[PATCH 01/14] VFS: Add additional RESOLVE_* flags [ver #18]

[Jeremy Allison]

On Fri, Mar 13, 2020 at 06:28:44PM +0000, Al Viro wrote:
> On Fri, Mar 13, 2020 at 08:59:01PM +1100, Aleksa Sarai wrote:
> > On 2020-03-12, Stefan Metzmacher <metze at samba.org> wrote:
> > > Am 12.03.20 um 17:24 schrieb Linus Torvalds:
> > > > But yes, if we have a major package like samba use it, then by all
> > > > means let's add linkat2(). How many things are we talking about? We
> > > > have a number of system calls that do *not* take flags, but do do
> > > > pathname walking. I'm thinking things like "mkdirat()"?)
> > > 
> > > I haven't looked them up in detail yet.
> > > Jeremy can you provide a list?
> > > 
> > > Do you think we could route some of them like mkdirat() and mknodat()
> > > via openat2() instead of creating new syscalls?
> > 
> > I have heard some folks asking for a way to create a directory and get a
> > handle to it atomically -- so arguably this is something that could be
> > inside openat2()'s feature set (O_MKDIR?). But I'm not sure how popular
> > of an idea this is.
> 
> For fuck sake, *NO*!
> 
> We don't need any more multiplexors from hell.  mkdir() and open() have
> deeply different interpretation of pathnames (and anyone who asks for
> e.g. traversals of dangling symlinks on mkdir() is insane).  Don't try to
> mix those; even O_TMPFILE had been a mistake.
> 
> Folks, we'd paid very dearly for the atomic_open() merge.  We are _still_
> paying for it - and keep finding bugs induced by the convoluted horrors
> in that thing (see yesterday pull from vfs.git#fixes for the latest crop).
> I hope to get into more or less sane shape (part - this cycle, with
> followups in the next one), but the last thing we need is more complexity
> in the area.

Can we disentangle the laudable desire to keep kernel internals
simple (which I completely agree with :-) from the desire to
keep user-space interfaces simple ?

Having some way of doing a mkdir() that returns an open fd
on the new directory *is* a very useful thing for many applications,
but I really don't care how the kernel implements it. We have so much
Linux-specific code already that one more thing won't matter :-).