Re: [PATCH 2/4] tools/nolibc: stdlib: avoid signed overflow in abs() and friends

[Willy Tarreau]

On Sun, Jul 26, 2026 at 03:13:34PM +0100, David Laight wrote:
> On Sun, 26 Jul 2026 17:13:03 +0700
> Ammar Faizi <ammarfaizi2@xxxxxxxxxxxxx> wrote:
> 
> > Negating the smallest negative value of a signed type overflows, which
> > is undefined behavior. The selftests are built with:
> > 
> >     -fsanitize=undefined -fsanitize-trap=all
> > 
> > so a caller passing INT_MIN does not merely get an unspecified answer,
> > it dies (on both x86-64 and i386):
> > 
> >     A simple test program:
> > 
> >       printf("x = %d\n", abs(INT_MIN));
> > 
> >     $ ./ab
> >     Illegal instruction (core dumped)
> > 
> >     (gdb) bt
> >     #0  0x0000000000401009 in main ()
> >     (gdb) x/6i main
> >        0x401000 <main>:     mov    $0x80000000,%eax
> >        0x401005 <main+5>:   neg    %eax
> >        0x401007 <main+7>:   jno    0x40100b <main+11>
> >     => 0x401009 <main+9>:   ud2  
> >        0x40100b <main+11>:  push   %rax
> >        0x40100c <main+12>:  mov    $0x80000000,%esi
> > 
> > Negate in the corresponding unsigned type instead. The value still
> > cannot be represented in the result type, so the minimum is returned
> > unchanged.
> > 
> > Cc: Yichun Zhang <yichun@xxxxxxxxxxxxx>
> > Cc: Alviro Iskandar Setiawan <alviro.iskandar@xxxxxxxxxxx>
> > Fixes: bf5e8a78bede ("tools/nolibc: add abs() and friends")
> > Signed-off-by: Ammar Faizi <ammarfaizi2@xxxxxxxxxxxxx>
> > ---
> >  tools/include/nolibc/stdlib.h | 12 +++++++++---
> >  1 file changed, 9 insertions(+), 3 deletions(-)
> > 
> > diff --git a/tools/include/nolibc/stdlib.h b/tools/include/nolibc/stdlib.h
> > index 1816c2368b68..8d86044f759f 100644
> > --- a/tools/include/nolibc/stdlib.h
> > +++ b/tools/include/nolibc/stdlib.h
> > @@ -32,22 +32,28 @@ static __attribute__((unused)) char itoa_buffer[21];
> >   * As much as possible, please keep functions alphabetically sorted.
> >   */
> >  
> > +/*
> > + * The absolute value of the smallest negative value is not representable in
> > + * the result type. Negate in the unsigned type so that the overflow is
> > + * defined and return it unchanged, like the other libcs do.
> > + */
> > +
> >  static __inline__
> >  int abs(int j)
> >  {
> > -	return j >= 0 ? j : -j;
> > +	return j >= 0 ? j : (int)-(unsigned int)j;
> 
> An alternative expression is -(j + 1) - 1
> gcc (and I think clang) optimise it to just -j.

This one would give -j -2, but ~(j - 1) would work, just like
(~j + 1). However here the benefit of the casts in Ammar's
version is that it's obvious that it's only playing with same
size casts with no extra operation.

Willy