Re: Have RESOLVE_* flags superseded AT_* flags for new syscalls?

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2020-03-02, Christian Brauner <christian.brauner@ubuntu.com> wrote:
> On Mon, Mar 02, 2020 at 01:05:04PM +0100, Christian Brauner wrote:
> > On Mon, Mar 02, 2020 at 12:52:39PM +0100, Christian Brauner wrote:
> > > On Mon, Mar 02, 2020 at 12:30:47PM +0100, Florian Weimer wrote:
> > > > * Christian Brauner:
> > > > 
> > > > > [Cc Florian since that ends up on libc's table sooner or later...]
> > > > 
> > > > I'm not sure what you are after here …
> > > 
> > > Exactly what you've commented below. Input on whether any of these
> > > changes would be either problematic if you e.g. were to implement
> > > openat() on top of openat2() in the future or if it would be problematic
> > > if we e.g. were to really deprecate AT_* flags for new syscalls.
> > > 
> > > > 
> > > > > On Fri, Feb 28, 2020 at 02:53:32PM +0000, David Howells wrote:
> > > > >> 	
> > > > >> I've been told that RESOLVE_* flags, which can be found in linux/openat2.h,
> > > > >> should be used instead of the equivalent AT_* flags for new system calls.  Is
> > > > >> this the case?
> > > > >
> > > > > Imho, it would make sense to use RESOLVE_* flags for new system calls
> > > > > and afair this was the original intention.
> > > > > The alternative is that RESOLVE_* flags are special to openat2(). But
> > > > > that seems strange, imho. The semantics openat2() has might be very
> > > > > useful for new system calls as well which might also want to support
> > > > > parts of AT_* flags (see fsinfo()). So we either end up adding new AT_*
> > > > > flags mirroring the new RESOLVE_* flags or we end up adding new
> > > > > RESOLVE_* flags mirroring parts of AT_* flags. And if that's a
> > > > > possibility I vote for RESOLVE_* flags going forward. The have better
> > > > > naming too imho.
> > > > >
> > > > > An argument against this could be that we might end up causing more
> > > > > confusion for userspace due to yet another set of flags. But maybe this
> > > > > isn't an issue as long as we restrict RESOLVE_* flags to new syscalls.
> > > > > When we introduce a new syscall userspace will have to add support for
> > > > > it anyway.
> > > > 
> > > > I missed the start of the dicussion and what this is about, sorry.
> > > > 
> > > > Regarding open flags, I think the key point for future APIs is to avoid
> > > > using the set of flags for both control of the operation itself
> > > > (O_NOFOLLOW/AT_SYMLINK_NOFOLLOW, O_NOCTTY) and properaties of the
> > > > resulting descriptor (O_RDWR, O_SYNC).  I expect that doing that would
> > 
> > Yeah, we have touched on that already and we have other APIs having
> > related problems. A clean way to avoid this problem is to require new
> > syscalls to either have two flag arguments, or - if appropriate -
> > suggest they make use of struct open_how that was implemented for
> > openat2().
> 
> By the way, if we really means business wrt to: separate resolution from
> fd-property falgs then shouldn't we either require O_NOFOLLOW for
> openat2() be specified in open_how->resolve or disallow O_NOFOLLOW for
> openat2() and introduce a new RESOLVE_* variant?

I think we agreed a while ago we aren't touching O_ flags for openat2()
because it would hamper adoption (this is the same reason we aren't
fixing the whole O_ACCMODE mess, and O_LARGEFILE, and the arch-specific
O_ flags, and O_TMPFILE, and __O_SYNC, and FASYNC/O_ASYNC, and
__FMODE_EXEC and __FMODE_NONOTIFY, and ...).

To be fair, we did fix O_PATH|O_TMPFILE and invalid mode combinations
but that's only because those were fairly broken.

But as I mentioned in a sister mail, I do agree that allowing O_NOFOLLOW
and RESOLVE_NO_TRAILING_SYMLINKS makes me feel a little uneasy. But
maybe it's totally fine and I'm worrying for no reason.

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

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