iconv: Fix incorrect UCS4 inner loop bounds (BZ#26923)

Previously, in UCS4 conversion routines we limit the number of
characters we examine to the minimum of the number of characters in the
input and the number of characters in the output. This is not the
correct behavior when __GCONV_IGNORE_ERRORS is set, as we do not consume
an output character when we skip a code unit. Instead, track the input
and output pointers and terminate the loop when either reaches its
limit.

This resolves assertion failures when resetting the input buffer in a step of
iconv, which assumes that the input will be fully consumed given sufficient
output space.

iconv/Makefile

@@ -44,7 +44,7 @@ CFLAGS-linereader.c += -DNO_TRANSLITERATION
 CFLAGS-simple-hash.c += -I../locale
 
 tests	= tst-iconv1 tst-iconv2 tst-iconv3 tst-iconv4 tst-iconv5 tst-iconv6 \
-	  tst-iconv7 tst-iconv-mt tst-iconv-opt
+	  tst-iconv7 tst-iconv8 tst-iconv-mt tst-iconv-opt
 
 others		= iconv_prog iconvconfig
 install-others-programs	= $(inst_bindir)/iconv

iconv/gconv_simple.c

@@ -239,11 +239,9 @@ ucs4_internal_loop (struct __gconv_step *step,
   int flags = step_data->__flags;
   const unsigned char *inptr = *inptrp;
   unsigned char *outptr = *outptrp;
-  size_t n_convert = MIN (inend - inptr, outend - outptr) / 4;
   int result;
-  size_t cnt;
 
-  for (cnt = 0; cnt < n_convert; ++cnt, inptr += 4)
+  for (; inptr + 4 <= inend && outptr + 4 <= outend; inptr += 4)
     {
       uint32_t inval;
 
@@ -307,11 +305,9 @@ ucs4_internal_loop_unaligned (struct __gconv_step *step,
   int flags = step_data->__flags;
   const unsigned char *inptr = *inptrp;
   unsigned char *outptr = *outptrp;
-  size_t n_convert = MIN (inend - inptr, outend - outptr) / 4;
   int result;
-  size_t cnt;
 
-  for (cnt = 0; cnt < n_convert; ++cnt, inptr += 4)
+  for (; inptr + 4 <= inend && outptr + 4 <= outend; inptr += 4)
     {
       if (__glibc_unlikely (inptr[0] > 0x80))
 	{
@@ -613,11 +609,9 @@ ucs4le_internal_loop (struct __gconv_step *step,
   int flags = step_data->__flags;
   const unsigned char *inptr = *inptrp;
   unsigned char *outptr = *outptrp;
-  size_t n_convert = MIN (inend - inptr, outend - outptr) / 4;
   int result;
-  size_t cnt;
 
-  for (cnt = 0; cnt < n_convert; ++cnt, inptr += 4)
+  for (; inptr + 4 <= inend && outptr + 4 <= outend; inptr += 4)
     {
       uint32_t inval;
 
@@ -684,11 +678,9 @@ ucs4le_internal_loop_unaligned (struct __gconv_step *step,
   int flags = step_data->__flags;
   const unsigned char *inptr = *inptrp;
   unsigned char *outptr = *outptrp;
-  size_t n_convert = MIN (inend - inptr, outend - outptr) / 4;
   int result;
-  size_t cnt;
 
-  for (cnt = 0; cnt < n_convert; ++cnt, inptr += 4)
+  for (; inptr + 4 <= inend && outptr + 4 <= outend; inptr += 4)
     {
       if (__glibc_unlikely (inptr[3] > 0x80))
 	{

iconv/tst-iconv8.c

@@ -0,0 +1,50 @@
+/* Test iconv behavior on UCS4 conversions with //IGNORE.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+/* Derived from BZ #26923 */
+#include <errno.h>
+#include <iconv.h>
+#include <stdio.h>
+#include <support/check.h>
+
+static int
+do_test (void)
+{
+  iconv_t cd = iconv_open ("UTF-8//IGNORE", "ISO-10646/UCS4/");
+  TEST_VERIFY_EXIT (cd != (iconv_t) -1);
+
+  /*
+   * Convert sequence beginning with an irreversible character into buffer that
+   * is too small.
+   */
+  char input[12] = "\xe1\x80\xa1" "AAAAAAAAA";
+  char *inptr = input;
+  size_t insize = sizeof (input);
+  char output[6];
+  char *outptr = output;
+  size_t outsize = sizeof (output);
+
+  TEST_VERIFY (iconv (cd, &inptr, &insize, &outptr, &outsize) == -1);
+  TEST_VERIFY (errno == E2BIG);
+
+  TEST_VERIFY_EXIT (iconv_close (cd) != -1);
+
+  return 0;
+}
+
+#include <support/test-driver.c>