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Subject[RFC][PATCH 12/20] tricky: elevate write count files are open()ed
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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 needs to be kept elevated
all the way into the may_open() call. Otherwise, when the
write is dropped, a ro->rw transisition could occur. This
would lead to having rw access on the newly created file,
while the vfsmount is ro. That is bad.

Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
---

lxc-dave/fs/file_table.c | 5 ++++-
lxc-dave/fs/namei.c | 22 ++++++++++++++++++----
lxc-dave/ipc/mqueue.c | 3 +++
3 files changed, 25 insertions(+), 5 deletions(-)

diff -puN fs/file_table.c~C-elevate-writers-opens-part1 fs/file_table.c
--- lxc/fs/file_table.c~C-elevate-writers-opens-part1 2006-06-16 15:58:06.000000000 -0700
+++ lxc-dave/fs/file_table.c 2006-06-16 15:58:06.000000000 -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 fs/namei.c~C-elevate-writers-opens-part1 fs/namei.c
--- lxc/fs/namei.c~C-elevate-writers-opens-part1 2006-06-16 15:58:06.000000000 -0700
+++ lxc-dave/fs/namei.c 2006-06-16 15:58:06.000000000 -0700
@@ -1512,8 +1512,17 @@ 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.
*/
@@ -1652,14 +1661,17 @@ do_last:
}

if (IS_ERR(nd->intent.open.file)) {
- mutex_unlock(&dir->d_inode->i_mutex);
error = PTR_ERR(nd->intent.open.file);
- goto exit_dput;
+ goto exit_mutex_unlock;
}

/* Negative dentry, just create the file */
if (!path.dentry->d_inode) {
+ error = mnt_want_write(nd->mnt);
+ if (error)
+ goto exit_mutex_unlock;
error = open_namei_create(nd, &path, flag, mode);
+ mnt_drop_write(nd->mnt);
if (error)
goto exit;
return 0;
@@ -1695,6 +1707,8 @@ ok:
goto exit;
return 0;

+exit_mutex_unlock:
+ mutex_unlock(&dir->d_inode->i_mutex);
exit_dput:
dput_path(&path, nd);
exit:
diff -puN ipc/mqueue.c~C-elevate-writers-opens-part1 ipc/mqueue.c
--- lxc/ipc/mqueue.c~C-elevate-writers-opens-part1 2006-06-16 15:58:06.000000000 -0700
+++ lxc-dave/ipc/mqueue.c 2006-06-16 15:58:06.000000000 -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);
}
diff -L fs/namei.c. -puN /dev/null /dev/null
_
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