Re: UBSAN report for main [so: 14] /usr/bin/whatis: non-zero (48) and zero offsets from null pointer in qsort.c

On 11.01.22 22:08, Stefan Esser wrote:
> diff --git a/lib/libc/stdlib/qsort.c b/lib/libc/stdlib/qsort.c
> index 5016fff7895f..51c41e802330 100644
> --- a/lib/libc/stdlib/qsort.c
> +++ b/lib/libc/stdlib/qsort.c
> @@ -108,6 +108,8 @@ local_qsort(void *a, size_t n, size_t es, cmp_t *cmp, void
> *thunk)
>  	int cmp_result;
>  	int swap_cnt;
> 
> +	if (__predict_false(a == NULL))
> +		return;
>  loop:
>  	swap_cnt = 0;
>  	if (n < 7) {
> 
> This would also work to prevent the NULL pointer arithmetik for
> ports that might also path a == NULL and n == 0 in certain cases.

The UB happens in this line, when "a == NULL" and "n == 0", right?

    for (pm = (char *)a + es; pm < (char *)a + n * es; pm += es)

This is arithmetic on a pointer (the NULL pointer) which is not part of an
array, which is UB.

Then, wouldn't "if (__predict_false(n == 0))" be more appropriate than checking
for "a == NULL" here? Testing for "a == NULL" might suppress UBSAN warnings of
valid bugs, i.e. when "qsort" is called with "a == NULL" and "n != 0". In that
case UBSAN _should_ trigger.

UBSAN should not trigger when n == 0, though. At least, when "a" does point to
a valid array. But what about the case of "a == NULL && n == 0"? Is that deemed
UB? It looks like at least FreeBSD's "qsort_s" implementation says it's legal.

a != NULL (pointing to valid array), n != 0  ->  "normal" case, no UB
a != NULL (pointing to valid array), n == 0  ->  should not trigger UB, and
                                                 doesn't in the current
                                                 implementation
a == NULL, n == 0                            ->  should not trigger UB?
                                                 (debatable)

So if "a == NULL && n == 0" was deemed legal, then there would be no bug in
"mansearch.c", right?

-Jan