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

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Message-ID: <20200910164234.GA25140@infradead.org>
Date:   Thu, 10 Sep 2020 17:42:34 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Rich Felker <dalias@...c.org>
Cc:     Christoph Hellwig <hch@...radead.org>, linux-api@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfs: add fchmodat2 syscall

On Thu, Sep 10, 2020 at 12:39:50PM -0400, Rich Felker wrote:
> On Thu, Sep 10, 2020 at 05:20:59PM +0100, Christoph Hellwig wrote:
> > On Thu, Sep 10, 2020 at 10:23:37AM -0400, Rich Felker wrote:
> > > 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.
> > 
> > And that is the goddamn problem.  We need to fix that _first_.
> 
> Can you clarify exactly what that is? Do you mean fixing the
> underlying fs backends, or just ensuring that the chmod for symlinks
> doesn't reach them by putting the check in chmod_common? I'm ok with
> any of these.

Either - we need to make sure the user can't change the permission
bits.

> > After that we can add sugarcoating using new syscalls if needed.
> 
> The new syscall is _not_ about this problem. It's about the missing
> flags argument and inability to implement fchmodat() without access to
> procfs. The above problem is just something you encounter and have to
> make a decision about in order to fix the missing flags problem and
> make a working AT_SYMLINK_NOFOLLOW.

And I'm generally supportive of that.  But we need to fix the damn
bug first an then do nice to haves.