Re: Fix modification of string literal by swprintf

[Thomas Preudhomme <thomas.preudhomme@foss.arm.com>]

Hi Corinna,

On 07/06/17 11:01, Corinna Vinschen wrote:
> On Jun  5 09:59, Thomas Preudhomme wrote:
>> Don't over-read memory returned by _DTOA_R, and never write to it
>> since the result might be a string literal.
>>
>> For example, when doing:
>>   swprintf(tt, 20, L"%.*f", 6, 0.0);
>>
>> we will get back "0".
>>
>> Instead, write the result returned by _DTOA_R to the output buffer.
>> After this, write the 0 chars directly to the the output buffer
>> (if there are any). This also has the (marginal) advantage that
>> we read/write less memory overall.
>>
>> The patch, contributed by Silviu Baranga was tested against libcxx testsuite
>> and showed no regression. Please find the patch in git format-patch format
>> in attachment.
>>
>> Best regards,
>>
>> Thomas
>
>> >From 7a31cfb01a0b089daf2bed93b742b6edbf4cba0c Mon Sep 17 00:00:00 2001
>> From: Silviu Baranga <silviu.baranga@arm.cm>
>> Date: Mon, 5 Jun 2017 09:54:42 +0100
>> Subject: [PATCH] Don't overread or write memory returned by _DTOA_R
>>
>> Don't over-read memory returned by _DTOA_R, and never write to it
>> since the result might be a string literal.
>>
>> For example, when doing:
>>   swprintf(tt, 20, L"%.*f", 6, 0.0);
>>
>> we will get back "0".
>>
>> Instead, write the result returned by _DTOA_R to the output buffer.
>> After this, write the 0 chars directly to the the output buffer
>> (if there are any). This also has the (marginal) advantage that
>> we read/write less memory overall.
>> ---
>>  newlib/libc/locale/setlocale.h |  2 +-
>>  newlib/libc/stdio/vfwprintf.c  | 24 ++++++++++++------------
>>  2 files changed, 13 insertions(+), 13 deletions(-)
>>
>> diff --git a/newlib/libc/locale/setlocale.h b/newlib/libc/locale/setlocale.h
>> index 85a38d5..1440d0e 100644
>> --- a/newlib/libc/locale/setlocale.h
>> +++ b/newlib/libc/locale/setlocale.h
>> @@ -227,7 +227,7 @@ __get_locale_r (struct _reent *r)
>>  _ELIDABLE_INLINE struct __locale_t *
>>  __get_current_locale (void)
>>  {
>> -  return _REENT->_locale ?: __get_global_locale ();
>> +  return _REENT->_locale ?: __get_global_locale (); // version for !_MB_CAPABLE?
>>  }
>
> This doesn't belong here.  Also, the code is already fine for !_MB_CAPABLE
> as well.

My bad, this was a note I made to myself when reading on MB_CAPABLE some time 
ago. This does not belong to the patch and is indeed wrong.

>
>>  /* Only access fixed "C" locale using this function.  Fake for !_MB_CAPABLE
>> diff --git a/newlib/libc/stdio/vfwprintf.c b/newlib/libc/stdio/vfwprintf.c
>> index f0179a0..1bec9b2 100644
>> --- a/newlib/libc/stdio/vfwprintf.c
>> +++ b/newlib/libc/stdio/vfwprintf.c
>> @@ -1627,13 +1627,20 @@ wcvt(struct _reent *data, _PRINTF_FLOAT_TYPE value, int ndigits, int flags,
>>
>>  	{
>>  	  char *digits, *bp, *rve;
>> -#ifndef _MB_CAPABLE
>>  	  int i;
>> -#endif
>>
>>  	  digits = _DTOA_R (data, value, mode, ndigits, decpt, &dsgn, &rve);
>>
>> +#ifdef _MB_CAPABLE
>> +	  _mbsnrtowcs_r (data, buf, (const char **) &digits, rve - digits,
>> +			 len, NULL);
>> +#else
>> +	  for (i = 0; i < rve - digits && i < len; ++i)
>> +	    buf[i] = (wchar_t) digits[i];
>> +#endif
>> +
>>  	  if ((ch != L'g' && ch != L'G') || flags & ALT) {	/* Print trailing zeros */
>> +		char *padding = rve;
>>  		bp = digits + ndigits;
>>  		if (ch == L'f' || ch == L'F') {
>>  			if (*digits == L'0' && value)
>> @@ -1642,18 +1649,11 @@ wcvt(struct _reent *data, _PRINTF_FLOAT_TYPE value, int ndigits, int flags,
>>  		}
>>  		if (value == 0)	/* kludge for __dtoa irregularity */
>>  			rve = bp;
>> -		while (rve < bp)
>> -			*rve++ = '0';
>> -	  }
>>
>> +		for (i = padding - digits; i < rve - digits && i < len; ++i)
>> +			buf[i] = L'0';
>
> Appending zeros here without incrementing rve...
>
>> +	  }
>>  	  *length = rve - digits; /* full length of the string */
>
> ...leads to incorrect setting of *length here.  Or am I missing
> something?

I'll transmit to Silviu for him to answer.

Thanks for the comments.

Best regards,

Thomas