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From: "mbuilov at gmail dot com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug string/28898] New: mbrtoc16 segfaults when counting wide
 characters
Date: Wed, 16 Feb 2022 06:41:00 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D28898

            Bug ID: 28898
           Summary: mbrtoc16 segfaults when counting wide characters
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: string
          Assignee: unassigned at sourceware dot org
          Reporter: mbuilov at gmail dot com
  Target Milestone: ---

The following test case demonstrates Segmentation fault.

The program works correctly if the first argument to mbrtoc16() is not NULL,
otherwise it crashes.


$ cat bug.c
#include <stdio.h>
#include <string.h>
#include <locale.h>
#include <uchar.h>

static const unsigned char utf8[] =3D {
  0xf0, 0x9d, 0x94, 0xa0, 0xd5, 0xae
};

int main(int argc, char *argv[])
{
        mbstate_t state;
        unsigned i =3D 0, n =3D 0;
        const char *const locale =3D setlocale(LC_ALL, "C.UTF-8");
        if (!locale) {
                fprintf(stderr, "failed to set locale\n");
                return -2;
        }
        memset(&state, 0, sizeof(state));
        while (i < sizeof(utf8)) {
                const size_t sz =3D mbrtoc16(NULL, (const char*)&utf8[i], 1,
&state);
                if (sz =3D=3D (size_t)-2) {
                        i +=3D 1;
                        continue;
                }
                if (sz !=3D (size_t)-3)
                        i +=3D 1;
                n++;
        }
        fprintf(stdout, "number of utf16 characters: %u\n", n);
        (void)argc, (void)argv;
        return 0;
}

-------------

The problem is in the following peace of code of mbrtoc16(), in
./wcsmbs/mbrtoc16.c:


  /* The standard text does not say that S being NULL means the state
     is reset even if the second half of a surrogate still have to be
     returned.  In fact, the error code description indicates
     otherwise.  Therefore always first try to return a second
     half.  */
  if (ps->__count & 0x80000000)
    {
      /* We have to return the second word for a surrogate.  */
      ps->__count &=3D 0x7fffffff;
      *pc16 =3D ps->__value.__wch;
      ps->__value.__wch =3D L'\0';
      return (size_t) -3;
    }


- the pc16 pointer is not checked for NULL before being dereferenced.

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