Subject: Re: gcc 4.6.3 miscompile on ppc32 (was Re: Regression in kernel 4.12-rc1 for Powerpc 32 - bisected to commit 3448890c32c3)

From: Segher Boessenkool <[email-masked]>
Date: Sun, 25 Jun 2017 16:44:09 -0500

On Sun, Jun 25, 2017 at 09:53:24PM +0100, Al Viro wrote:
> Confirmed.  It manages to bugger the loop immediately after the (successful)
> copying of iovec array in rw_copy_check_uvector(); both with and without
> INLINE_COPY_FROM_USER it has (just before the call of copy_from_user()) r27
> set to nr_segs * sizeof(struct iovec).  The call is made, we check that it
> has succeeded and that's when it hits the fan: without INLINE_COPY_FROM_USER
> we have (interleaved with unrelated insns)
>         addi 27,27,-8
>         srwi 27,27,3
>         addi 27,27,1
>         mtctr 27
> Weird, but manages to pass nr_segs to mtctr.

This weirdosity is https://gcc.gnu.org/PR67288 .  Those three instructions
are not the same as just  srwi 27,27,3  in case r27 is 0; GCC does not
figure out this cannot happen here.

> _With_ INLINE_COPY_FROM_USER we
> get this:
>         lis 9,0x2000
>         mtctr 9
> In other words, the loop will try to go through 8192 iterations.  No idea where
> that number has come from, but it sure as hell is wrong.

8192*65535, even.  This is as if r27 was 0 always.

Do you have a short stand-alone testcase?  4.6 is ancient, of course, but
the actual problem may still exist in more recent compilers (if it _is_
a compiler problem; if it's not, you *really* want to know :-) )

Segher