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

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Message-ID: <CANn89iJdGZq0HW3+uGLCMtekC7G5cPnHChCJFCUhvzuzPuhsrA@mail.gmail.com>
Date: Wed, 2 Jul 2025 06:59:55 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Jiayuan Chen <jiayuan.chen@...ux.dev>
Cc: netdev@...r.kernel.org, mrpre@....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

On Wed, Jul 2, 2025 at 6:42 AM Jiayuan Chen <jiayuan.chen@...ux.dev> wrote:
>
> 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
> '''

Can you explain how one skb could have more bytes (skb->len) than size_goal ?

If we are under this condition, we already have a prior bug ?

Please describe how you caught this issue.