[glibc] wcrtomb: Make behavior POSIX compliant

[glibc] wcrtomb: Make behavior POSIX compliant

[Siddhesh Poyarekar]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=9bcd12d223a8990254b65e2dada54faa5d2742f3

commit 9bcd12d223a8990254b65e2dada54faa5d2742f3
Author: Siddhesh Poyarekar <siddhesh@sourceware.org>
Date:   Fri May 13 19:10:15 2022 +0530

    wcrtomb: Make behavior POSIX compliant
    
    The GNU implementation of wcrtomb assumes that there are at least
    MB_CUR_MAX bytes available in the destination buffer passed to wcrtomb
    as the first argument.  This is not compatible with the POSIX
    definition, which only requires enough space for the input wide
    character.
    
    This does not break much in practice because when users supply buffers
    smaller than MB_CUR_MAX (e.g. in ncurses), they compute and dynamically
    allocate the buffer, which results in enough spare space (thanks to
    usable_size in malloc and padding in alloca) that no actual buffer
    overflow occurs.  However when the code is built with _FORTIFY_SOURCE,
    it runs into the hard check against MB_CUR_MAX in __wcrtomb_chk and
    hence fails.  It wasn't evident until now since dynamic allocations
    would result in wcrtomb not being fortified but since _FORTIFY_SOURCE=3,
    that limitation is gone, resulting in such code failing.
    
    To fix this problem, introduce an internal buffer that is MB_LEN_MAX
    long and use that to perform the conversion and then copy the resultant
    bytes into the destination buffer.  Also move the fortification check
    into the main implementation, which checks the result after conversion
    and aborts if the resultant byte count is greater than the destination
    buffer size.
    
    One complication is that applications that assume the MB_CUR_MAX
    limitation to be gone may not be able to run safely on older glibcs if
    they use static destination buffers smaller than MB_CUR_MAX; dynamic
    allocations will always have enough spare space that no actual overruns
    will occur.  One alternative to fixing this is to bump symbol version to
    prevent them from running on older glibcs but that seems too strict a
    constraint.  Instead, since these users will only have made this
    decision on reading the manual, I have put a note in the manual warning
    them about the pitfalls of having static buffers smaller than
    MB_CUR_MAX and running them on older glibc.
    
    Benchmarking:
    
    The wcrtomb microbenchmark shows significant increases in maximum
    execution time for all locales, ranging from 10x for ar_SA.UTF-8 to
    1.5x-2x for nearly everything else.  The mean execution time however saw
    practically no impact, with some results even being quicker, indicating
    that cache locality has a much bigger role in the overhead.
    
    Given that the additional copy uses a temporary buffer inside wcrtomb,
    it's likely that a hot path will end up putting that buffer (which is
    responsible for the additional overhead) in a similar place on stack,
    giving the necessary cache locality to negate the overhead.  However in
    situations where wcrtomb ends up getting called at wildly different
    spots on the call stack (or is on different call stacks, e.g. with
    threads or different execution contexts) and is still a hotspot, the
    performance lag will be visible.
    
    Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

Diff:
---
 debug/tst-fortify.c |  7 ++++++-
 debug/wcrtomb_chk.c |  8 ++------
 include/wchar.h     |  4 ++++
 manual/charset.texi | 11 ++++++-----
 wcsmbs/wcrtomb.c    | 31 +++++++++++++++++++++++--------
 5 files changed, 41 insertions(+), 20 deletions(-)

diff --git a/debug/tst-fortify.c b/debug/tst-fortify.c
index 03c9867714..8e94643bf2 100644
--- a/debug/tst-fortify.c
+++ b/debug/tst-fortify.c
@@ -1478,10 +1478,15 @@ do_test (void)
 	 character which has a multibyte representation which does not
 	 fit.  */
       CHK_FAIL_START
-      char smallbuf[2];
+      char smallbuf[1];
       if (wcrtomb (smallbuf, L'\x100', &s) != 2)
 	FAIL ();
       CHK_FAIL_END
+
+      /* Same input with a large enough buffer and we're good.  */
+      char bigenoughbuf[2];
+      if (wcrtomb (bigenoughbuf, L'\x100', &s) != 2)
+	FAIL ();
 #endif
 
       wchar_t wenough[10];
diff --git a/debug/wcrtomb_chk.c b/debug/wcrtomb_chk.c
index 8b6d026560..28c3ea0d2d 100644
--- a/debug/wcrtomb_chk.c
+++ b/debug/wcrtomb_chk.c
@@ -1,4 +1,5 @@
 /* Copyright (C) 2005-2022 Free Software Foundation, Inc.
+   Copyright The GNU Toolchain Authors.
    This file is part of the GNU C Library.
 
