Re: [PATCH net-next v4] tcp: Correct signedness in skb remaining space calculation

[David Howells <dhowells@redhat.com>]

Jiayuan Chen <jiayuan.chen@linux.dev> wrote:

> The types of the variables involved are:
> '''
> copy: ssize_t (s64 on 64-bit systems)
> 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.

Ewww.

Would it be better to explicitly force the subtraction to be signed, e.g.:

		skb = tcp_write_queue_tail(sk);
		if (skb)
			copy = size_goal - (ssize_t)skb->len;

rather than relying on getting it right with an implicit conversion to a
signed int of the same size?

If not, is it worth sticking in a comment to note the potential issue?

David