.
+ * @param timeout_ms The amount of time, in milliseconds, for which the caller
+ * should block when waiting for a sample. -1 causes the caller to block
+ * indefinitely.
+ * @return A pointer to a reserved region of the user ring buffer; NULL, and
+ * errno being set if a sample could not be reserved.
+ *
+ * This function is *not* thread safe, and callers must synchronize
+ * accessing this function if there are multiple producers
+ *
+ * If **timeout_ms** is -1, the function will block indefinitely until a sample
+ * becomes available. Otherwise, **timeout_ms** must be non-negative, or errno
+ * is set to EINVAL, and NULL is returned. If **timeout_ms** is 0, no blocking
+ * will occur and the function will return immediately after attempting to
+ * reserve a sample.
+ *
+ * If **size** is larger than the size of the entire ring buffer, errno is set
+ * to E2BIG and NULL is returned. If the ring buffer could accommodate
+ * **size**, but currently does not have enough space, the caller will block
+ * until at most **timeout_ms** has elapsed. If insufficient space is available
+ * at that time, errno is set to ENOSPC, and NULL is returned.
+ *
+ * The kernel guarantees that it will wake up this thread to check if
+ * sufficient space is available in the ring buffer at least once per
+ * invocation of the **bpf_ringbuf_drain()** helper function, provided that at
+ * least one sample is consumed, and the BPF program did not invoke the
+ * function with BPF_RB_NO_WAKEUP. A wakeup may occur sooner than that, but the
+ * kernel does not guarantee this.
+ *
+ * When a sample of size **size** is found within **timeout_ms**, a pointer to
+ * the sample is returned. After initializing the sample, callers must invoke
+ * **user_ring_buffer__submit()** to post the sample to the ring buffer.
+ * Otherwise, the sample must be freed with **user_ring_buffer__discard()**.
+ */
+LIBBPF_API void *user_ring_buffer__reserve_blocking(struct user_ring_buffer *rb,
+ __u32 size,
+ int timeout_ms);
+
+/* @brief **user_ring_buffer__submit()** submits a previously reserved sample
+ * into the ring buffer.
+ * @param rb The user ring buffer.
+ * @param sample A reserved sample.
+ *
+ * It is not necessary to synchronize amongst multiple producers when invoking
+ * this function.
+ */
+LIBBPF_API void user_ring_buffer__submit(struct user_ring_buffer *rb, void *sample);
+
+/* @brief **user_ring_buffer__discard()** discards a previously reserved sample.
+ * @param rb The user ring buffer.
+ * @param sample A reserved sample.
+ *
+ * It is not necessary to synchronize amongst multiple producers when invoking
+ * this function.
+ */
+LIBBPF_API void user_ring_buffer__discard(struct user_ring_buffer *rb, void *sample);
+
+/* @brief **user_ring_buffer__free()** frees a ring buffer that was previously
+ * created with **user_ring_buffer__new()**.
+ * @param rb The user ring buffer being freed.
+ */
+LIBBPF_API void user_ring_buffer__free(struct user_ring_buffer *rb);
+
/* Perf buffer APIs */
struct perf_buffer;

diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index 2b928dc21af0..c1d6aa7c82b6 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -368,3 +368,13 @@ LIBBPF_1.0.0 {
libbpf_bpf_prog_type_str;
perf_buffer__buffer;
};
+
+LIBBPF_1.1.0 {
+ global:
+ user_ring_buffer__discard;
+ user_ring_buffer__free;
+ user_ring_buffer__new;
+ user_ring_buffer__reserve;
+ user_ring_buffer__reserve_blocking;
+ user_ring_buffer__submit;
+} LIBBPF_1.0.0;
diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
index 6d495656f554..f3a8e8e74eb8 100644
--- a/tools/lib/bpf/libbpf_probes.c
+++ b/tools/lib/bpf/libbpf_probes.c
@@ -231,6 +231,7 @@ static int probe_map_create(enum bpf_map_type map_type)
return btf_fd;
break;
case BPF_MAP_TYPE_RINGBUF:
+ case BPF_MAP_TYPE_USER_RINGBUF:
key_size = 0;
value_size = 0;
max_entries = 4096;
diff --git a/tools/lib/bpf/libbpf_version.h b/tools/lib/bpf/libbpf_version.h
index 2fb2f4290080..e944f5bce728 100644
--- a/tools/lib/bpf/libbpf_version.h
+++ b/tools/lib/bpf/libbpf_version.h
@@ -4,6 +4,6 @@
#define __LIBBPF_VERSION_H

#define LIBBPF_MAJOR_VERSION 1
-#define LIBBPF_MINOR_VERSION 0
+#define LIBBPF_MINOR_VERSION 1

#endif /* __LIBBPF_VERSION_H */
diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c
index 8bc117bcc7bc..a5712007c30d 100644
--- a/tools/lib/bpf/ringbuf.c
+++ b/tools/lib/bpf/ringbuf.c
@@ -16,6 +16,7 @@
#include <asm/barrier.h>
#include <sys/mman.h>
#include <sys/epoll.h>
+#include <time.h>

