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

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Message-ID: <c910cfc4b58e9e2e1ceaca9d4dc7d68b679caa48@linux.dev>
Date: Wed, 02 Jul 2025 15:27:56 +0000
From: "Jiayuan Chen" <jiayuan.chen@...ux.dev>
To: "Eric Dumazet" <edumazet@...gle.com>
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

July 2, 2025 at 22:02, "Eric Dumazet" <edumazet@...gle.com> wrote:

> 
> On Wed, Jul 2, 2025 at 6:59 AM Eric Dumazet <edumazet@...gle.com> wrote:
> 
> > 
> > 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.
> > 
> 
> Also, not sure why copy variable had to be changed from "int" to "ssize_t"
> 
> A nicer patch (without a cast) would be to make it an "int" again/
>

I encountered this issue because I had tcp_repair enabled, which uses
tcp_init_tso_segs to reset the MSS.
However, it seems that tcp_bound_to_half_wnd also dynamically adjusts
the value to be smaller than the current size_goal.

Looking at the commit history, it's indeed unnecessary to define the
copy variable as type ssize_t.