[RFC bpf-next 15/23] net, xdp: allow metadata > 32

Larysa Zaremba
2023-08-24T19:34:27Z

From: Aleksander Lobakin <aleksander.lobakin@intel.com>

When using XDP hints, metadata sometimes has to be much bigger
than 32 bytes. Relax the restriction, allow metadata larger than 32 bytes
and make __skb_metadata_differs() work with bigger lengths.

Now size of metadata is only limited by the fact it is stored as u8
in skb_shared_info, so maximum possible value is 255. Other important
conditions, such as having enough space for xdp_frame building, are already
checked in bpf_xdp_adjust_meta().

The requirement of having its length aligned to 4 bytes is still
valid.

Signed-off-by: Aleksander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
---
include/linux/skbuff.h | 13 ++++++++-----
include/net/xdp.h | 7 ++++++-
2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index aa57e2eca33b..0e455678cb8a 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -4216,10 +4216,13 @@ static inline bool __skb_metadata_differs(const struct sk_buff *skb_a,
{
const void *a = skb_metadata_end(skb_a);
const void *b = skb_metadata_end(skb_b);
- /* Using more efficient varaiant than plain call to memcmp(). */
-#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) && BITS_PER_LONG == 64
u64 diffs = 0;

+ if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) ||
+ BITS_PER_LONG != 64)
+ goto slow;
+
+ /* Using more efficient variant than plain call to memcmp(). */
switch (meta_len) {
#define __it(x, op) (x -= sizeof(u##op))
#define __it_diff(a, b, op) (*(u##op *)__it(a, op)) ^ (*(u##op *)__it(b, op))
@@ -4239,11 +4242,11 @@ static inline bool __skb_metadata_differs(const struct sk_buff *skb_a,
fallthrough;
case 4: diffs |= __it_diff(a, b, 32);
break;
+ default:
+slow:
+ return memcmp(a - meta_len, b - meta_len, meta_len);
}
return diffs;
-#else
- return memcmp(a - meta_len, b - meta_len, meta_len);
-#endif
}

static inline bool skb_metadata_differs(const struct sk_buff *skb_a,
diff --git a/include/net/xdp.h b/include/net/xdp.h
index 495c4d2a2c50..05234f156a73 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h
@@ -369,7 +369,12 @@ xdp_data_meta_unsupported(const struct xdp_buff *xdp)

static inline bool xdp_metalen_invalid(unsigned long metalen)
{
- return (metalen & (sizeof(__u32) - 1)) || (metalen > 32);
+ typeof(metalen) meta_max;
+
+ meta_max = type_max(typeof_member(struct skb_shared_info, meta_len));
+ BUILD_BUG_ON(!__builtin_constant_p(meta_max));
+
+ return !IS_ALIGNED(metalen, sizeof(u32)) || metalen > meta_max;
}

struct xdp_attachment_info {
--
2.41.0