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From: "goldstein.w.n at gmail dot com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug string/28895] New: Wrong result in strncmp-avx2 and strncmp-evex
Date: Tue, 15 Feb 2022 13:37:07 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D28895

            Bug ID: 28895
           Summary: Wrong result in strncmp-avx2 and strncmp-evex
           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: ---

Wrong result for strncmp-avx2 and strncmp-evex when it hits the page cross =
case
followed by the loop page cross case immediately afterwards with a short
length.

See following test case:

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

#define PAGE_SIZE 4096
static void *
make_buf(uint64_t sz) {
    void * p =3D mmap(NULL, 2 * PAGE_SIZE + sz, PROT_NONE,
                    MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
    assert(p !=3D NULL);
    assert(!mprotect(p + PAGE_SIZE, sz, PROT_READ | PROT_WRITE));
    return p + PAGE_SIZE;
}

int
main(int argc, char ** argv) {
    char * s1 =3D make_buf(0xd000 + 0x4000);
    char * s2 =3D make_buf(0xd000 + 0x4000);

    s1 +=3D 0xffa;
    s2 +=3D 0xfed;

    strcpy(s1, "tst-tlsmod%");
    strcpy(s2, "tst-tls-manydynamic73mod");

    if (strncmp(s1, s2, 10) !=3D 0) {
        printf("Error!\n");
    }
}
```


Issue (for both evex and avx2)

```
        .p2align 4,, 10
L(return_page_cross_end_check):
        tzcntl  %ecx, %ecx
```

`ecx` can have garbabe in it from before the start of the string.

Fix is to use the `r10` mask to and off these bad matches (strcmp was doing
so. It was an oversight not to to do so in strncmp).

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