Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 10 Sep 2020 18:16:15 +0200
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Rich Felker <dalias@libc.org>
Cc:     linux-api@vger.kernel.org,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] vfs: add fchmodat2 syscall
Message-ID: <20200910161615.GA1180022@kroah.com>
References: <20200910142335.GG3265@brightrain.aerifal.cx>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200910142335.GG3265@brightrain.aerifal.cx>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Sep 10, 2020 at 10:23:37AM -0400, Rich Felker wrote:
> POSIX defines fchmodat as having a 4th argument, flags, that can be
> AT_SYMLINK_NOFOLLOW. Support for changing the access mode of symbolic
> links is optional (EOPNOTSUPP allowed if not supported), but this flag
> is important even on systems where symlinks do not have access modes,
> since it's the only way to safely change the mode of a file which
> might be asynchronously replaced with a symbolic link, without a race
> condition whereby the link target is changed.
> 
> It's possible to emulate AT_SYMLINK_NOFOLLOW in userspace, and both
> musl libc and glibc do this, by opening an O_PATH file descriptor and
> performing chmod on the corresponding magic symlink in /proc/self/fd.
> However, this requires procfs to be mounted and accessible.
> 
> It was determined (see glibc issue #14578 and commit a492b1e5ef) that,
> on some filesystems, performing chmod on the link itself produces a
> change in the inode's access mode, but returns an EOPNOTSUPP error.
> This is non-conforming and wrong. Rather than try to fix all the
> broken filesystem backends, block attempts to change the symlink
> access mode via fchmodat2 at the frontend layer. This matches the
> userspace emulation done in libc implementations. No change is made to
> the underlying chmod_common(), so it's still possible to attempt
> changes via procfs, if desired. If at some point all filesystems have
> been fixed, this could be relaxed to allow filesystems to make their
> own decision whether changing access mode of links is supported.

A new syscall just because we have broken filesystems seems really odd,
why not just fix the filesystems instead?

thanks,

greg k-h