[SRU][F][PATCH 1/2] net-zerocopy: Refactor frag-is-remappable test.

[Koichiro Den]

From: Arjun Roy <arjunroy at google.com>

Refactor frag-is-remappable test for tcp receive zerocopy. This is
part of a patch set that introduces short-circuited hybrid copies
for small receive operations, which results in roughly 33% fewer
syscalls for small RPC scenarios.

Signed-off-by: Arjun Roy <arjunroy at google.com>
Signed-off-by: Eric Dumazet <edumazet at google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil at google.com>
Signed-off-by: Jakub Kicinski <kuba at kernel.org>
(backported from commit 98917cf0d6eda01e8c3c34d35398d46b247b6fd3)
[koichiroden: Adjusted context due to missing commit 18fb76ed5386
 ("net-zerocopy: Copy straggler unaligned data for TCP Rx. zerocopy.")]
CVE-2024-26640
Signed-off-by: Koichiro Den <koichiro.den at canonical.com>
---
 net/ipv4/tcp.c | 34 ++++++++++++++++++++++++++--------
 1 file changed, 26 insertions(+), 8 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 54399256a438..7510e1937734 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1753,6 +1753,26 @@ int tcp_mmap(struct file *file, struct socket *sock,
 }
 EXPORT_SYMBOL(tcp_mmap);
 
+static bool can_map_frag(const skb_frag_t *frag)
+{
+	return skb_frag_size(frag) == PAGE_SIZE && !skb_frag_off(frag);
+}
+
+static int find_next_mappable_frag(const skb_frag_t *frag,
+				   int remaining_in_skb)
+{
+	int offset = 0;
+
+	if (likely(can_map_frag(frag)))
+		return 0;
+
+	while (offset < remaining_in_skb && !can_map_frag(frag)) {
+		offset += skb_frag_size(frag);
+		++frag;
+	}
+	return offset;
+}
+
 static int tcp_zerocopy_receive(struct sock *sk,
 				struct tcp_zerocopy_receive *zc)
 {
@@ -1795,6 +1815,8 @@ static int tcp_zerocopy_receive(struct sock *sk,
 	}
 	ret = 0;
 	while (length + PAGE_SIZE <= zc->length) {
+		int mappable_offset;
+
 		if (zc->recv_skip_hint < PAGE_SIZE) {
 			if (skb) {
 				skb = skb->next;
@@ -1815,15 +1837,11 @@ static int tcp_zerocopy_receive(struct sock *sk,
 				frags++;
 			}
 		}
-		if (skb_frag_size(frags) != PAGE_SIZE || skb_frag_off(frags)) {
-			int remaining = zc->recv_skip_hint;
 
-			while (remaining && (skb_frag_size(frags) != PAGE_SIZE ||
-					     skb_frag_off(frags))) {
-				remaining -= skb_frag_size(frags);
-				frags++;
-			}
-			zc->recv_skip_hint -= remaining;
+		mappable_offset = find_next_mappable_frag(frags,
+							  zc->recv_skip_hint);
+		if (mappable_offset) {
+			zc->recv_skip_hint = mappable_offset;
 			break;
 		}
 		ret = vm_insert_page(vma, address + length,
-- 
2.43.0