On Sun, Jul 26, 2026 at 03:13:34PM +0100, David Laight wrote: > On Sun, 26 Jul 2026 17:13:03 +0700 > Ammar Faizi <[email protected]> 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 <[email protected]> > > Cc: Alviro Iskandar Setiawan <[email protected]> > > Fixes: bf5e8a78bede ("tools/nolibc: add abs() and friends") > > Signed-off-by: Ammar Faizi <[email protected]> > > --- > > 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