#include "libbpf.h"
#include "libbpf_internal.h"
@@ -39,6 +40,23 @@ struct ring_buffer {
int ring_cnt;
};

+struct user_ring_buffer {
+ struct epoll_event event;
+ unsigned long *consumer_pos;
+ unsigned long *producer_pos;
+ void *data;
+ unsigned long mask;
+ size_t page_size;
+ int map_fd;
+ int epoll_fd;
+};
+
+/* 8-byte ring buffer header structure */
+struct ringbuf_hdr {
+ __u32 len;
+ __u32 pad;
+};
+
static void ringbuf_unmap_ring(struct ring_buffer *rb, struct ring *r)
{
if (r->consumer_pos) {
@@ -300,3 +318,255 @@ int ring_buffer__epoll_fd(const struct ring_buffer *rb)
{
return rb->epoll_fd;
}
+
+static void user_ringbuf_unmap_ring(struct user_ring_buffer *rb)
+{
+ if (rb->consumer_pos) {
+ munmap(rb->consumer_pos, rb->page_size);
+ rb->consumer_pos = NULL;
+ }
+ if (rb->producer_pos) {
+ munmap(rb->producer_pos, rb->page_size + 2 * (rb->mask + 1));
+ rb->producer_pos = NULL;
+ }
+}
+
+void user_ring_buffer__free(struct user_ring_buffer *rb)
+{
+ if (!rb)
+ return;
+
+ user_ringbuf_unmap_ring(rb);
+
+ if (rb->epoll_fd >= 0)
+ close(rb->epoll_fd);
+
+ free(rb);
+}
+
+static int user_ringbuf_map(struct user_ring_buffer *rb, int map_fd)
+{
+ struct bpf_map_info info;
+ __u32 len = sizeof(info);
+ void *tmp;
+ struct epoll_event *rb_epoll;
+ int err;
+
+ memset(&info, 0, sizeof(info));
+
+ err = bpf_obj_get_info_by_fd(map_fd, &info, &len);
+ if (err) {
+ err = -errno;
+ pr_warn("user ringbuf: failed to get map info for fd=%d: %d\n", map_fd, err);
+ return err;
+ }
+
+ if (info.type != BPF_MAP_TYPE_USER_RINGBUF) {
+ pr_warn("user ringbuf: map fd=%d is not BPF_MAP_TYPE_USER_RINGBUF\n", map_fd);
+ return -EINVAL;
+ }
+
+ rb->map_fd = map_fd;
+ rb->mask = info.max_entries - 1;
+
+ /* Map read-only consumer page */
+ tmp = mmap(NULL, rb->page_size, PROT_READ, MAP_SHARED, map_fd, 0);
+ if (tmp == MAP_FAILED) {
+ err = -errno;
+ pr_warn("user ringbuf: failed to mmap consumer page for map fd=%d: %d\n",
+ map_fd, err);
+ return err;
+ }
+ rb->consumer_pos = tmp;
+
+ /* Map read-write the producer page and data pages. We map the data
+ * region as twice the total size of the ring buffer to allow the
+ * simple reading and writing of samples that wrap around the end of
+ * the buffer. See the kernel implementation for details.
+ */
+ tmp = mmap(NULL, rb->page_size + 2 * info.max_entries,
+ PROT_READ | PROT_WRITE, MAP_SHARED, map_fd, rb->page_size);
+ if (tmp == MAP_FAILED) {
+ err = -errno;
+ pr_warn("user ringbuf: failed to mmap data pages for map fd=%d: %d\n",
+ map_fd, err);
+ return err;
+ }
+
+ rb->producer_pos = tmp;
+ rb->data = tmp + rb->page_size;
+
+ rb_epoll = &rb->event;
+ rb_epoll->events = EPOLLOUT;
+ if (epoll_ctl(rb->epoll_fd, EPOLL_CTL_ADD, map_fd, rb_epoll) < 0) {
+ err = -errno;
+ pr_warn("user ringbuf: failed to epoll add map fd=%d: %d\n", map_fd, err);
+ return err;
+ }
+
+ return 0;
+}
+
+struct user_ring_buffer *
+user_ring_buffer__new(int map_fd, const struct user_ring_buffer_opts *opts)
+{
+ struct user_ring_buffer *rb;
+ int err;
+
+ if (!OPTS_VALID(opts, user_ring_buffer_opts))
+ return errno = EINVAL, NULL;
+
+ rb = calloc(1, sizeof(*rb));
+ if (!rb)
+ return errno = ENOMEM, NULL;
+
+ rb->page_size = getpagesize();
+
+ rb->epoll_fd = epoll_create1(EPOLL_CLOEXEC);
+ if (rb->epoll_fd < 0) {
+ err = -errno;
+ pr_warn("user ringbuf: failed to create epoll instance: %d\n", err);
+ goto err_out;
+ }
+
+ err = user_ringbuf_map(rb, map_fd);
+ if (err)
+ goto err_out;
+
+ return rb;
+
+err_out:
+ user_ring_buffer__free(rb);
+ return errno = -err, NULL;
+}
+
+static void user_ringbuf__commit(struct user_ring_buffer *rb, void *sample, bool discard)
+{
+ __u32 new_len;
