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Re: Fix modification of string literal by swprintf


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


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