linux-kernel - Re: Have RESOLVE_* flags superseded AT

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Date:   Mon, 02 Mar 2020 12:30:47 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     David Howells <dhowells@...hat.com>, linux-api@...r.kernel.org,
        viro@...iv.linux.org.uk, metze@...ba.org,
        torvalds@...ux-foundation.org, cyphar@...har.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Have RESOLVE_* flags superseded AT_* flags for new syscalls?

* Christian Brauner:

> [Cc Florian since that ends up on libc's table sooner or later...]

I'm not sure what you are after here …

> 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
help code that has to re-create an equivalent descriptor.  The operation
flags are largely irrelevant to that if you can get the descriptor by
other means.

>>  (*) It has been suggested that AT_SYMLINK_NOFOLLOW should be the default, but
>>      only RESOLVE_NO_SYMLINKS exists.
>
> I'd be very much in favor of not following symlinks being the default.
> That's usually a source of a lot of security issues.

But that's inconsistent with the rest of the system.  And for example,
if you make /etc/resolv.conf a symbolic link, a program which uses a new
I/O library (with the new interfaces) will not be able to read it.

AT_SYMLINK_NOFOLLOW only applies to the last pathname component anyway,
so it's relatively little protection.

Thanks,
Florian