Re: [LSF/MM/BPF TOPIC] Allowing linkat() to replace the destination

[Al Viro <viro@zeniv.linux.org.uk>]

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.