This is the first really tricky patch in the series. It
elevates the writer count on a mount each time a
non-special file is opened for write.
This is not completely apparent in the patch because the
two if() conditions in may_open() above the
mnt_want_write() call are, combined, equivalent to
special_file().
There is also an elevated count around the vfs_create()
call in open_namei(). The count does not need to be
kept elevated all the way into the may_open() call because
after creation, the write bits of the acc_mode are cleared.
This keeps may_open() from ever failing. Howver, this may
open one potential race where a change from a r/w to a r/o
mount could occur between the mnt_drop_write() and may_open()
allowing a user to obtain a r/w file on what is now a r/w
mount. But, this functionality does not yet exist.
Signed-off-by: Dave Hansen <haveblue@xxxxxxxxxx>
---
lxc-dave/fs/file_table.c | 5 ++++-
lxc-dave/fs/namei.c | 29 ++++++++++++++++++++++++++---
lxc-dave/ipc/mqueue.c | 3 +++
3 files changed, 33 insertions(+), 4 deletions(-)
diff -puN fs/namei.c~elevate-writers-opens-part1 fs/namei.c
--- lxc/fs/namei.c~elevate-writers-opens-part1 2006-06-07 16:53:20.000000000 -0700
+++ lxc-dave/fs/namei.c 2006-06-07 16:53:20.000000000 -0700
@@ -1511,8 +1511,17 @@ int may_open(struct nameidata *nd, int a
return -EACCES;
flag &= ~O_TRUNC;
- } else if (IS_RDONLY(inode) && (flag & FMODE_WRITE))
- return -EROFS;
+ } else if (flag & FMODE_WRITE) {
+ /*
+ * effectively: !special_file()
+ * balanced by __fput()
+ */
+ error = mnt_want_write(nd->mnt);
+ if (error)
+ return error;
+ if (IS_RDONLY(inode))
+ return -EROFS;
+ }
/*
* An append-only file must be opened in append mode for writing.
*/
@@ -1642,10 +1651,24 @@ do_last:
if (!path.dentry->d_inode) {
if (!IS_POSIXACL(dir->d_inode))
mode &= ~current->fs->umask;
- error = vfs_create(dir->d_inode, path.dentry, mode, nd);
+ /*
+ * this serves dual roles, it makes sure there is no
+ * r/o mount, and keeps the write count for what is
+ * the newly created file
+ */
+ error = mnt_want_write(nd->mnt);
+ if (!error)
+ error = vfs_create(dir->d_inode, path.dentry, mode, nd);
mutex_unlock(&dir->d_inode->i_mutex);
dput(nd->dentry);
nd->dentry = path.dentry;
+ /*
+ * Unconditionally drop the write access because
+ * the acc_mode=0 set below will keep may_open()
+ * from ever failing if there was a r/o mount
+ * between here and there
+ */
+ mnt_drop_write(nd->mnt);
if (error)
goto exit;
/* Don't check for write permission, don't truncate */
diff -puN fs/open.c~elevate-writers-opens-part1 fs/open.c
diff -puN include/linux/mount.h~elevate-writers-opens-part1 include/linux/mount.h
diff -puN fs/file_table.c~elevate-writers-opens-part1 fs/file_table.c
--- lxc/fs/file_table.c~elevate-writers-opens-part1 2006-06-07 16:53:20.000000000 -0700
+++ lxc-dave/fs/file_table.c 2006-06-07 16:53:20.000000000 -0700
@@ -180,8 +180,11 @@ void fastcall __fput(struct file *file)
if (unlikely(inode->i_cdev != NULL))
cdev_put(inode->i_cdev);
fops_put(file->f_op);
- if (file->f_mode & FMODE_WRITE)
+ if (file->f_mode & FMODE_WRITE) {
put_write_access(inode);
+ if(!special_file(inode->i_mode))
+ mnt_drop_write(mnt);
+ }
file_kill(file);
file->f_dentry = NULL;
file->f_vfsmnt = NULL;
diff -puN ipc/mqueue.c~elevate-writers-opens-part1 ipc/mqueue.c
--- lxc/ipc/mqueue.c~elevate-writers-opens-part1 2006-06-07 16:53:20.000000000 -0700
+++ lxc-dave/ipc/mqueue.c 2006-06-07 16:53:20.000000000 -0700
@@ -679,6 +679,9 @@ asmlinkage long sys_mq_open(const char _
goto out;
filp = do_open(dentry, oflag);
} else {
+ error = mnt_want_write(mqueue_mnt);
+ if (error)
+ goto out;
filp = do_create(mqueue_mnt->mnt_root, dentry,
oflag, mode, u_attr);
}
_
-
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