Re: Race-free unlinking of directory entries

[Christoph Hellwig]

On Mon, Apr 09, 2018 at 12:10:09PM +0200, Pali Rohár wrote:
> I do not think it is enough, specially in cases when hard links are in
> use. Example:
> 
> fd = open("/a")
> link("/a", "/b")
> link("/a", "/c")
> unlink("/a")
> 
> And now I want to call funlink for "fd". Which file should be unlinked?
> "/b" or "/c" or none or both?

'/a' as that is what fd refers to.

> 
> Another example:
> 
> fd = open("/a")
> link("/a", "/b")
> unlink("/a")
> 
> Calling funlink for fd should unlink "/b" or it should fail?

It should fail, as '/a' doesn't refer to name that is visible in the
namespace.

> And another example:
> 
> fd = open("/a")
> rename("/a", "/b")
> 
> What should funlink do for fd now?

remove the directory entry refering to '/b' as that is what fd refers
to.

> Note that there is difference which symlink is shown in /proc/<pid>/fd
> between last two examples -- even those two examples results in the same
> state (if there is no race). For last example <fd> points to "/b", for
> previous <fd> points to "/a (deleted)".

Yes, that is fully expected.