Re: [RFC][PATCH 2/7] get mount write in __dentry_open()

[Miklos Szeredi]

> 
> 
> The first patch fixes an actual bug.  I think the
> reset will reduce the chance for any future bugs
> to creep in.
> 
> --
> 
> The r/o bind mount patches require matching mnt_want_write()
> at filp creation time with a mnt_drop_write() at __fput().
> 
> We used to do this in may_open(), but Miklos pointed out
> that __dentry_open() is used as well to create filps.  We
> don't currently do mnt_want_write() for these.
> 
> If a filp on a writeable file is created this way, and
> destroyed via __fput() we'll get a mount count imbalance.
> 
> This patch moves the mount write count acquisition from
> may_open() into __dentry_open(), where we should catch
> many more of the users.
> 
> Signed-off-by: Dave Hansen <haveblue@xxxxxxxxxx>
> ---
> 
>  lxc-dave/fs/namei.c |   12 ------------
>  lxc-dave/fs/open.c  |   45 ++++++++++++++++++++++++++++++++++++++-------
>  2 files changed, 38 insertions(+), 19 deletions(-)
> 
> diff -puN fs/namei.c~get-write-in-__dentry_open fs/namei.c
> --- lxc/fs/namei.c~get-write-in-__dentry_open	2007-10-03 14:44:52.000000000 -0700
> +++ lxc-dave/fs/namei.c	2007-10-04 18:02:48.000000000 -0700
> @@ -1621,14 +1621,6 @@ int may_open(struct nameidata *nd, int a
>  			return -EACCES;
>  
>  		flag &= ~O_TRUNC;
> -	} else if (flag & FMODE_WRITE) {
> -		/*
> -		 * effectively: !special_file()
> -		 * balanced by __fput()
> -		 */
> -		error = mnt_want_write(nd->mnt);
> -		if (error)
> -			return error;
>  	}

Maybe readonly should still be checked here, so that the order of
error checking doesn't change.  If racing with a read-only remount the
order is irrelevant anyway.  Something like this?

	} else if (flag & FMODE_WRITE && __mnt_is_readonly(nd->mnt)) {
		return -EROFS
	}

>  	error = vfs_permission(nd, acc_mode);
> @@ -1778,11 +1770,7 @@ do_last:
>  
>  	/* 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);

This is still needed, isn't it?

And they should be added around do_truncate() as well, since you
remove the protection from may_open().

This one introduces an interesting race between ro-remount and
open(O_TRUNC), where the truncate can succeed but the open fail with
EROFS.  Is that a problem?

>  		if (error)
>  			goto exit;
>  		return 0;
> diff -puN fs/open.c~get-write-in-__dentry_open fs/open.c
> --- lxc/fs/open.c~get-write-in-__dentry_open	2007-10-03 14:44:52.000000000 -0700
> +++ lxc-dave/fs/open.c	2007-10-04 18:02:48.000000000 -0700
> @@ -766,22 +766,51 @@ out:
>  	return error;
>  }
>  
> +/*
> + * You have to be very careful that these write
> + * counts get cleaned up in error cases and
> + * upon __fput().  This should probably never
> + * be called outside of __dentry_open().
> + */
> +static inline int __get_file_write_access(struct inode *inode,
> +					  struct vfsmount *mnt)
> +{
> +	int error;
> +	error = get_write_access(inode);
> +	if (error)
> +		return error;
> +	/*
> +	 * Do not take mount writer counts on
> +	 * special files since no writes to
> +	 * the mount itself will occur.
> +	 */
> +	if (special_file(inode->i_mode))
> +		return 0;
> +
> +	/*
> +	 * Balanced in __fput()
> +	 */
> +	error = mnt_want_write(mnt);
> +	if (error)
> +		put_write_access(inode);
> +	return error;
> +}
> +
>  static struct file *__dentry_open(struct dentry *dentry, struct vfsmount *mnt,
>  					int flags, struct file *f,
>  					int (*open)(struct inode *, struct file *))
>  {
>  	struct inode *inode;
> -	int error;
> +	int error = 0;
>  
>  	f->f_flags = flags;
>  	f->f_mode = ((flags+1) & O_ACCMODE) | FMODE_LSEEK |
>  				FMODE_PREAD | FMODE_PWRITE;
>  	inode = dentry->d_inode;
> -	if (f->f_mode & FMODE_WRITE) {
> -		error = get_write_access(inode);
> -		if (error)
> -			goto cleanup_file;
> -	}
> +	if (f->f_mode & FMODE_WRITE)
> +		error = __get_file_write_access(inode, mnt);
> +	if (error)
> +		goto cleanup_file;
>  
>  	f->f_mapping = inode->i_mapping;
>  	f->f_path.dentry = dentry;
> @@ -820,8 +849,10 @@ static struct file *__dentry_open(struct
>  
>  cleanup_all:
>  	fops_put(f->f_op);
> -	if (f->f_mode & FMODE_WRITE)
> +	if (f->f_mode & FMODE_WRITE) {
>  		put_write_access(inode);
> +		mnt_drop_write(mnt);

Shouldn't this be conditional on !special_file()?

Miklos
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