Re: [PATCH 3/5] bpf: Add bpf_user_ringbuf_drain() helper

Andrii Nakryiko
2022-08-11T23:23:06Z

On Mon, Aug 8, 2022 at 8:54 AM David Vernet <void@manifault.com> wrote:
>
> Now that we have a BPF_MAP_TYPE_USER_RINGBUF map type, we need to add a
> helper function that allows BPF programs to drain samples from the ring
> buffer, and invoke a callback for each. This patch adds a new
> bpf_user_ringbuf_drain() helper that provides this abstraction.
>
> In order to support this, we needed to also add a new PTR_TO_DYNPTR
> register type to reflect a dynptr that was allocated by a helper function
> and passed to a BPF program. The verifier currently only supports
> PTR_TO_DYNPTR registers that are also DYNPTR_TYPE_LOCAL and MEM_ALLOC.

This commit message is a bit too laconic. There is a lot of
implications of various parts of this patch, it would be great to
highlight most important ones. Please consider elaborating a bit more.

>
> Signed-off-by: David Vernet <void@manifault.com>
> ---
> include/linux/bpf.h | 6 +-
> include/uapi/linux/bpf.h | 8 ++
> kernel/bpf/helpers.c | 2 +
> kernel/bpf/ringbuf.c | 157 +++++++++++++++++++++++++++++++--
> kernel/bpf/verifier.c | 57 +++++++++++-
> tools/include/uapi/linux/bpf.h | 8 ++
> 6 files changed, 228 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 20c26aed7896..4d0d5f2a3ac8 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. */
> @@ -2410,6 +2411,7 @@ 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);
> @@ -2554,7 +2556,7 @@ 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 a341f877b230..ca125648d7fd 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -5332,6 +5332,13 @@ union bpf_attr {
> * **-EACCES** if the SYN cookie is not valid.
> *
> * **-EPROTONOSUPPORT** if CONFIG_IPV6 is not builtin.
> + *
> + * 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.

please specify what's the expected signature of callback_fn

> + * Return
> + * An error if a sample could not be drained.

Negative error, right? And might be worth it briefly describing what
are the situation(s) when you won't be able to drain a sample?

Also please clarify if having no sample to drain is an error or not?

It's also important to specify that if no error (and -ENODATA
shouldn't be an error, actually) occurred then we get number of
consumed samples back.

> */
> #define __BPF_FUNC_MAPPER(FN) \
> FN(unspec), \
> @@ -5542,6 +5549,7 @@ union bpf_attr {
> FN(tcp_raw_gen_syncookie_ipv6), \
> FN(tcp_raw_check_syncookie_ipv4), \
> FN(tcp_raw_check_syncookie_ipv6), \
> + 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 1f961f9982d2..b8097876014c 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -1647,6 +1647,8 @@ 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 29e2de42df15..fc589fc8eb7c 100644
> --- a/kernel/bpf/ringbuf.c
> +++ b/kernel/bpf/ringbuf.c
> @@ -291,16 +291,33 @@ static unsigned long ringbuf_avail_data_sz(struct bpf_ringbuf *rb)
> }
>
> static __poll_t ringbuf_map_poll(struct bpf_map *map, struct file *filp,
> - struct poll_table_struct *pts)
> + struct poll_table_struct *pts,
> + bool kernel_producer)
> {
> struct bpf_ringbuf_map *rb_map;
> + bool buffer_empty;
>
> rb_map = container_of(map, struct bpf_ringbuf_map, map);
> poll_wait(filp, &rb_map->rb->waitq, pts);
>
> - if (ringbuf_avail_data_sz(rb_map->rb))
> - return EPOLLIN | EPOLLRDNORM;
> - return 0;
> + buffer_empty = !ringbuf_avail_data_sz(rb_map->rb);
> +
> + if (kernel_producer)
> + return buffer_empty ? 0 : EPOLLIN | EPOLLRDNORM;
> + else
> + return buffer_empty ? EPOLLOUT | EPOLLWRNORM : 0;
> +}
> +
> +static __poll_t ringbuf_map_poll_kern(struct bpf_map *map, struct file *filp,
> + struct poll_table_struct *pts)
> +{
> + return ringbuf_map_poll(map, filp, pts, true);
> +}
> +
> +static __poll_t ringbuf_map_poll_user(struct bpf_map *map, struct file *filp,
> + struct poll_table_struct *pts)
> +{
> + return ringbuf_map_poll(map, filp, pts, false);
> }

This is an even stronger case where I think we should keep two
implementations completely separate. Please keep existing
ringbuf_map_poll as is and just add a user variant as a separate
implementation

