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bpf: Add skb dynptrs

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Add skb dynptrs, which are dynptrs whose underlying pointer points
to a skb. The dynptr acts on skb data. skb dynptrs have two main
benefits. One is that they allow operations on sizes that are not
statically known at compile-time (eg variable-sized accesses).
Another is that parsing the packet data through dynptrs (instead of
through direct access of skb->data and skb->data_end) can be more
ergonomic and less brittle (eg does not need manual if checking for
being within bounds of data_end).

For bpf prog types that don't support writes on skb data, the dynptr is
read-only (bpf_dynptr_write() will return an error)

For reads and writes through the bpf_dynptr_read() and bpf_dynptr_write()
interfaces, reading and writing from/to data in the head as well as from/to
non-linear paged buffers is supported. Data slices through the
bpf_dynptr_data API are not supported; instead bpf_dynptr_slice() and
bpf_dynptr_slice_rdwr() (added in subsequent commit) should be used.

For examples of how skb dynptrs can be used, please see the attached
selftests.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://lore.kernel.org/r/20230301154953.641654-8-joannelkoong@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Joanne Koong
2023-03-01 07:49:50 -08:00
committed by Alexei Starovoitov
parent d96d937d7c
commit b5964b968a
8 changed files with 261 additions and 19 deletions
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13
include/uapi/linux/bpf.h
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@@ -5325,11 +5325,17 @@ union bpf_attr {
* Description
* Write *len* bytes from *src* into *dst*, starting from *offset*
* into *dst*.
* *flags* is currently unused.
*
* *flags* must be 0 except for skb-type dynptrs.
*
* For skb-type dynptrs:
* * For *flags*, please see the flags accepted by
* **bpf_skb_store_bytes**\ ().
* Return
* 0 on success, -E2BIG if *offset* + *len* exceeds the length
* of *dst*'s data, -EINVAL if *dst* is an invalid dynptr or if *dst*
* is a read-only dynptr or if *flags* is not 0.
* is a read-only dynptr or if *flags* is not correct. For skb-type dynptrs,
* other errors correspond to errors returned by **bpf_skb_store_bytes**\ ().
*
* void *bpf_dynptr_data(const struct bpf_dynptr *ptr, u32 offset, u32 len)
* Description
@@ -5337,6 +5343,9 @@ union bpf_attr {
*
* *len* must be a statically known value. The returned data slice
* is invalidated whenever the dynptr is invalidated.
*
* skb type dynptrs may not use bpf_dynptr_data. They should
* instead use bpf_dynptr_slice and bpf_dynptr_slice_rdwr.
* Return
* Pointer to the underlying dynptr data, NULL if the dynptr is
* read-only, if the dynptr is invalid, or if the offset and length