Regression in kernel 4.12-rc1 for Powerpc 32 - bisected to commit 3448890c32c3

[Al Viro]

On Fri, Jun 23, 2017 at 01:49:16PM -0500, Larry Finger wrote:

> > BTW, could you try to check what happens if you kill the
> > 	if (__builtin_constant_p(n) && (n <= 8))
> > bits in raw_copy_{to,from}_user()?  The usefulness of those (in __copy_from_user()
> > originally) had always been dubious and the things are simpler without them.
> > If _that_ turns out to cure breakage, I would be very surprised, though.
> > 
> Sorry I was gone so long. Installing jessie on this box resulted in a crash
> on boot. Lubuntu 14.04 yielded a desktop with a functioning cursor, but
> nothing else. Finally, Ubuntu 12.04 resulted in a working system. I hate
> Unity, but I guess I'm stuck for now.

Ho-hum...  Jessie is 3.16, so whatever is crashing there, it's something
different...  Ubuntu 12.04 is what, 3.2?

> I know how easy it is to screw up a long bisection by booting the wrong
> kernel. To help that problem and to work around the yaconf/yboot nonsense on
> the MAC, my /etc/yaconf has always had generic kernel stanzas with only
> default, old, and original kernels mentioned. From there I use a local
> script to finish a kernel installation by moving the default links to the
> old ones and creating the new default links pointing to the current kernel.
> With those long-tested scripts, I'm sure that I am booting the one I want.
> 
> With the new installation, kernel 4.12-rc6 failed, as did 3448890c with the
> backported 46f401c4 added.
> 
> Replacing "if (__builtin_constant_p(n) && (n <= 8))" with "if (0)" had no effect.

OK, that simplifies things a bit.  Just to make sure we are on the same page:

* f2ed8bebee69 + cherry-pick of 46f401c4 boots (Ubuntu 12.04 userland)
* 3448890c32c3 + cherry-pick of 46f401c4 fails (Ubuntu 12.04 userland), ditto
  with removal of constant-size bits in raw_copy_..._user().  Failure appears
  to be on udev getting EFAULT on some syscalls.
* straight Ubuntu 12.04 works
* jessie crashes on boot.

Could you post the boot logs of the first two?