>
> BTF_ID_LIST_SINGLE(ringbuf_map_btf_ids, struct, bpf_ringbuf_map)
> @@ -309,7 +326,7 @@ const struct bpf_map_ops ringbuf_map_ops = {
> .map_alloc = ringbuf_map_alloc,
> .map_free = ringbuf_map_free,
> .map_mmap = ringbuf_map_mmap_kern,
> - .map_poll = ringbuf_map_poll,
> + .map_poll = ringbuf_map_poll_kern,
> .map_lookup_elem = ringbuf_map_lookup_elem,
> .map_update_elem = ringbuf_map_update_elem,
> .map_delete_elem = ringbuf_map_delete_elem,
> @@ -323,6 +340,7 @@ const struct bpf_map_ops user_ringbuf_map_ops = {
> .map_alloc = ringbuf_map_alloc,
> .map_free = ringbuf_map_free,
> .map_mmap = ringbuf_map_mmap_user,
> + .map_poll = ringbuf_map_poll_user,
> .map_lookup_elem = ringbuf_map_lookup_elem,
> .map_update_elem = ringbuf_map_update_elem,
> .map_delete_elem = ringbuf_map_delete_elem,
> @@ -605,3 +623,132 @@ const struct bpf_func_proto bpf_ringbuf_discard_dynptr_proto = {
> .arg1_type = ARG_PTR_TO_DYNPTR | DYNPTR_TYPE_RINGBUF | OBJ_RELEASE,
> .arg2_type = ARG_ANYTHING,
> };
> +
> +static int __bpf_user_ringbuf_poll(struct bpf_ringbuf *rb, void **sample,
> + u32 *size)

"poll" part is quite confusing as it has nothing to do with epoll and
the other poll implementation. Maybe "peek"? It peek into next sample
without consuming it, seems appropriate

nit: this declaration can also stay on single line

> +{
> + unsigned long cons_pos, prod_pos;
> + u32 sample_len, total_len;
> + u32 *hdr;
> + int err;
> + int busy = 0;

nit: combine matching types:

u32 sample_len, total_len, *hdr;
int err, busy = 0;

> +
> + /* If another consumer is already consuming a sample, wait for them to
> + * finish.
> + */
> + if (!atomic_try_cmpxchg(&rb->busy, &busy, 1))
> + return -EBUSY;
> +
> + /* Synchronizes with smp_store_release() in user-space. */
> + prod_pos = smp_load_acquire(&rb->producer_pos);

I think we should enforce that prod_pos is a multiple of 8

> + /* Synchronizes with smp_store_release() in
> + * __bpf_user_ringbuf_sample_release().
> + */
> + cons_pos = smp_load_acquire(&rb->consumer_pos);
> + if (cons_pos >= prod_pos) {
> + atomic_set(&rb->busy, 0);
> + return -ENODATA;
> + }
> +
> + hdr = (u32 *)((uintptr_t)rb->data + (cons_pos & rb->mask));
> + sample_len = *hdr;

do we need smp_load_acquire() here? libbpf's ring_buffer
implementation uses load_acquire here

> +
> + /* Check that the sample can fit into a dynptr. */
> + err = bpf_dynptr_check_size(sample_len);
> + if (err) {
> + atomic_set(&rb->busy, 0);
> + return err;
> + }
> +
> + /* Check that the sample fits within the region advertised by the
> + * consumer position.
> + */
> + total_len = sample_len + BPF_RINGBUF_HDR_SZ;

round up to closest multiple of 8? All the pointers into ringbuf data
area should be 8-byte aligned

> + if (total_len > prod_pos - cons_pos) {
> + atomic_set(&rb->busy, 0);
> + return -E2BIG;
> + }
> +
> + /* Check that the sample fits within the data region of the ring buffer.
> + */
> + if (total_len > rb->mask + 1) {
> + atomic_set(&rb->busy, 0);
> + return -E2BIG;
> + }
> +
> + /* consumer_pos is updated when the sample is released.
> + */
> +

nit: unnecessary empty line

and please keep single-line comments as single-line, no */ on separate
line in such case

> + *sample = (void *)((uintptr_t)rb->data +
> + ((cons_pos + BPF_RINGBUF_HDR_SZ) & rb->mask));
> + *size = sample_len;
> +
> + return 0;
> +}
> +
> +static void
> +__bpf_user_ringbuf_sample_release(struct bpf_ringbuf *rb, size_t size,
> + u64 flags)

try to keep single lines if they are under 100 characters

> +{
> +
> +

empty lines, why?

> + /* To release the ringbuffer, just increment the producer position to
> + * signal that a new sample can be consumed. The busy bit is cleared by
> + * userspace when posting a new sample to the ringbuffer.
> + */
> + smp_store_release(&rb->consumer_pos, rb->consumer_pos + size +
> + BPF_RINGBUF_HDR_SZ);
> +
> + if (flags & BPF_RB_FORCE_WAKEUP || !(flags & BPF_RB_NO_WAKEUP))

please use () around bit operator expressions

> + irq_work_queue(&rb->work);
> +
> + atomic_set(&rb->busy, 0);

set busy before scheduling irq_work? why delaying?

