Fix fseek handling for mmap files after ungetc or fflush (bug 32529)

As discussed in bug 32529, fseek fails on files opened for reading
using mmap after ungetc.  The implementation of fseek for such files
has an offset computation that's also incorrect after fflush.  A
combined fix addresses both problems (with tests for both included as
well) and it seems reasonable to consider them a single bug.

Tested for x86_64.

2 files changed, 60 insertions, 0 deletions

diff --git a/stdio-common/Makefile b/stdio-common/Makefile
index 26c1b88..f80b9ec 100644
--- a/stdio-common/Makefile
+++ b/stdio-common/Makefile
@@ -262,6 +262,7 @@ tests := \
   tst-freopen64-7 \
   tst-freopen7 \
   tst-fseek \
+  tst-fseek-mmap \
   tst-fwrite \
   tst-fwrite-memstrm \
   tst-fwrite-overflow \
diff --git a/stdio-common/tst-fseek-mmap.c b/stdio-common/tst-fseek-mmap.c
new file mode 100644
index 0000000..86fa99a
--- /dev/null
+++ b/stdio-common/tst-fseek-mmap.c
@@ -0,0 +1,59 @@
+/* Test fseek on files using mmap (bug 32529).
+   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-fseek-mmap", &filename);
+  TEST_VERIFY_EXIT (fd != -1);
+  xclose (fd);
+
+  /* Test fseek after ungetc (bug 32529).  */
+  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 (fseek (fp, 0, SEEK_CUR), 0);
+  xfclose (fp);
+
+  /* Test fseek positioning after fflush (another issue covered by the
+     same fix).  */
+  fp = xfopen (filename, "rm");
+  TEST_COMPARE (fgetc (fp), 't');
+  TEST_COMPARE (fflush (fp), 0);
+  TEST_COMPARE (ftell (fp), 1);
+  TEST_COMPARE (fseek (fp, 0, SEEK_CUR), 0);
+  TEST_COMPARE (ftell (fp), 1);
+  TEST_COMPARE (fgetc (fp), 'e');
+  xfclose (fp);
+
+  return 0;
+}
+
+#include <support/test-driver.c>