linux-kernel - Re: Have RESOLVE_* flags superseded AT

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Date:   Sun, 1 Mar 2020 02:54:11 +1100
From:   Aleksa Sarai <cyphar@...har.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, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, fweimer@...hat.com
Subject: Re: Have RESOLVE_* flags superseded AT_* flags for new syscalls?

On 2020-03-01, Aleksa Sarai <cyphar@...har.com> wrote:
> On 2020-02-28, Christian Brauner <christian.brauner@...ntu.com> wrote:
> > 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.
> 
> I can see the argument for merging AT_ flags into RESOLVE_ flags (fewer
> flag arguments for syscalls is usually a good thing) ... but I don't
> really like it. There are a couple of problems right off the bat:
> 
>  * The prefix RESOLVE_ implies that the flag is specifically about path
>    resolution. While you could argue that AT_EMPTY_PATH is at least
>    *related* to path resolution, flags like AT_REMOVEDIR and
>    AT_RECURSIVE aren't.
> 
>  * That point touches on something I see as a more fundamental problem
>    in the AT_ flags -- they were intended to be generic flags for all of
>    the ...at(2) syscalls. But then AT_ grew things like AT_STATX_ and
>    AT_REMOVEDIR (both of which are necessary features to have for their
>    respective syscalls, but now those flag bits are dead for other
>    syscalls -- not to mention the whole AT_SYMLINK_{NO,}FOLLOW thing).
> 
>  * While the above might be seen as minor quibbles, the really big
>    issue is that even the flags which are "similar" (AT_SYMLINK_NOFOLLOW
>    and RESOLVE_NO_SYMLINKS) have different semantics (by design -- in my
>    view, AT_SYMLINK_{NO,}FOLLOW / O_NOFOLLOW / lstat(2) has always had
>    the wrong semantics if the intention was to be a way to safely avoid
>    resolving symlinks).
> 
> But maybe I'm just overthinking what a merge of AT_ and RESOLVE_ would
> look like -- would it on.

Eugh, dropped the rest of that sentence:

... would it only be the few AT_ flags which are strictly related to
path resolution (such as AT_EMPTY_PATH)? If so wouldn't that just mean
we end up with two flag arguments for new syscalls?

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

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