tc: bpf: add checksum helpers

Commit 608cd71a9c7c ("tc: bpf: generalize pedit action") has added the
possibility to mangle packet data to BPF programs in the tc pipeline.
This patch adds two helpers bpf_l3_csum_replace() and bpf_l4_csum_replace()
for fixing up the protocol checksums after the packet mangling.

It also adds 'flags' argument to bpf_skb_store_bytes() helper to avoid
unnecessary checksum recomputations when BPF programs adjusting l3/l4
checksums and documents all three helpers in uapi header.

Moreover, a sample program is added to show how BPF programs can make use
of the mangle and csum helpers.

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 37 insertions, 1 deletions

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 0db8580f3cca..23df3e7f8e7d 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -168,7 +168,43 @@ enum bpf_func_id {
         BPF_FUNC_map_delete_elem, /* int map_delete_elem(&map, &key) */
         BPF_FUNC_get_prandom_u32, /* u32 prandom_u32(void) */
         BPF_FUNC_get_smp_processor_id, /* u32 raw_smp_processor_id(void) */
-        BPF_FUNC_skb_store_bytes, /* int skb_store_bytes(skb, offset, from, len) */
+
+        /**
+         * skb_store_bytes(skb, offset, from, len, flags) - store bytes into packet
+         * @skb: pointer to skb
+         * @offset: offset within packet from skb->data
+         * @from: pointer where to copy bytes from
+         * @len: number of bytes to store into packet
+         * @flags: bit 0 - if true, recompute skb->csum
+         *         other bits - reserved
+         * Return: 0 on success
+         */
+        BPF_FUNC_skb_store_bytes,
+
+        /**
+         * l3_csum_replace(skb, offset, from, to, flags) - recompute IP checksum
+         * @skb: pointer to skb
+         * @offset: offset within packet where IP checksum is located
+         * @from: old value of header field
+         * @to: new value of header field
+         * @flags: bits 0-3 - size of header field
+         *         other bits - reserved
+         * Return: 0 on success
+         */
+        BPF_FUNC_l3_csum_replace,
+
+        /**
+         * l4_csum_replace(skb, offset, from, to, flags) - recompute TCP/UDP checksum
+         * @skb: pointer to skb
+         * @offset: offset within packet where TCP/UDP checksum is located
+         * @from: old value of header field
+         * @to: new value of header field
+         * @flags: bits 0-3 - size of header field
+         *         bit 4 - is pseudo header
+         *         other bits - reserved
+         * Return: 0 on success
+         */
+        BPF_FUNC_l4_csum_replace,
         __BPF_FUNC_MAX_ID,
 };