From: Miklos Szeredi <email@example.com>
To: Chengguang Xu <firstname.lastname@example.org>
Cc: overlayfs <email@example.com>
Subject: Re: [RFC PATCH 1/2] ovl: skip checking lower file's write permisson on truncate
Date: Tue, 20 Jul 2021 17:19:16 +0200 [thread overview]
Message-ID: <CAJfpegvmBggw3bgumMwDF_V_dgn=gvC+a+8oCgYfZ+Qu55Ufirstname.lastname@example.org> (raw)
On Tue, 20 Jul 2021 at 16:35, Miklos Szeredi <email@example.com> wrote:
> On Sat, 24 Apr 2021 at 16:04, Chengguang Xu <firstname.lastname@example.org> 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.
Applied with the following description:
It is possible that a directory tree is shared between multiple overlay
instances as a lower layer. In this case when one instance executes a file
residing on the lower layer, the other instance denies a truncate(2) call
on this file.
This only happens for truncate(2) and not for open(2) with the O_TRUNC
Fix this interference and inconsistency by removing the preliminary
i_writecount check before copy-up.
This means that unlike on normal filesystems truncate(argv) will now
succeed. If this ever causes a regression in a real world use case this
needs to be revisited.
One way to fix this properly would be to keep a correct i_writecount in the
overlay inode, but that is difficult due to memory mapping code only
dealing with the real file/inode.
next prev parent reply other threads:[~2021-07-20 15:46 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-24 14:03 [RFC PATCH 1/2] ovl: skip checking lower file's write permisson on truncate Chengguang Xu
2021-04-24 14:03 ` [RFC PATCH 2/2] ovl: enhance write permission check for writable open Chengguang Xu
2021-07-21 13:14 ` Miklos Szeredi
2021-04-28 12:18 ` 回复:[RFC PATCH 1/2] ovl: skip checking lower file's write permisson on truncate Chengguang Xu
2021-07-20 14:35 ` [RFC " Miklos Szeredi
2021-07-20 15:19 ` Miklos Szeredi [this message]
2021-07-20 16:01 ` Chengguang Xu
2021-07-20 16:01 ` Miklos Szeredi
2021-07-20 16:04 ` Chengguang Xu
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