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.
David
> }
>
> static __inline__
> long labs(long j)
> {
> - return j >= 0 ? j : -j;
> + return j >= 0 ? j : (long)-(unsigned long)j;
> }
>
> static __inline__
> long long llabs(long long j)
> {
> - return j >= 0 ? j : -j;
> + return j >= 0 ? j : (long long)-(unsigned long long)j;
> }
>
> /* must be exported, as it's used by libgcc for various divide functions */