+ struct ringbuf_hdr *hdr;
+ uintptr_t hdr_offset;
+
+ hdr_offset = rb->mask + 1 + (sample - rb->data) - BPF_RINGBUF_HDR_SZ;
+ hdr = rb->data + (hdr_offset & rb->mask);
+
+ new_len = hdr->len & ~BPF_RINGBUF_BUSY_BIT;
+ if (discard)
+ new_len |= BPF_RINGBUF_DISCARD_BIT;
+
+ /* Synchronizes with smp_load_acquire() in __bpf_user_ringbuf_peek() in
+ * the kernel.
+ */
+ __atomic_exchange_n(&hdr->len, new_len, __ATOMIC_ACQ_REL);
+}
+
+void user_ring_buffer__discard(struct user_ring_buffer *rb, void *sample)
+{
+ user_ringbuf__commit(rb, sample, true);
+}
+
+void user_ring_buffer__submit(struct user_ring_buffer *rb, void *sample)
+{
+ user_ringbuf__commit(rb, sample, false);
+}
+
+void *user_ring_buffer__reserve(struct user_ring_buffer *rb, __u32 size)
+{
+ __u32 avail_size, total_size, max_size;
+ /* 64-bit to avoid overflow in case of extreme application behavior */
+ __u64 cons_pos, prod_pos;
+ struct ringbuf_hdr *hdr;
+
+ /* Synchronizes with smp_store_release() in __bpf_user_ringbuf_peek() in
+ * the kernel.
+ */
+ cons_pos = smp_load_acquire(rb->consumer_pos);
+ /* Synchronizes with smp_store_release() in user_ringbuf__commit() */
+ prod_pos = smp_load_acquire(rb->producer_pos);
+
+ max_size = rb->mask + 1;
+ avail_size = max_size - (prod_pos - cons_pos);
+ /* Round up total size to a multiple of 8. */
+ total_size = (size + BPF_RINGBUF_HDR_SZ + 7) / 8 * 8;
+
+ if (total_size > max_size)
+ return errno = E2BIG, NULL;
+
+ if (avail_size < total_size)
+ return errno = ENOSPC, NULL;
+
+ hdr = rb->data + (prod_pos & rb->mask);
+ hdr->len = size | BPF_RINGBUF_BUSY_BIT;
+ hdr->pad = 0;
+
+ /* Synchronizes with smp_load_acquire() in __bpf_user_ringbuf_peek() in
+ * the kernel.
+ */
+ smp_store_release(rb->producer_pos, prod_pos + total_size);
+
+ return (void *)rb->data + ((prod_pos + BPF_RINGBUF_HDR_SZ) & rb->mask);
+}
+
+static __u64 ns_elapsed_timespec(const struct timespec *start, const struct timespec *end)
+{
+ __u64 start_ns, end_ns, ns_per_s = 1000000000;
+
+ start_ns = (__u64)start->tv_sec * ns_per_s + start->tv_nsec;
+ end_ns = (__u64)end->tv_sec * ns_per_s + end->tv_nsec;
+
+ return end_ns - start_ns;
+}
+
+void *user_ring_buffer__reserve_blocking(struct user_ring_buffer *rb, __u32 size, int timeout_ms)
+{
+ int err;
+ struct timespec start;
+ __u64 ns_per_ms = 1000000, ns_elapsed = 0, timeout_ns;
+
+ if (timeout_ms < 0 && timeout_ms != -1)
+ return errno = EINVAL, NULL;
+
+ if (timeout_ms != -1) {
+ err = clock_gettime(CLOCK_MONOTONIC, &start);
+ if (err)
+ return NULL;
+ }
+
+ timeout_ns = timeout_ms * ns_per_ms;
+ do {
+ __u64 ns_remaining = timeout_ns - ns_elapsed;
+ int cnt, ms_remaining;
+ void *sample;
+ struct timespec curr;
+
+ sample = user_ring_buffer__reserve(rb, size);
+ if (sample)
+ return sample;
+ else if (errno != ENOSPC)
+ return NULL;
+
+ ms_remaining = timeout_ms == -1 ? -1 : ns_remaining / ns_per_ms;
+ /* The kernel guarantees at least one event notification
+ * delivery whenever at least one sample is drained from the
+ * ring buffer in an invocation to bpf_ringbuf_drain(). Other
+ * additional events may be delivered at any time, but only one
+ * event is guaranteed per bpf_ringbuf_drain() invocation,
+ * provided that a sample is drained, and the BPF program did
+ * not pass BPF_RB_NO_WAKEUP to bpf_ringbuf_drain().
+ */
+ cnt = epoll_wait(rb->epoll_fd, &rb->event, 1, ms_remaining);
+ if (cnt < 0)
+ return NULL;
+
+ if (timeout_ms == -1)
+ continue;
+
+ err = clock_gettime(CLOCK_MONOTONIC, &curr);
+ if (err)
+ return NULL;
+
+ ns_elapsed = ns_elapsed_timespec(&start, &curr);
+ } while (ns_elapsed <= timeout_ns);
+
+ errno = ENOSPC;
+ return NULL;
+}
--
2.37.1