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Date: Wed, 22 Mar 2023 12:56:34 -0400
From: Rich Felker <dalias@...c.org>
To: Bruno Haible <bruno@...sp.org>
Cc: musl@...ts.openwall.com
Subject: Re: swprintf produces garbage after a null wide character
On Wed, Mar 22, 2023 at 05:24:18PM +0100, Bruno Haible wrote:
> When swprintf is meant to produce output with a null wide character, in
> musl libc 1.2.3, it produces a correct return value, but fills the entire
> destination buffer with null wide characters. I.e. in this case, the number
> of written wide characters is larger than the return value + 1.
>
> How to reproduce:
> ==================================== foo.c ====================================
> #include <stdio.h>
> #include <wchar.h>
>
> int main ()
> {
> {
> wchar_t buf[5] = { 0xBEEF, 0xBEEF, 0xBEEF, 0xBEEF, 0xBEEF };
> int ret = swprintf (buf, 4, L"%c", '\0');
> printf ("ret = %d, buf[0] = 0x%x, buf[1] = 0x%x, buf[2] = 0x%x, buf[3] = 0x%x, buf[4] = 0x%x\n",
> ret,
> (unsigned int) buf[0], (unsigned int) buf[1],
> (unsigned int) buf[2], (unsigned int) buf[3],
> (unsigned int) buf[4]);
> }
> {
> wchar_t buf[5] = { 0xBEEF, 0xBEEF, 0xBEEF, 0xBEEF, 0xBEEF };
> int ret = swprintf (buf, 4, L"%cz", '\0');
> printf ("ret = %d, buf[0] = 0x%x, buf[1] = 0x%x, buf[2] = 0x%x, buf[3] = 0x%x, buf[4] = 0x%x\n",
> ret,
> (unsigned int) buf[0], (unsigned int) buf[1],
> (unsigned int) buf[2], (unsigned int) buf[3],
> (unsigned int) buf[4]);
> }
> return 0;
> }
> /*
> glibc, Solaris 11, Cygwin:
> ret = 1, buf[0] = 0x0, buf[1] = 0x0, buf[2] = 0xbeef, buf[3] = 0xbeef, buf[4] = 0xbeef
> ret = 2, buf[0] = 0x0, buf[1] = 0x7a, buf[2] = 0x0, buf[3] = 0xbeef, buf[4] = 0xbeef
> musl libc:
> ret = 1, buf[0] = 0x0, buf[1] = 0x0, buf[2] = 0x0, buf[3] = 0x0, buf[4] = 0xbeef
> ret = 2, buf[0] = 0x0, buf[1] = 0x0, buf[2] = 0x0, buf[3] = 0x0, buf[4] = 0xbeef
> FreeBSD 13, NetBSD 9, OpenBSD 7.2, macOS 12.5, AIX 7.1:
> ret = 1, buf[0] = 0x0, buf[1] = 0xbeef, buf[2] = 0xbeef, buf[3] = 0xbeef, buf[4] = 0xbeef
> ret = 2, buf[0] = 0x0, buf[1] = 0xbeef, buf[2] = 0xbeef, buf[3] = 0xbeef, buf[4] = 0xbeef
> */
> ===============================================================================
>
> $ gcc -Wall foo.c
> $ ./a.out
>
> Expected output:
> ret = 1, buf[0] = 0x0, buf[1] = 0x0, buf[2] = 0xbeef, buf[3] = 0xbeef, buf[4] = 0xbeef
> ret = 2, buf[0] = 0x0, buf[1] = 0x7a, buf[2] = 0x0, buf[3] = 0xbeef, buf[4] = 0xbeef
>
> Actual output:
> ret = 1, buf[0] = 0x0, buf[1] = 0x0, buf[2] = 0x0, buf[3] = 0x0, buf[4] = 0xbeef
> ret = 2, buf[0] = 0x0, buf[1] = 0x0, buf[2] = 0x0, buf[3] = 0x0, buf[4] = 0xbeef
Thanks. This one is an easy one-line fix. Caught by the bad mbtowc
behavior of returning 0 rather than 1 when converting a nul character
(thereby not advancing past it in the buffer).
Rich
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