Re: [PATCH v3 2/4] bpf: Add bpf_user_ringbuf_drain() helper

Andrii Nakryiko
2022-08-24T21:23:07Z

On Thu, Aug 18, 2022 at 3:12 PM David Vernet <void@manifault.com> wrote:
>
> In a prior change, we added a new BPF_MAP_TYPE_USER_RINGBUF map type which
> will allow user-space applications to publish messages to a ringbuffer that
> is consumed by a BPF program in kernel-space. In order for this map-type to
> be useful, it will require a BPF helper function that BPF programs can
> invoke to drain samples from the ringbuffer, and invoke callbacks on those
> samples. This change adds that capability via a new BPF helper function:
>
> bpf_user_ringbuf_drain(struct bpf_map *map, void *callback_fn, void *ctx,
> u64 flags)
>
> BPF programs may invoke this function to run callback_fn() on a series of
> samples in the ringbuffer. callback_fn() has the following signature:
>
> long callback_fn(struct bpf_dynptr *dynptr, void *context);
>
> Samples are provided to the callback in the form of struct bpf_dynptr *'s,
> which the program can read using BPF helper functions for querying
> struct bpf_dynptr's.
>
> In order to support bpf_ringbuf_drain(), a new PTR_TO_DYNPTR register
> type is added to the verifier to reflect a dynptr that was allocated by
> a helper function and passed to a BPF program. Unlike PTR_TO_STACK
> dynptrs which are allocated on the stack by a BPF program, PTR_TO_DYNPTR
> dynptrs need not use reference tracking, as the BPF helper is trusted to
> properly free the dynptr before returning. The verifier currently only
> supports PTR_TO_DYNPTR registers that are also DYNPTR_TYPE_LOCAL.
>
> Note that while the corresponding user-space libbpf logic will be added in
> a subsequent patch, this patch does contain an implementation of the
> .map_poll() callback for BPF_MAP_TYPE_USER_RINGBUF maps. This .map_poll()
> callback guarantees that an epoll-waiting user-space producer will
> receive at least one event notification whenever at least one sample is
> drained in an invocation of bpf_user_ringbuf_drain(), provided that the
> function is not invoked with the BPF_RB_NO_WAKEUP flag.
>
> Sending an event notification for every sample is not an option, as it
> could cause the system to hang due to invoking irq_work_queue() in
> too-frequent succession. So as to try and optimize for the common case,
> however, bpf_user_ringbuf_drain() will also send an event notification
> whenever a sample being drained causes the ringbuffer to no longer be
> full. This heuristic may not help some user-space producers, as a
> producer can publish samples of varying size, and there may not be
> enough space in the ringbuffer after the first sample is drained which
> causes it to no longer be full. In this case, the producer may have to
> wait until bpf_ringbuf_drain() returns to receive an event notification.
>
> Signed-off-by: David Vernet <void@manifault.com>
> ---
> include/linux/bpf.h | 11 +-
> include/uapi/linux/bpf.h | 36 ++++++
> kernel/bpf/helpers.c | 2 +
> kernel/bpf/ringbuf.c | 210 ++++++++++++++++++++++++++++++++-
> kernel/bpf/verifier.c | 72 +++++++++--
> tools/include/uapi/linux/bpf.h | 36 ++++++
> 6 files changed, 352 insertions(+), 15 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index a627a02cf8ab..515d712fd4a5 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -401,7 +401,7 @@ enum bpf_type_flag {
> /* DYNPTR points to memory local to the bpf program. */
> DYNPTR_TYPE_LOCAL = BIT(8 + BPF_BASE_TYPE_BITS),
>
> - /* DYNPTR points to a ringbuf record. */
> + /* DYNPTR points to a kernel-produced ringbuf record. */
> DYNPTR_TYPE_RINGBUF = BIT(9 + BPF_BASE_TYPE_BITS),
>
> /* Size is known at compile time. */
> @@ -606,6 +606,7 @@ enum bpf_reg_type {
> PTR_TO_MEM, /* reg points to valid memory region */
> PTR_TO_BUF, /* reg points to a read/write buffer */
> PTR_TO_FUNC, /* reg points to a bpf program function */
> + PTR_TO_DYNPTR, /* reg points to a dynptr */
> __BPF_REG_TYPE_MAX,
>
> /* Extended reg_types. */
> @@ -1333,6 +1334,11 @@ struct bpf_array {
> #define BPF_MAP_CAN_READ BIT(0)
> #define BPF_MAP_CAN_WRITE BIT(1)
>
> +/* Maximum number of user-producer ringbuffer samples that can be drained in
> + * a call to bpf_user_ringbuf_drain().
> + */
> +#define BPF_MAX_USER_RINGBUF_SAMPLES BIT(17)

