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Date:   Wed, 24 Jun 2020 20:14:37 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Christoph Hellwig <hch@lst.de>, Al Viro <viro@zeniv.linux.org.uk>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 03/11] fs: add new read_uptr and write_uptr file
 operations
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On Wed, Jun 24, 2020 at 11:11:50AM -0700, Linus Torvalds wrote:
> What I mean was *not* something like uptr_t.
> 
> Just keep the existing "set_fs()". It's not harmful if it's only used
> occasionally. We should rename it once it's rare enough, though.
> 
> Then, make the following changes:
> 
>  - all the normal user access functions stop caring. They use
> TASK_SIZE_MAX and are done with it. They basically stop reacting to
> set_fs().
> 
>  - then, we can have a few *very* specific cases (like setsockopt,
> maybe some random read/write) that we teach to use the new set_fs()
> thing.
> 
> So in *those* cases, we'd basically just do "oh, ok, we are supposed
> to use a kernel pointer" based on the setfs value.
> 
> IOW, I mean tto do something much more gradual. No new interfaces, no
> new types, just a couple of (very clearly marked!) cases of the legacy
> set_fs() behavior.

So we'd need new user copy functions for just those cases, and make
sure everything below the potential get_fs-NG uses them.  But without
any kind of tape safety to easily validate all users below actually
use them?  I just don't see how that makes sense.

FYI, I think the only users where we really need it are setsockopt
and a s390-specific driver from my audits so far.  Everything else
shouldn't need anything like that.