linux-kernel - Re: [PATCH v2 2/2] vfs: add fchmodat2 syscall

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Message-ID: <20200916061943.GC142621@kroah.com>
Date:   Wed, 16 Sep 2020 08:19:43 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Rich Felker <dalias@...c.org>
Cc:     linux-api@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] vfs: add fchmodat2 syscall

On Tue, Sep 15, 2020 at 08:23:38PM -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.
> 
> Signed-off-by: Rich Felker <dalias@...c.org>

No kselftest for this new system call, or man page?  How do we know this
actually works and what the expected outcome should be?

thanks,

greg k-h