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.

diff --git a/libio/fileops.c b/libio/fileops.c
index e868d520b6f65b398658508c10ffcef36210618c..b00a5d24ab3adfe0e4508b8f32c0cf4d2c1e8ee1 100644 (file)
--- a/libio/fileops.c
+++ b/libio/fileops.c
@@ -1107,11 +1107,18 @@ _IO_file_seekoff_mmap (FILE *fp, off64_t offset, int dir, int mode)
   if (mode == 0)
     return fp->_offset - (fp->_IO_read_end - fp->_IO_read_ptr);
 
+  if (_IO_in_backup (fp))
+    {
+      if (dir == _IO_seek_cur)
+       offset += fp->_IO_read_ptr - fp->_IO_read_end;
+      _IO_switch_to_main_get_area (fp);
+    }
+
   switch (dir)
     {
     case _IO_seek_cur:
       /* Adjust for read-ahead (bytes is buffer). */
-      offset += fp->_IO_read_ptr - fp->_IO_read_base;
+      offset += fp->_offset - (fp->_IO_read_end - fp->_IO_read_ptr);
       break;
     case _IO_seek_set:
       break;

diff --git a/stdio-common/Makefile b/stdio-common/Makefile
index 26c1b881cc0cfca586f176f3394e67231623ed72..f80b9ec57b26c3a3a52cdb0485d4301a75876a10 100644 (file)
--- 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 (file)
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>