linux-kernel - Re: [PATCH] vfs: add faccessat2 syscall

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Message-ID: <20200419095651.djee6mgneyf3qi3v@yavin.dot.cyphar.com>
Date:   Sun, 19 Apr 2020 19:56:51 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Stefan Metzmacher <metze@...ba.org>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        Al Viro <viro@...IV.linux.org.uk>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, Eric Sandeen <sandeen@...deen.net>
Subject: Re: [PATCH] vfs: add faccessat2 syscall

On 2020-04-18, Stefan Metzmacher <metze@...ba.org> wrote:
> > POSIX defines faccessat() as having a fourth "flags" argument, while the
> > linux syscall doesn't have it.  Glibc tries to emulate AT_EACCESS and
> > AT_SYMLINK_NOFOLLOW, but AT_EACCESS emulation is broken.
> > 
> > Add a new faccessat(2) syscall with the added flags argument and implement
> > both flags.
> > 
> > The value of AT_EACCESS is defined in glibc headers to be the same as
> > AT_REMOVEDIR.  Use this value for the kernel interface as well, together
> > with the explanatory comment.
> 
> It would be nice if resolv_flags would also be passed in addition to the
> at flags.
> See:https://lore.kernel.org/linux-api/CAHk-=wiaL6zznNtCHKg6+MJuCqDxO=yVfms3qR9A0czjKuSSiA@mail.gmail.com/
> 
> We should avoid expecting yet another syscall in near future.

If faccessat2() supported AT_EMPTY_PATH (which I'd be in favour of),
there's no need to add resolve flags because you could open the file as
O_PATH with whatever resolve flags you want, and then check it with
faccessat2().

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

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