nit: I don't think using BIT() is appropriate here. 128 * 1024 would
be better, IMO. This is not inherently required to be a single bit
constant.

> +
> static inline u32 bpf_map_flags_to_cap(struct bpf_map *map)
> {
> u32 access_flags = map->map_flags & (BPF_F_RDONLY_PROG | BPF_F_WRONLY_PROG);
> @@ -2411,6 +2417,7 @@ extern const struct bpf_func_proto bpf_loop_proto;
> extern const struct bpf_func_proto bpf_copy_from_user_task_proto;
> extern const struct bpf_func_proto bpf_set_retval_proto;
> extern const struct bpf_func_proto bpf_get_retval_proto;
> +extern const struct bpf_func_proto bpf_user_ringbuf_drain_proto;
>
> const struct bpf_func_proto *tracing_prog_func_proto(
> enum bpf_func_id func_id, const struct bpf_prog *prog);
> @@ -2555,7 +2562,7 @@ enum bpf_dynptr_type {
> BPF_DYNPTR_TYPE_INVALID,
> /* Points to memory that is local to the bpf program */
> BPF_DYNPTR_TYPE_LOCAL,
> - /* Underlying data is a ringbuf record */
> + /* Underlying data is a kernel-produced ringbuf record */
> BPF_DYNPTR_TYPE_RINGBUF,
> };
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 3aee7681fa68..25c599d9adf8 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -5356,6 +5356,41 @@ union bpf_attr {
> * Return
> * Current *ktime*.
> *
> + * long bpf_user_ringbuf_drain(struct bpf_map *map, void *callback_fn, void *ctx, u64 flags)
> + * Description
> + * Drain samples from the specified user ringbuffer, and invoke the
> + * provided callback for each such sample:
> + *
> + * long (\*callback_fn)(struct bpf_dynptr \*dynptr, void \*ctx);
> + *
> + * If **callback_fn** returns 0, the helper will continue to try
> + * and drain the next sample, up to a maximum of
> + * BPF_MAX_USER_RINGBUF_SAMPLES samples. If the return value is 1,
> + * the helper will skip the rest of the samples and return. Other
> + * return values are not used now, and will be rejected by the
> + * verifier.
> + * Return
> + * The number of drained samples if no error was encountered while
> + * draining samples. If a user-space producer was epoll-waiting on
> + * this map, and at least one sample was drained, they will
> + * receive an event notification notifying them of available space
> + * in the ringbuffer. If the BPF_RB_NO_WAKEUP flag is passed to
> + * this function, no wakeup notification will be sent. If there
> + * are no samples in the ringbuffer, 0 is returned.
> + *
> + * On failure, the returned value is one of the following:
> + *
> + * **-EBUSY** if the ringbuffer is contended, and another calling
> + * context was concurrently draining the ringbuffer.
> + *
> + * **-EINVAL** if user-space is not properly tracking the
> + * ringbuffer due to the producer position not being aligned to 8

s/ringbuffer/ring buffer/ everywhere to be more human-readable and
consistent with bpf_ringbuf_xxx() descriptions?

> + * bytes, a sample not being aligned to 8 bytes, the producer
> + * position not matching the advertised length of a sample, or the
> + * sample size being larger than the ringbuffer.
> + *
> + * **-E2BIG** if user-space has tried to publish a sample that
> + * cannot fit within a struct bpf_dynptr.

