[Bug string/29591] New: wcsnlen length can overflow in page cross case.

["goldstein.w.n at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29591

            Bug ID: 29591
           Summary: wcsnlen length can overflow in page cross case.
           Product: glibc
           Version: 2.36
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: string
          Assignee: unassigned at sourceware dot org
          Reporter: goldstein.w.n at gmail dot com
  Target Milestone: ---

```
#include <assert.h>
#include <stdint.h>
#include <string.h>
#include <sys/mman.h>
#include <wchar.h>

#define PAGE_SIZE 4096
static void *
make_buf(uint64_t sz) {
    void * p = mmap(NULL, 2 * PAGE_SIZE + sz, PROT_NONE,
                    MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
    assert(p != NULL);
    assert(!mprotect(p + PAGE_SIZE, sz, PROT_READ | PROT_WRITE));
    return p + PAGE_SIZE;
}

size_t __wcsnlen_avx2(wchar_t const *, size_t);
size_t __wcsnlen_evex(wchar_t const *, size_t);

int
main(int argc, char ** argv) {
    wchar_t * buf  = (wchar_t *)make_buf(PAGE_SIZE);
    wchar_t * wstr = buf + (PAGE_SIZE / sizeof(wchar_t)) - 2;

    /* Expect wstr len == 1.  */
    memset(wstr, -1, sizeof(wchar_t));
    memset(wstr + 1, 0, sizeof(wchar_t));


    assert(__wcsnlen_evex(wstr, (1UL << 63)) == 1);
    assert(__wcsnlen_avx2(wstr, (1UL << 63)) == 1);
}
```

Build statically so `__wcsnlen_avx2` / `__wcsnlen_evex` are defined.


Output:
```
wcsnlen: wcsnlen.c:31: main: Assertion `__wcsnlen_avx2(wstr, (1UL << 63)) == 1'
failed.
Aborted (core dumped)
```

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