[PATCH 5/6] Fix fflush handling for mmap files after ungetc (bug 32535)

[Joseph Myers]

As discussed in bug 32535, fflush fails on files opened for reading
using mmap after ungetc.  Fix the logic to handle this case and still
compute the file offset correctly.

Tested for x86_64.
---
 libio/fileops.c                | 12 ++++++---
 stdio-common/Makefile          |  1 +
 stdio-common/tst-fflush-mmap.c | 48 ++++++++++++++++++++++++++++++++++
 3 files changed, 57 insertions(+), 4 deletions(-)
 create mode 100644 stdio-common/tst-fflush-mmap.c

diff --git a/libio/fileops.c b/libio/fileops.c
index 97875d1eaf..7e370d12b2 100644
--- a/libio/fileops.c
+++ b/libio/fileops.c
@@ -858,17 +858,21 @@ libc_hidden_ver (_IO_new_file_sync, _IO_file_sync)
 int
 _IO_file_sync_mmap (FILE *fp)
 {
+  off64_t o = fp->_offset - (fp->_IO_read_end - fp->_IO_read_ptr);
   if (fp->_IO_read_ptr != fp->_IO_read_end)
     {
-      if (__lseek64 (fp->_fileno, fp->_IO_read_ptr - fp->_IO_buf_base,
-		     SEEK_SET)
-	  != fp->_IO_read_ptr - fp->_IO_buf_base)
+      if (_IO_in_backup (fp))
+	{
+	  _IO_switch_to_main_get_area (fp);
+	  o -= fp->_IO_read_end - fp->_IO_read_base;
+	}
+      if (__lseek64 (fp->_fileno, o, SEEK_SET) != o)
 	{
 	  fp->_flags |= _IO_ERR_SEEN;
 	  return EOF;
 	}
     }
-  fp->_offset = fp->_IO_read_ptr - fp->_IO_buf_base;
+  fp->_offset = o;
   fp->_IO_read_end = fp->_IO_read_ptr = fp->_IO_read_base;
   return 0;
 }
diff --git a/stdio-common/Makefile b/stdio-common/Makefile
index 06c9eaf426..703e8af7e8 100644
--- a/stdio-common/Makefile
+++ b/stdio-common/Makefile
@@ -239,6 +239,7 @@ tests := \
   tst-fdopen2 \
   tst-ferror \
   tst-fflush-all-input \
+  tst-fflush-mmap \
   tst-fgets \
   tst-fgets2 \
   tst-fileno \
diff --git a/stdio-common/tst-fflush-mmap.c b/stdio-common/tst-fflush-mmap.c
new file mode 100644
index 0000000000..fd3671f198
--- /dev/null
+++ b/stdio-common/tst-fflush-mmap.c
@@ -0,0 +1,48 @@
+/* Test fflush after ungetc on files using mmap (bug 32535).
+   Copyright (C) 2025 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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+
+#include <support/check.h>
+#include <support/temp_file.h>
+#include <support/xstdio.h>
+#include <support/xunistd.h>
+
+int
+do_test (void)
+{
+  char *filename = NULL;
+  int fd = create_temp_file ("tst-fflush-mmap", &filename);
+  TEST_VERIFY_EXIT (fd != -1);
+  xclose (fd);
+
+  /* Test fflush after ungetc (bug 32535).  */
+  FILE *fp = xfopen (filename, "w");
+  TEST_VERIFY (0 <= fputs ("test", fp));
+  xfclose (fp);
+
+  fp = xfopen (filename, "rm");
+  TEST_COMPARE (fgetc (fp), 't');
+  TEST_COMPARE (ungetc ('u', fp), 'u');
+  TEST_COMPARE (fflush (fp), 0);
+  xfclose (fp);
+
+  return 0;
+}
+
+#include <support/test-driver.c>
-- 
2.43.0


-- 
Joseph S. Myers
josmyers@redhat.com