Re: [RFC PATCH 1/2] ovl: skip checking lower file's write permisson on truncate

From: Chengguang Xu <cgxu519@mykernel.net>
To: "Miklos Szeredi" <miklos@szeredi.hu>
Cc: "overlayfs" <linux-unionfs@vger.kernel.org>
Subject: Re: [RFC PATCH 1/2] ovl: skip checking lower file's write permisson on truncate
Date: Wed, 21 Jul 2021 00:01:00 +0800	[thread overview]
Message-ID: <17ac4a6258a.1113fdbb760287.4207934850282026708@mykernel.net> (raw)
In-Reply-To: <CAJfpegvmBggw3bgumMwDF_V_dgn=gvC+a+8oCgYfZ+Qu55U=vw@mail.gmail.com>

 ---- 在 星期二, 2021-07-20 23:19:16 Miklos Szeredi <miklos@szeredi.hu> 撰写 ----
 > On Tue, 20 Jul 2021 at 16:35, Miklos Szeredi <miklos@szeredi.hu> wrote:
 > >
 > > On Sat, 24 Apr 2021 at 16:04, Chengguang Xu <cgxu519@mykernel.net> wrote:
 > > >
 > > > Lower files may be shared in overlayfs so strictly checking write
 > > > perssmion on lower file will cause interferes between different
 > > > overlayfs instances.
 > >
 > > How so?
 > >
 > > i_writecount on lower inode is not modified by overlayfs (at least not
 > > in this codepath).  Which means that there should be no interference
 > > between overlayfs instances sharing a lower directory tree.
 > 
 > I'm beginning to see what you are worrying about.
 > 
 > So on one instance a file on lower gets executed and on another
 > instance sharing the lower layer the file is truncated.  The truncate
 > is currently denied due to the negative i_writecount on the lower
 > file.  Also behavior is inconsistent between open(path, O_TRUNC) and
 > truncate(path) even though the two should be equivalent.

Yeah, that's it.
Thanks for applying the patch and supplementary description.

Thanks,
Chengguang