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

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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.
This commit is contained in:
Joseph Myers 2025-01-28 22:35:21 +00:00
parent 94251ae99e
commit 0dcc0b2f63
3 changed files with 68 additions and 1 deletions
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9
libio/fileops.c
View File

@ -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;

1
stdio-common/Makefile
View File

@ -262,6 +262,7 @@ tests := \
tst-freopen64-7 \
tst-freopen7 \
tst-fseek \
tst-fseek-mmap \
tst-fwrite \
tst-fwrite-memstrm \
tst-fwrite-overflow \

59
stdio-common/tst-fseek-mmap.c Normal file
View File

@ -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>