> +}
> +
> +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;
> + int err;
> + bpf_callback_t callback = (bpf_callback_t)callback_fn;
> + u64 wakeup_flags = BPF_RB_NO_WAKEUP | BPF_RB_FORCE_WAKEUP;
> +
> + if (unlikely(flags & ~wakeup_flags))
> + return -EINVAL;
> +
> + /* The two wakeup flags are mutually exclusive. */
> + if (unlikely((flags & wakeup_flags) == wakeup_flags))
> + return -EINVAL;

we don't check this for existing ringbuf, so maybe let's keep it
consistent? FORCE_WAKEUP is just stronger than NO_WAKEUP

> +
> + rb = container_of(map, struct bpf_ringbuf_map, map)->rb;
> + do {
> + u32 size;
> + void *sample;
> +
> + err = __bpf_user_ringbuf_poll(rb, &sample, &size);
> +

nit: don't keep empty line between function call and error check

> + if (!err) {

so -ENODATA is a special error and you should stop if you get that.
But for any other error we should propagate error back, not just
silently consuming it

maybe

err = ...
if (err) {
if (err == -ENODATA)
break;
return err;
}

?

> + struct bpf_dynptr_kern dynptr;
> +
> + bpf_dynptr_init(&dynptr, sample, BPF_DYNPTR_TYPE_LOCAL,
> + 0, size);

we should try to avoid unnecessary re-initialization of dynptr, let's
initialize it once and then just update data and size field inside the
loop?

> + ret = callback((u64)&dynptr,
> + (u64)(uintptr_t)callback_ctx, 0, 0, 0);
> +
> + __bpf_user_ringbuf_sample_release(rb, size, flags);
> + num_samples++;
> + }
> + } while (err == 0 && num_samples < 4096 && ret == 0);
> +

4096 is pretty arbitrary. Definitely worth noting it somewhere and it
seems somewhat low, tbh...

ideally we'd cond_resched() from time to time, but that would require
BPF program to be sleepable, so we can't do that :(

> + return num_samples;
> +}
> +
> +const struct bpf_func_proto bpf_user_ringbuf_drain_proto = {
> + .func = bpf_user_ringbuf_drain,
> + .ret_type = RET_INTEGER,
> + .arg1_type = ARG_CONST_MAP_PTR,
> + .arg2_type = ARG_PTR_TO_FUNC,
> + .arg3_type = ARG_PTR_TO_STACK_OR_NULL,
> + .arg4_type = ARG_ANYTHING,
> +};
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 4a9562c62af0..99dfdc3ed187 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -555,6 +555,7 @@ static const char *reg_type_str(struct bpf_verifier_env *env,
> [PTR_TO_BUF] = "buf",
> [PTR_TO_FUNC] = "func",
> [PTR_TO_MAP_KEY] = "map_key",
> + [PTR_TO_DYNPTR] = "dynptr_ptr",
> };
>
> if (type & PTR_MAYBE_NULL) {
> @@ -5656,6 +5657,12 @@ static const struct bpf_reg_types stack_ptr_types = { .types = { PTR_TO_STACK }
> static const struct bpf_reg_types const_str_ptr_types = { .types = { PTR_TO_MAP_VALUE } };
> static const struct bpf_reg_types timer_types = { .types = { PTR_TO_MAP_VALUE } };
> static const struct bpf_reg_types kptr_types = { .types = { PTR_TO_MAP_VALUE } };
> +static const struct bpf_reg_types dynptr_types = {
> + .types = {
> + PTR_TO_STACK,
> + PTR_TO_DYNPTR | MEM_ALLOC | DYNPTR_TYPE_LOCAL,
> + }
> +};
>
> static const struct bpf_reg_types *compatible_reg_types[__BPF_ARG_TYPE_MAX] = {
> [ARG_PTR_TO_MAP_KEY] = &map_key_value_types,
> @@ -5682,7 +5689,7 @@ static const struct bpf_reg_types *compatible_reg_types[__BPF_ARG_TYPE_MAX] = {
> [ARG_PTR_TO_CONST_STR] = &const_str_ptr_types,
> [ARG_PTR_TO_TIMER] = &timer_types,
> [ARG_PTR_TO_KPTR] = &kptr_types,
> - [ARG_PTR_TO_DYNPTR] = &stack_ptr_types,
> + [ARG_PTR_TO_DYNPTR] = &dynptr_types,
> };
>
> static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
> @@ -6025,6 +6032,13 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
> err = check_mem_size_reg(env, reg, regno, true, meta);
> break;
> case ARG_PTR_TO_DYNPTR:
> + /* We only need to check for initialized / uninitialized helper
> + * dynptr args if the dynptr is not MEM_ALLOC, as the assumption
> + * is that if it is, that a helper function initialized the
> + * dynptr on behalf of the BPF program.
> + */
> + if (reg->type & MEM_ALLOC)

Isn't PTR_TO_DYNPTR enough indication? Do we need MEM_ALLOC modifier?
Normal dynptr created and used inside BPF program on the stack are
actually PTR_TO_STACK, so that should be enough distinction? Or am I
missing something?