"sample size being larger than the ringbuffer" is documented above for
-EINVAL, so it's ambiguous if it's E2BIG or EINVAL?

> */
> #define __BPF_FUNC_MAPPER(FN) \
> FN(unspec), \
> @@ -5567,6 +5602,7 @@ union bpf_attr {
> FN(tcp_raw_check_syncookie_ipv4), \
> FN(tcp_raw_check_syncookie_ipv6), \
> FN(ktime_get_tai_ns), \
> + FN(user_ringbuf_drain), \
> /* */
>
> /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 3c1b9bbcf971..9141eae0ca67 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -1661,6 +1661,8 @@ bpf_base_func_proto(enum bpf_func_id func_id)
> return &bpf_dynptr_write_proto;
> case BPF_FUNC_dynptr_data:
> return &bpf_dynptr_data_proto;
> + case BPF_FUNC_user_ringbuf_drain:
> + return &bpf_user_ringbuf_drain_proto;
> default:
> break;
> }
> diff --git a/kernel/bpf/ringbuf.c b/kernel/bpf/ringbuf.c
> index 0a8de712ecbe..3818398e57de 100644
> --- a/kernel/bpf/ringbuf.c
> +++ b/kernel/bpf/ringbuf.c
> @@ -38,6 +38,22 @@ struct bpf_ringbuf {
> struct page **pages;
> int nr_pages;
> spinlock_t spinlock ____cacheline_aligned_in_smp;
> + /* For user-space producer ringbuffers, an atomic_t busy bit is used to
> + * synchronize access to the ringbuffer in the kernel, rather than the
> + * spinlock that is used for kernel-producer ringbuffers. This is done
> + * because the ringbuffer must hold a lock across a BPF program's

ditto about ringbuffer -> ring buffer (though here it's probably fine
to just use short ringbuf), Gmail also doesn't like "ringbuffer" ;)

> + * callback:
> + *
> + * __bpf_user_ringbuf_peek() // lock acquired
> + * -> program callback_fn()
> + * -> __bpf_user_ringbuf_sample_release() // lock released
> + *
> + * It is unsafe and incorrect to hold an IRQ spinlock across what could
> + * be a long execution window, so we instead simply disallow concurrent
> + * access to the ringbuffer by kernel consumers, and return -EBUSY from
> + * __bpf_user_ringbuf_peek() if the busy bit is held by another task.
> + */

[...]

> + if (flags & BPF_RINGBUF_DISCARD_BIT) {
> + /* If the discard bit is set, the sample should be ignored, and
> + * we can instead try to read the next one.
> + *
> + * Synchronizes with smp_load_acquire() in the user-space
> + * producer, and smp_load_acquire() in
> + * __bpf_user_ringbuf_peek() above.
> + */
> + smp_store_release(&rb->consumer_pos, cons_pos + total_len);
> + goto retry;

so given fast enough user-space producer, we can make kernel spend a
lot of time looping and retrying here if we just commit discarded
samples. And we won't be taking into account
BPF_MAX_USER_RINGBUF_SAMPLES for those discards. That seems like a bit
of a hole in the logic... would it be better to return with -EAGAIN
for discard samples and let drain logic skip over them?

