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

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

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



> 
> On Wed, Jul 2, 2025 at 6:59 AM Eric Dumazet <edumazet@google.com> wrote:
> 
> > 
> > On Wed, Jul 2, 2025 at 6:42 AM Jiayuan Chen <jiayuan.chen@linux.dev> wrote:
> > 
> >  July 2, 2025 at 19:00, "Jiayuan Chen" <jiayuan.chen@linux.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.