[PATCH net-next v1] tcp: Correct signedness in skb remaining space calculation

[Jiayuan Chen <jiayuan.chen@linux.dev>]

The calculation for the remaining space, 'copy = size_goal - skb->len',
was prone to an integer promotion bug that prevented copy from ever being
negative.

The variable types involved are:
copy: ssize_t (long)
size_goal: int
skb->len: unsigned int

Due to C's type promotion rules, the signed size_goal is converted to an
unsigned int to match skb->len before the subtraction. The result is an
unsigned int.

When this unsigned int result is then assigned to the s64 copy variable,
it is zero-extended, preserving its non-negative value. Consequently,
copy is always >= 0.

The intended logic is that a negative copy value indicates that the tail
skb lacks sufficient space for appending new data, which should trigger
the allocation of a new skb. Because of this bug, the condition copy <= 0
was never met, causing the code to always append to the tail skb.

Fixes: 270a1c3de47e ("tcp: Support MSG_SPLICE_PAGES")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 net/ipv4/tcp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 8a3c99246d2e..ed942cd17351 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1180,7 +1180,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 
 		skb = tcp_write_queue_tail(sk);
 		if (skb)
-			copy = size_goal - skb->len;
+			copy = size_goal - (ssize_t)skb->len;
 
 		trace_tcp_sendmsg_locked(sk, msg, skb, size_goal);
 
-- 
2.47.1