    The GNU C Library is free software; you can redistribute it and/or
@@ -25,10 +26,5 @@
 size_t
 __wcrtomb_chk (char *s, wchar_t wchar, mbstate_t *ps, size_t buflen)
 {
-  /* We do not have to implement the full wctomb semantics since we
-     know that S cannot be NULL when we come here.  */
-  if (buflen < MB_CUR_MAX)
-    __chk_fail ();
-
-  return __wcrtomb (s, wchar, ps);
+  return __wcrtomb_internal (s, wchar, ps, buflen);
 }
diff --git a/include/wchar.h b/include/wchar.h
index 4267985625..db83297bca 100644
--- a/include/wchar.h
+++ b/include/wchar.h
@@ -172,6 +172,10 @@ libc_hidden_proto (__mbrtowc)
 libc_hidden_proto (__mbrlen)
 extern size_t __wcrtomb (char *__restrict __s, wchar_t __wc,
 			 __mbstate_t *__restrict __ps) attribute_hidden;
+extern size_t __wcrtomb_internal (char *__restrict __s, wchar_t __wc,
+				  __mbstate_t *__restrict __ps,
+				  size_t __s_size)
+     attribute_hidden;
 extern size_t __mbsrtowcs (wchar_t *__restrict __dst,
 			   const char **__restrict __src,
 			   size_t __len, __mbstate_t *__restrict __ps)
diff --git a/manual/charset.texi b/manual/charset.texi
index a9b5cb4a37..427db3bc80 100644
--- a/manual/charset.texi
+++ b/manual/charset.texi
@@ -883,11 +883,12 @@ the string @var{s}.  This includes all bytes representing shift
 sequences.
 
 One word about the interface of the function: there is no parameter
-specifying the length of the array @var{s}.  Instead the function
-assumes that there are at least @code{MB_CUR_MAX} bytes available since
-this is the maximum length of any byte sequence representing a single
-character.  So the caller has to make sure that there is enough space
-available, otherwise buffer overruns can occur.
+specifying the length of the array @var{s}, so the caller has to make sure
+that there is enough space available, otherwise buffer overruns can occur.
+This version of @theglibc{} does not assume that @var{s} is at least
+@var{MB_CUR_MAX} bytes long, but programs that need to run on @glibcadj{}
+versions that have this assumption documented in the manual must comply
+with this limit.
 
 @pindex wchar.h
 @code{wcrtomb} was introduced in @w{Amendment 1} to @w{ISO C90} and is
diff --git a/wcsmbs/wcrtomb.c b/wcsmbs/wcrtomb.c
index e17438989f..c0cce3792f 100644
--- a/wcsmbs/wcrtomb.c
+++ b/wcsmbs/wcrtomb.c
@@ -1,4 +1,5 @@
 /* Copyright (C) 1996-2022 Free Software Foundation, Inc.
+   Copyright The GNU Toolchain Authors.
    This file is part of the GNU C Library.
 
    The GNU C Library is free software; you can redistribute it and/or
@@ -20,6 +21,7 @@
 #include <errno.h>
 #include <gconv.h>
 #include <stdlib.h>
+#include <string.h>
 #include <wchar.h>
 #include <wcsmbsload.h>
 
@@ -34,7 +36,7 @@
 static mbstate_t state;
 
 size_t
-__wcrtomb (char *s, wchar_t wc, mbstate_t *ps)
+__wcrtomb_internal (char *s, wchar_t wc, mbstate_t *ps, size_t s_size)
 {
   char buf[MB_LEN_MAX];
   struct __gconv_step_data data;
@@ -52,14 +54,11 @@ __wcrtomb (char *s, wchar_t wc, mbstate_t *ps)
   /* A first special case is if S is NULL.  This means put PS in the
      initial state.  */
   if (s == NULL)
-    {
-      s = buf;
-      wc = L'\0';
-    }
+    wc = L'\0';
 
   /* Tell where we want to have the result.  */
-  data.__outbuf = (unsigned char *) s;
-  data.__outbufend = (unsigned char *) s + MB_CUR_MAX;
+  data.__outbuf = (unsigned char *) buf;
+  data.__outbufend = (unsigned char *) buf + sizeof buf;
 
   /* Get the conversion functions.  */
   fcts = get_gconv_fcts (_NL_CURRENT_DATA (LC_CTYPE));
@@ -101,7 +100,17 @@ __wcrtomb (char *s, wchar_t wc, mbstate_t *ps)
 
   if (status == __GCONV_OK || status == __GCONV_EMPTY_INPUT
       || status == __GCONV_FULL_OUTPUT)
-    result = data.__outbuf - (unsigned char *) s;
+    {
+      result = data.__outbuf - (unsigned char *) buf;
+
+      if (s != NULL)
+	{
+	  if (result > s_size)
+	    __chk_fail ();
+
+	  memcpy (s, buf, result);
+	}
+    }
   else
     {
       result = (size_t) -1;
@@ -110,5 +119,11 @@ __wcrtomb (char *s, wchar_t wc, mbstate_t *ps)
 
   return result;
 }
+
+size_t
+__wcrtomb (char *s, wchar_t wc, mbstate_t *ps)
+{
+  return __wcrtomb_internal (s, wc, ps, (size_t) -1);
+}
 weak_alias (__wcrtomb, wcrtomb)
 libc_hidden_weak (wcrtomb)