bpf: Add skb dynptrs

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>

1 files changed, 11 insertions, 2 deletions

diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 62ce1f5d1b1d..d0351d30e551 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -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