> + break;
> if (arg_type & MEM_UNINIT) {
> if (!is_dynptr_reg_valid_uninit(env, reg)) {
> verbose(env, "Dynptr has to be an uninitialized dynptr\n");
> @@ -6197,7 +6211,9 @@ static int check_map_func_compatibility(struct bpf_verifier_env *env,
> goto error;
> break;
> case BPF_MAP_TYPE_USER_RINGBUF:
> - goto error;
> + if (func_id != BPF_FUNC_user_ringbuf_drain)
> + goto error;
> + break;
> case BPF_MAP_TYPE_STACK_TRACE:
> if (func_id != BPF_FUNC_get_stackid)
> goto error;
> @@ -6317,6 +6333,10 @@ static int check_map_func_compatibility(struct bpf_verifier_env *env,
> if (map->map_type != BPF_MAP_TYPE_RINGBUF)
> goto error;
> break;
> + case BPF_FUNC_user_ringbuf_drain:
> + if (map->map_type != BPF_MAP_TYPE_USER_RINGBUF)
> + goto error;
> + break;
> case BPF_FUNC_get_stackid:
> if (map->map_type != BPF_MAP_TYPE_STACK_TRACE)
> goto error;
> @@ -6901,6 +6921,29 @@ static int set_find_vma_callback_state(struct bpf_verifier_env *env,
> return 0;
> }
>
> +static int set_user_ringbuf_callback_state(struct bpf_verifier_env *env,
> + struct bpf_func_state *caller,
> + struct bpf_func_state *callee,
> + int insn_idx)
> +{
> + /* bpf_user_ringbuf_drain(struct bpf_map *map, void *callback_fn, void
> + * callback_ctx, u64 flags);
> + * callback_fn(struct bpf_dynptr_t* dynptr, void *callback_ctx);
> + */
> + __mark_reg_not_init(env, &callee->regs[BPF_REG_0]);
> + callee->regs[BPF_REG_1].type = PTR_TO_DYNPTR | DYNPTR_TYPE_LOCAL | MEM_ALLOC;
> + __mark_reg_known_zero(&callee->regs[BPF_REG_1]);
> + callee->regs[BPF_REG_2] = caller->regs[BPF_REG_3];
> +
> + /* unused */
> + __mark_reg_not_init(env, &callee->regs[BPF_REG_3]);
> + __mark_reg_not_init(env, &callee->regs[BPF_REG_4]);
> + __mark_reg_not_init(env, &callee->regs[BPF_REG_5]);
> +
> + callee->in_callback_fn = true;
> + return 0;
> +}
> +
> static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
> {
> struct bpf_verifier_state *state = env->cur_state;
> @@ -7345,6 +7388,10 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
> }
> }
> 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)
> @@ -7477,11 +7524,15 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
> /* Find the id of the dynptr we're acquiring a reference to */
> for (i = 0; i < MAX_BPF_FUNC_REG_ARGS; i++) {
> if (arg_type_is_dynptr(fn->arg_type[i])) {
> + struct bpf_reg_state *reg = &regs[BPF_REG_1 + i];
> +
> if (dynptr_id) {
> verbose(env, "verifier internal error: multiple dynptr args in func\n");
> return -EFAULT;
> }
> - dynptr_id = stack_slot_get_id(env, &regs[BPF_REG_1 + i]);
> +
> + if (!(reg->type & MEM_ALLOC))
> + dynptr_id = stack_slot_get_id(env, reg);

this part has changed in bpf-next

> }
> }
> /* For release_reference() */
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index ce0ce713faf9..e5e4ab12177c 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -5332,6 +5332,13 @@ union bpf_attr {
> * **-EACCES** if the SYN cookie is not valid.
> *
> * **-EPROTONOSUPPORT** if CONFIG_IPV6 is not builtin.
> + *
> + * 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.
> + * Return
> + * An error if a sample could not be drained.
> */
> #define __BPF_FUNC_MAPPER(FN) \
> FN(unspec), \
> @@ -5542,6 +5549,7 @@ union bpf_attr {
> FN(tcp_raw_gen_syncookie_ipv6), \
> FN(tcp_raw_check_syncookie_ipv4), \
> FN(tcp_raw_check_syncookie_ipv6), \
> + FN(bpf_user_ringbuf_drain), \
> /* */
>
> /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> --
> 2.30.2
>