bpf: Clarify role of BPF_F_RECOMPUTE_CSUM

BPF_F_RECOMPUTE_CSUM doesn't update the actual L3 and L4 checksums in
the packet, but simply updates skb->csum (according to skb->ip_summed).
This patch clarifies that to avoid confusions.

Signed-off-by: Paul Chaignon <[email protected]>
Link: https://lore.kernel.org/r/ff6895d42936f03dbb82334d8bcfd50e00c79086.1744102490.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: pchaigno

include/uapi/linux/bpf.h

@@ -1995,11 +1995,15 @@ union bpf_attr {
  * long bpf_skb_store_bytes(struct sk_buff *skb, u32 offset, const void *from, u32 len, u64 flags)
  * 	Description
  * 		Store *len* bytes from address *from* into the packet
- * 		associated to *skb*, at *offset*. *flags* are a combination of
- * 		**BPF_F_RECOMPUTE_CSUM** (automatically recompute the
- * 		checksum for the packet after storing the bytes) and
- * 		**BPF_F_INVALIDATE_HASH** (set *skb*\ **->hash**, *skb*\
- * 		**->swhash** and *skb*\ **->l4hash** to 0).
+ * 		associated to *skb*, at *offset*. The *flags* are a combination
+ * 		of the following values:
+ *
+ * 		**BPF_F_RECOMPUTE_CSUM**
+ * 			Automatically update *skb*\ **->csum** after storing the
+ * 			bytes.
+ * 		**BPF_F_INVALIDATE_HASH**
+ * 			Set *skb*\ **->hash**, *skb*\ **->swhash** and *skb*\
+ * 			**->l4hash** to 0.
  *
  * 		A call to this helper is susceptible to change the underlying
  * 		packet buffer. Therefore, at load time, all checks on pointers

tools/include/uapi/linux/bpf.h

@@ -1995,11 +1995,15 @@ union bpf_attr {
  * long bpf_skb_store_bytes(struct sk_buff *skb, u32 offset, const void *from, u32 len, u64 flags)
  * 	Description
  * 		Store *len* bytes from address *from* into the packet
- * 		associated to *skb*, at *offset*. *flags* are a combination of
- * 		**BPF_F_RECOMPUTE_CSUM** (automatically recompute the
- * 		checksum for the packet after storing the bytes) and
- * 		**BPF_F_INVALIDATE_HASH** (set *skb*\ **->hash**, *skb*\
- * 		**->swhash** and *skb*\ **->l4hash** to 0).
+ * 		associated to *skb*, at *offset*. The *flags* are a combination
+ * 		of the following values:
+ *
+ * 		**BPF_F_RECOMPUTE_CSUM**
+ * 			Automatically update *skb*\ **->csum** after storing the
+ * 			bytes.
+ * 		**BPF_F_INVALIDATE_HASH**
+ * 			Set *skb*\ **->hash**, *skb*\ **->swhash** and *skb*\
+ * 			**->l4hash** to 0.
  *
  * 		A call to this helper is susceptible to change the underlying
  * 		packet buffer. Therefore, at load time, all checks on pointers