> + }
> +
> + if (flags & BPF_RINGBUF_BUSY_BIT) {
> + err = -ENODATA;
> + goto err_unlock;
> + }
> +
> + *sample = (void *)((uintptr_t)rb->data +
> + (uintptr_t)((cons_pos + BPF_RINGBUF_HDR_SZ) & rb->mask));
> + *size = sample_len;
> + return 0;
> +
> +err_unlock:
> + atomic_set(&rb->busy, 0);
> + return err;
> +}
> +
> +static void __bpf_user_ringbuf_sample_release(struct bpf_ringbuf *rb, size_t size, u64 flags)
> +{
> + u64 producer_pos, consumer_pos;
> +
> + /* Synchronizes with smp_store_release() in user-space producer. */
> + producer_pos = smp_load_acquire(&rb->producer_pos);
> +
> + /* Using smp_load_acquire() is unnecessary here, as the busy-bit
> + * prevents another task from writing to consumer_pos after it was read
> + * by this task with smp_load_acquire() in __bpf_user_ringbuf_peek().
> + */
> + consumer_pos = rb->consumer_pos;
> + /* Synchronizes with smp_load_acquire() in user-space producer. */
> + smp_store_release(&rb->consumer_pos, consumer_pos + size + BPF_RINGBUF_HDR_SZ);
> +
> + /* Prevent the clearing of the busy-bit from being reordered before the
> + * storing of the updated rb->consumer_pos value.
> + */
> + smp_mb__before_atomic();
> + atomic_set(&rb->busy, 0);
> +
> + if (!(flags & BPF_RB_NO_WAKEUP)) {
> + /* As a heuristic, if the previously consumed sample caused the
> + * ringbuffer to no longer be full, send an event notification
> + * to any user-space producer that is epoll-waiting.
> + */
> + if (producer_pos - consumer_pos == ringbuf_total_data_sz(rb))

I'm a bit confused here. This will be true only if user-space producer
filled out entire ringbuf data *exactly* to the last byte with a
single record. Or am I misunderstanding this?

If my understanding is correct, how is this a realistic use case and
how does this heuristic help at all?

> + irq_work_queue(&rb->work);
> +
> + }
> +}
> +
> +BPF_CALL_4(bpf_user_ringbuf_drain, struct bpf_map *, map,
> + void *, callback_fn, void *, callback_ctx, u64, flags)
> +{
> + struct bpf_ringbuf *rb;
> + long num_samples = 0, ret = 0;
> + bpf_callback_t callback = (bpf_callback_t)callback_fn;
> + u64 wakeup_flags = BPF_RB_NO_WAKEUP;
> +
> + if (unlikely(flags & ~wakeup_flags))

hm... so if we specify BPF_RB_FORCE_WAKEUP we'll reject this? Why? Why
not allow both? And why use u64 variable to store BPF_RB_NO_WAKEUP
constant, just use constant right here?

> + return -EINVAL;
> +
> + rb = container_of(map, struct bpf_ringbuf_map, map)->rb;
> + do {
> + int err;
> + u32 size;
> + void *sample;
> + struct bpf_dynptr_kern dynptr;
> +

[...]

> @@ -7323,22 +7366,35 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
> }
> break;
> case BPF_FUNC_dynptr_data:
> + helper_allocated_dynptr = false;
> for (i = 0; i < MAX_BPF_FUNC_REG_ARGS; i++) {
> if (arg_type_is_dynptr(fn->arg_type[i])) {
> - if (meta.ref_obj_id) {
> - verbose(env, "verifier internal error: meta.ref_obj_id already set\n");
> + struct bpf_reg_state *reg = &regs[BPF_REG_1 + i];
> +
> + if (helper_allocated_dynptr || meta.ref_obj_id) {
> + verbose(env, "verifier internal error: multiple dynptrs not supported\n");
> return -EFAULT;
> }
> - /* Find the id of the dynptr we're tracking the reference of */
> - meta.ref_obj_id = stack_slot_get_id(env, &regs[BPF_REG_1 + i]);
> +
> + if (base_type(reg->type) == PTR_TO_DYNPTR)
> + helper_allocated_dynptr = true;
> + else
> + /* Find the id of the dynptr we're
> + * tracking the reference of
> + */
> + meta.ref_obj_id = stack_slot_get_id(env, reg);
> break;
> }
> }
> - if (i == MAX_BPF_FUNC_REG_ARGS) {
> + if (!helper_allocated_dynptr && i == MAX_BPF_FUNC_REG_ARGS) {

we still expect to get to break in the loop above, right? so there is
no need to special-case !helper_allocated_dynptr, is there?

> verbose(env, "verifier internal error: no dynptr in bpf_dynptr_data()\n");
> return -EFAULT;
> }
> break;
> + case BPF_FUNC_user_ringbuf_drain:
> + err = __check_func_call(env, insn, insn_idx_p, meta.subprogno,
> + set_user_ringbuf_callback_state);
> + break;
> }
>
> if (err) …