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

[Omar Sandoval <osandov@osandov.com>]

On Fri, Jan 17, 2020 at 04:59:04PM +0000, Al Viro wrote:
> On Fri, Jan 17, 2020 at 08:36:16AM -0800, Omar Sandoval wrote:
> 
> > 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.
> 
> Umm...  EXDEV in rename() comes when _parents_ are on different mounts.
> rename() over a mountpoint is EBUSY if it has mounts in caller's
> namespace, but it succeeds (and detaches all mounts on the victim
> in any namespaces) otherwise.
> 
> When are you returning EXDEV?

EXDEV was a thinko, the patch does what rename does:


+	if (is_local_mountpoint(new_dentry)) {
+		error = -EBUSY;
+		goto out;
+	}

...

+	if (target) {
+		dont_mount(new_dentry);
+		detach_mounts(new_dentry);
+	}

Anyways, my point is that the rename semantics cover 90% of AT_REPLACE.
Before I resend the patches, I'll write up the documentation and we can
see what other corner cases I missed.

> Incidentally, mounts _are_ traversed on
> the link source, so what should that variant do when /tmp/foo is
> a mountpoint and you feed it "/tmp/foo" both for source and target?

EBUSY and noop both seem reasonable in this case, so we pick one and
document it.