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bpf: add bpf_skb_adjust_room flag BPF_F_ADJ_ROOM_FIXED_GSO

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bpf_skb_adjust_room adjusts gso_size of gso packets to account for the
pushed or popped header room.

This is not allowed with UDP, where gso_size delineates datagrams. Add
an option to avoid these updates and allow this call for datagrams.

It can also be used with TCP, when MSS is known to allow headroom,
e.g., through MSS clamping or route MTU.

Changes v1->v2:
  - document flag BPF_F_ADJ_ROOM_FIXED_GSO
  - do not expose BPF_F_ADJ_ROOM_MASK through uapi, as it may change.

Link: https://patchwork.ozlabs.org/patch/1052497/
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Willem de Bruijn
2019-03-22 14:32:55 -04:00
committed by Alexei Starovoitov
parent 14aa31929b
commit 2278f6cc15
2 changed files with 34 additions and 13 deletions
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9
include/uapi/linux/bpf.h
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@@ -1486,8 +1486,10 @@ union bpf_attr {
* * **BPF_ADJ_ROOM_NET**: Adjust room at the network layer
* (room space is added or removed below the layer 3 header).
*
* All values for *flags* are reserved for future usage, and must
* be left at zero.
* There is one supported flag at this time:
*
* * **BPF_F_ADJ_ROOM_FIXED_GSO**: Do not adjust gso_size.
* Adjusting mss in this way is not allowed for datagrams.
*
* A call to this helper is susceptible to change the underlaying
* packet buffer. Therefore, at load time, all checks on pointers
@@ -2627,6 +2629,9 @@ enum bpf_func_id {
/* Current network namespace */
#define BPF_F_CURRENT_NETNS (-1L)
/* BPF_FUNC_skb_adjust_room flags. */
#define BPF_F_ADJ_ROOM_FIXED_GSO (1ULL << 0)
/* Mode for BPF_FUNC_skb_adjust_room helper. */
enum bpf_adj_room_mode {
BPF_ADJ_ROOM_NET,