linux-kernel - Re: [fs] 36e2c7421f: kernel-selftests.splice.short_splice

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Date:   Sat, 19 Sep 2020 07:07:39 +0200
From:   Christoph Hellwig <hch@....de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Christoph Hellwig <hch@....de>, Al Viro <viro@...iv.linux.org.uk>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        lkp@...el.com, kernel test robot <rong.a.chen@...el.com>
Subject: Re: [fs] 36e2c7421f:
 kernel-selftests.splice.short_splice_read.sh.fail

On Fri, Sep 18, 2020 at 02:49:19PM -0700, Kees Cook wrote:
> In response to my recent bug fix for splice vs sysfs binary handler[1],
> I added splice testing for other pseudo filesystems[2], for which the
> test output is seen above.
> 
> What is the final verdict on the "should splice have a fallback mode?"
> question[3]? Right now /proc and /sys reject splice attempts (which, as
> I mentioned in the thread, is fine by me, since it would have blocked
> the bug I had to fix from ever being exposed in the first place).

The verdict is: without a set_fs()-like mechanism that allows uaccess
routines to operate on kernel buffers, or even worse a
compat_alloc_user_space-like mechanism we can't have an entirely
generic fallback.

> Should I update the test to _expect_ that splice should fail?

I think so.  We can updated individual file operations to support splice
where actually used applications except it (even when they shouldn't),
but I'd rather not do it just for a test case.