linux-kernel - Re: Can ovl_drop_write() be called earlier in ovl_dentry

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Message-ID: <CAJfpegsuVxOAGenfxeSsQEKZR+_+OfR6uOxm2USGvbAuCWRhvg@mail.gmail.com>
Date:	Mon, 1 Jun 2015 16:22:40 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	David Howells <dhowells@...hat.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	"linux-unionfs@...r.kernel.org" <linux-unionfs@...r.kernel.org>
Subject: Re: Can ovl_drop_write() be called earlier in ovl_dentry_open()

On Mon, Jun 1, 2015 at 3:52 PM, David Howells <dhowells@...hat.com> wrote:
> In ovl_dentry_open(), ovl_drop_write() is called after vfs_open() - but is
> this actually necessary?  Can't we just drop it post-copyup?  After all,
> that's all we wanted the write lock for, right?

Hmm,  that could result in a race where remount r/o of upper fs comes
in between copy-up and vfs_open() so copy-up succeeds but the actual
open fails.  It's harmless, though, and not  very likely.  So I guess
your patch is OK.

Thanks,
Miklos




>
> David
> ---
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -356,16 +356,14 @@ static int ovl_dentry_open(struct dentry *dentry, struct inode *inode,
>                         err = ovl_copy_up_last(dentry, NULL, true);
>                 else
>                         err = ovl_copy_up(dentry);
> +               ovl_drop_write(dentry);
>                 if (err)
> -                       goto out_drop_write;
> +                       goto out;
>
>                 ovl_path_upper(dentry, &realpath);
>         }
>
>         err = vfs_open(&realpath, d_backing_inode(realpath.dentry), file, cred);
> -out_drop_write:
> -       if (want_write)
> -               ovl_drop_write(dentry);
>  out:
>         return err;
>  }
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