Re: [fs] 36e2c7421f:

From: Christoph Hellwig
Date: Sat Sep 19 2020 - 01:13:44 EST

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.