[PATCH net-next 02/10] net: add skb_crc32c()
From: Eric Biggers <ebiggers@kernel.org>
Date: 2025-05-11 00:41:31
Also in:
linux-nvme, linux-rdma, linux-sctp, lkml
Subsystem:
networking [general], the rest · Maintainers:
"David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds
From: Eric Biggers <redacted>
Add skb_crc32c(), which calculates the CRC32C of a sk_buff. It will
replace __skb_checksum(), which unnecessarily supports arbitrary
checksums. Compared to __skb_checksum(), skb_crc32c():
- Uses the correct type for CRC32C values (u32, not __wsum).
- Does not require the caller to provide a skb_checksum_ops struct.
- Is faster because it does not use indirect calls and does not use
the very slow crc32c_combine().
According to commit 2817a336d4d5 ("net: skb_checksum: allow custom
update/combine for walking skb") which added __skb_checksum(), the
original motivation for the abstraction layer was to avoid code
duplication for CRC32C and other checksums in the future. However:
- No additional checksums showed up after CRC32C. __skb_checksum()
is only used with the "regular" net checksum and CRC32C.
- Indirect calls are expensive. Commit 2544af0344ba ("net: avoid
indirect calls in L4 checksum calculation") worked around this
using the INDIRECT_CALL_1 macro. But that only avoided the indirect
call for the net checksum, and at the cost of an extra branch.
- The checksums use different types (__wsum and u32), causing casts
to be needed.
- It made the checksums of fragments be combined (rather than
chained) for both checksums, despite this being highly
counterproductive for CRC32C due to how slow crc32c_combine() is.
This can clearly be seen in commit 4c2f24549644 ("sctp: linearize
early if it's not GSO") which tried to work around this performance
bug. With a dedicated function for each checksum, we can instead
just use the proper strategy for each checksum.
Signed-off-by: Eric Biggers <redacted>
---
include/linux/skbuff.h | 1 +
net/core/skbuff.c | 73 ++++++++++++++++++++++++++++++++++++++++++
2 files changed, 74 insertions(+)
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index f3e72be6f634..33b33bb18aa6 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h@@ -4192,10 +4192,11 @@ extern const struct skb_checksum_ops *crc32c_csum_stub __read_mostly; __wsum __skb_checksum(const struct sk_buff *skb, int offset, int len, __wsum csum, const struct skb_checksum_ops *ops); __wsum skb_checksum(const struct sk_buff *skb, int offset, int len, __wsum csum); +u32 skb_crc32c(const struct sk_buff *skb, int offset, int len, u32 crc); static inline void * __must_check __skb_header_pointer(const struct sk_buff *skb, int offset, int len, const void *data, int hlen, void *buffer) {
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index d73ad79fe739..b9900cc16a24 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c@@ -62,10 +62,11 @@ #include <linux/bitfield.h> #include <linux/if_vlan.h> #include <linux/mpls.h> #include <linux/kcov.h> #include <linux/iov_iter.h> +#include <linux/crc32.h> #include <net/protocol.h> #include <net/dst.h> #include <net/sock.h> #include <net/checksum.h>
@@ -3626,10 +3627,82 @@ __wsum skb_copy_and_csum_bits(const struct sk_buff *skb, int offset, BUG_ON(len); return csum; } EXPORT_SYMBOL(skb_copy_and_csum_bits); +#ifdef CONFIG_NET_CRC32C +u32 skb_crc32c(const struct sk_buff *skb, int offset, int len, u32 crc) +{ + int start = skb_headlen(skb); + int i, copy = start - offset; + struct sk_buff *frag_iter; + + if (copy > 0) { + copy = min(copy, len); + crc = crc32c(crc, skb->data + offset, copy); + len -= copy; + if (len == 0) + return crc; + offset += copy; + } + + if (WARN_ON_ONCE(!skb_frags_readable(skb))) + return 0; + + for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) { + int end; + skb_frag_t *frag = &skb_shinfo(skb)->frags[i]; + + WARN_ON(start > offset + len); + + end = start + skb_frag_size(frag); + copy = end - offset; + if (copy > 0) { + u32 p_off, p_len, copied; + struct page *p; + u8 *vaddr; + + copy = min(copy, len); + skb_frag_foreach_page(frag, + skb_frag_off(frag) + offset - start, + copy, p, p_off, p_len, copied) { + vaddr = kmap_atomic(p); + crc = crc32c(crc, vaddr + p_off, p_len); + kunmap_atomic(vaddr); + } + len -= copy; + if (len == 0) + return crc; + offset += copy; + } + start = end; + } + + skb_walk_frags(skb, frag_iter) { + int end; + + WARN_ON(start > offset + len); + + end = start + frag_iter->len; + copy = end - offset; + if (copy > 0) { + copy = min(copy, len); + crc = skb_crc32c(frag_iter, offset - start, copy, crc); + len -= copy; + if (len == 0) + return crc; + offset += copy; + } + start = end; + } + BUG_ON(len); + + return crc; +} +EXPORT_SYMBOL(skb_crc32c); +#endif /* CONFIG_NET_CRC32C */ + __sum16 __skb_checksum_complete_head(struct sk_buff *skb, int len) { __sum16 sum; sum = csum_fold(skb_checksum(skb, 0, len, skb->csum));
--
2.49.0