On 2020-03-13, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Fri, Mar 13, 2020 at 08:59:01PM +1100, Aleksa Sarai wrote:
> > On 2020-03-12, Stefan Metzmacher <metze@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.

I agree that O_TMPFILE is a mess, and you're right that it wouldn't be a
good idea to fold it into open*(). But what is your opinion on a
hypothetical mkdirat2() which would let you get an fd to the directory
that was just created?

> We really don't need openat2() turning into another one.  Syscall table
> slots are not in a short supply, and the level of review one gets from
> "new syscall added" is higher than from "make fubar(2) recognize a new
> member in options->union_full_of_crap if it has RESOLVE_TO_WANK_WITH_RIGHT_HAND
> set in options->flags, affecting its behaviour in some odd ways".
> Which is a good thing, damnit.

You're quite right, and I don't intend openat2() to become another
ioctl-but-with-even-more-fun-semantics.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>