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@xxxxxx/ > > > > > > though it only includes filesystem support for btrfs. > > > > > > This could be useful for cachefiles: > > > > > > > > > https://lore.kernel.org/linux-fsdevel/3326.1579019665@xxxxxxxxxxxxxxxxxxxxxx/ > > > > > > 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.
