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

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Message-ID: <c9c5d36bc516e70171d1bb1974806e16020fbff1@linux.dev>
Date: Wed, 02 Jul 2025 13:41:44 +0000
From: "Jiayuan Chen" <jiayuan.chen@...ux.dev>
To: netdev@...r.kernel.org
Cc: mrpre@....com, "Eric Dumazet" <edumazet@...gle.com>, "Neal Cardwell"
 <ncardwell@...gle.com>, "Kuniyuki Iwashima" <kuniyu@...gle.com>, "David
 S. Miller" <davem@...emloft.net>, "David Ahern" <dsahern@...nel.org>,
 "Jakub Kicinski" <kuba@...nel.org>, "Paolo Abeni" <pabeni@...hat.com>,
 "Simon Horman" <horms@...nel.org>, "David Howells" <dhowells@...hat.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v1] tcp: Correct signedness in skb remaining
 space calculation

July 2, 2025 at 19:00, "Jiayuan Chen" <jiayuan.chen@...ux.dev> wrote:

> 
> 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.
> 

To better explain this problem, consider the following example:
'''
#include <sys/types.h>
#include <stdio.h>
int size_goal = 536;
unsigned int skblen = 1131;

void main() {
	ssize_t copy = 0;
	copy = size_goal - skblen;
	printf("wrong: %zd\n", copy);

	copy = size_goal - (ssize_t)skblen;
	printf("correct: %zd\n", copy);
	return;
}
'''
Output:
'''
wrong: 4294966701
correct: -595
'''