linux-kernel - Re: [PATCH 03/11] fs: add new read_uptr and write

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Date:   Wed, 8 Jul 2020 05:14:55 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>
Cc:     David Laight <David.Laight@...lab.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 03/11] fs: add new read_uptr and write_uptr file
 operations

On Sat, Jun 27, 2020 at 09:33:03AM -0700, Linus Torvalds wrote:
> The real problem with
> "set_fs()" has been that we've occasionally had bugs where we ended up
> running odd paths that we really didn't _intend_ to run with kernel
> pointers. The classic example is the SCSI "write as ioctl" example,
> where a write to a SCSI generic device would do various odd things and
> follow pointers and what-not. Then you get into real trouble when
> "splice()" ends up truiong to write a kernel buffer, and because of
> "set_fs()" suddenly the sg code started accessing kernel memory
> willy-nilly.

So the semantics of this interface can create chaos fast if not used
carefully and conservatively.

Christoph, it would be great if you're future series can include some
version of a verbiage for the motivation for the culling of set_fs().
Maybe it was just me, but the original motivation wasn't clear at first
and took some thread digging to get it.

  Luis