[glibc] Avoid -Wstringop-overflow= warning in iconv module.

[Stefan Liebler <stli@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=876cdf517d1c464ce3d8f7eaf4199565e5592f16

commit 876cdf517d1c464ce3d8f7eaf4199565e5592f16
Author: Stefan Liebler <stli@linux.ibm.com>
Date:   Fri Jun 3 14:52:51 2022 +0200

    Avoid -Wstringop-overflow= warning in iconv module.
    
    On s390x when compiling with GCC 12, I get this warning:
    utf8-utf16-z9.c:
    ../iconv/loop.c: In function ‘__from_utf8_loop_etf3eh_single’:
    ../iconv/loop.c:445:22: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
      445 |     bytebuf[inlen++] = *inptr++;
          |     ~~~~~~~~~~~~~~~~~^~~~~~~~~~
    ../iconv/loop.c:381:17: note: at offset 4 into destination object ‘bytebuf’ of size 4
      381 |   unsigned char bytebuf[MAX_NEEDED_INPUT];
          |                 ^~~~~~~
    ../iconv/loop.c:445:22: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
      445 |     bytebuf[inlen++] = *inptr++;
          |     ~~~~~~~~~~~~~~~~~^~~~~~~~~~
    ../iconv/loop.c:381:17: note: at offset 5 into destination object ‘bytebuf’ of size 4
      381 |   unsigned char bytebuf[MAX_NEEDED_INPUT];
          |                 ^~~~~~~
    
    This patch tells the compiler that inend is always behind inptr which
    avoids the warning.  Note that the SINGLE function is only used to
    implement the mb*towc*() or wc*tomb*() functions.  Those functions use
    inptr and inend pointing to a variable on stack, compute the inend pointer
    or explicitly check the arguments which always leads to inptr < inend.
    
    Special notes for backporters (according to Siddhesh Poyarekar):
    If someone wants to backport this patch to release branches, they should
    also backport the following wcrtomb change. Otherwise the assumptions
    assumed by this patch are not true.
    
    commit 9bcd12d223a8990254b65e2dada54faa5d2742f3
    Author: Siddhesh Poyarekar <siddhesh@sourceware.org>
    Date:   Fri May 13 19:10:15 2022 +0530
    
        wcrtomb: Make behavior POSIX compliant
    
    Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

Diff:
---
 iconv/loop.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/iconv/loop.c b/iconv/loop.c
index f8727a637a..09ade3b765 100644
--- a/iconv/loop.c
+++ b/iconv/loop.c
@@ -435,11 +435,17 @@ SINGLE(LOOPFCT) (struct __gconv_step *step,
     return __GCONV_FULL_OUTPUT;
 
   /*  Now add characters from the normal input buffer.  */
-  if (inlen >= MAX_NEEDED_INPUT)
+  if (inlen >= MAX_NEEDED_INPUT || inptr >= inend)
     /* Avoid a -Wstringop-overflow= warning when this loop is
        unrolled.  The compiler cannot otherwise see that this is
        unreachable because it depends on (state->__count & 7) not
-       being too large after a previous conversion step.  */
+       being too large after a previous conversion step.
+       Starting with GCC 12, we also have mark the inptr >= inend
+       case as unreachable to omit the warning.  Note that this SINGLE
+       function is only used to implement the mb*towc*() or wc*tomb*()
+       functions.  Those functions use inptr and inend pointing to a
+       variable on stack, compute the inend pointer or explicitly check
+       the arguments which always leads to inptr < inend.  */
     __builtin_unreachable ();
   do
     bytebuf[inlen++] = *inptr++;