From: Omar Sandoval <osandov@osandov.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Trond Myklebust <trondmy@hammerspace.com>,
"amir73il@gmail.com" <amir73il@gmail.com>,
"dhowells@redhat.com" <dhowells@redhat.com>,
"lsf-pc@lists.linux-foundation.org"
<lsf-pc@lists.linux-foundation.org>, "hch@lst.de" <hch@lst.de>,
"miklos@szeredi.hu" <miklos@szeredi.hu>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>
Subject: Re: [LSF/MM/BPF TOPIC] Allowing linkat() to replace the destination
Date: Fri, 17 Jan 2020 08:36:16 -0800 [thread overview]
Message-ID: <20200117163616.GA282555@vader> (raw)
In-Reply-To: <20200117154657.GK8904@ZenIV.linux.org.uk>
On Fri, Jan 17, 2020 at 03:46:57PM +0000, Al Viro wrote:
> On Fri, Jan 17, 2020 at 02:33:01PM +0000, Trond Myklebust wrote:
> > On Fri, 2020-01-17 at 12:49 +0000, David Howells wrote:
> > > It may be worth a discussion of whether linkat() could be given a
> > > flag to
> > > allow the destination to be replaced or if a new syscall should be
> > > made for
> > > this - or whether it should be disallowed entirely.
> > >
> > > A set of patches has been posted by Omar Sandoval that makes this
> > > possible:
> > >
> > >
> > > https://lore.kernel.org/linux-fsdevel/cover.1524549513.git.osandov@fb.com/
> > >
> > > though it only includes filesystem support for btrfs.
> > >
> > > This could be useful for cachefiles:
> > >
> > >
> > > https://lore.kernel.org/linux-fsdevel/3326.1579019665@warthog.procyon.org.uk/
> > >
> > > and overlayfs.
> > >
> >
> > That seems to me like a "just go ahead and do it if you can justify it"
> > kind of thing. It has plenty of precedent, and fits easily into the
> > existing syscall, so why do we need a face-to-face discussion?
>
> Unfortunately, it does *not* fit easily. And IMO that's linux-abi fodder more
> than anything else. The problem is in coming up with sane semantics - there's
> a plenty of corner cases with that one. What to do when destination is
> a dangling symlink, for example? Or has something mounted on it (no, saying
> "we'll just reject directories" is not enough). What should happen when
> destination is already a hardlink to the same object?
>
> It's less of a horror than rename() would've been, but that's not saying
> much.
The semantics I implemented in my series were basically "linkat with
AT_REPLACE replaces the target iff rename would replace the target".
Therefore, symlinks are replaced, not followed, and mountpoints get
EXDEV. In my opinion that's both sane and unsurprising.
next prev parent reply other threads:[~2020-01-17 16:36 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-17 12:49 [LSF/MM/BPF TOPIC] Allowing linkat() to replace the destination David Howells
2020-01-17 14:33 ` Trond Myklebust
2020-01-17 15:46 ` Al Viro
2020-01-17 16:12 ` Trond Myklebust
2020-01-17 16:48 ` Al Viro
2020-01-17 16:36 ` Omar Sandoval [this message]
2020-01-17 16:59 ` Al Viro
2020-01-17 17:28 ` Omar Sandoval
2020-01-17 18:17 ` Al Viro
2020-01-17 20:22 ` Omar Sandoval
2020-01-17 22:22 ` Al Viro
2020-01-17 23:54 ` Omar Sandoval
2020-01-18 0:47 ` Al Viro
2020-01-18 1:17 ` Omar Sandoval
2020-01-18 2:20 ` Al Viro
2020-01-21 23:05 ` Omar Sandoval
2020-01-22 6:57 ` Amir Goldstein
2020-01-22 22:10 ` Omar Sandoval
2020-01-23 3:47 ` Al Viro
2020-01-23 7:16 ` Dave Chinner
2020-01-23 7:47 ` Amir Goldstein
2020-01-24 21:25 ` Dave Chinner
2020-01-31 5:24 ` Darrick J. Wong
2020-01-31 5:29 ` hch
2020-01-31 7:00 ` Amir Goldstein
2020-01-31 20:33 ` Omar Sandoval
2020-01-31 21:55 ` Amir Goldstein
2020-01-28 1:27 ` Omar Sandoval
2020-01-28 14:35 ` David Howells
2020-01-31 5:31 ` hch
2020-01-31 8:04 ` David Howells
2020-01-31 8:56 ` Amir Goldstein
2020-01-22 9:53 ` David Howells
2020-01-17 14:47 ` David Howells
2020-01-17 14:56 ` Trond Myklebust
2020-01-17 16:01 ` Al Viro
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