linux-kernel - Re: race between vfs_rename and do

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Message-ID: <CAJfpegv03YpTPiDnLwbaewQX_KZws5nutays+vso2BVJ1v1+TA@mail.gmail.com>
Date:   Wed, 16 Feb 2022 10:28:20 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Xavier Roche <xavier.roche@...olia.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: race between vfs_rename and do_linkat (mv and link)

On Tue, 15 Feb 2022 at 17:20, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Tue, Feb 15, 2022 at 04:17:11PM +0000, Matthew Wilcox wrote:
> > On Tue, Feb 15, 2022 at 04:06:06PM +0000, Al Viro wrote:
> > > On Tue, Feb 15, 2022 at 01:37:40PM +0000, Al Viro wrote:
> > > > On Tue, Feb 15, 2022 at 10:56:29AM +0100, Miklos Szeredi wrote:
> > > >
> > > > > Doing "lock_rename() + lookup last components" would fix this race.
> > >
> > > "Fucking ugly" is inadequate for the likely results of that approach.
> > > It's guaranteed to be a source of headache for pretty much ever after.

So this is a fairly special situation.  How about adding a new rwsem
(could possibly be global or per-fs)?

 - acquired for read in lock_rename() before inode locks
 - acquired for write in do_linkat before inode locks, but only on retry

Thanks,
Miklos