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

From: Michael Ellerman <mpe@ellerman.id.au>
To: Larry Finger <Larry.Finger@lwfinger.net>,
	Al Viro <viro@ZenIV.linux.org.uk>
Cc: Thorsten Leemhuis <regressions@leemhuis.info>,
	linuxppc-dev@lists.ozlabs.org,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: Regression in kernel 4.12-rc1 for Powerpc 32 - bisected to commit 3448890c32c3
Date: Mon, 26 Jun 2017 23:40:00 +1000	[thread overview]
Message-ID: <87wp7ydg0f.fsf@concordia.ellerman.id.au> (raw)
In-Reply-To: <3e093314-5148-2e14-33a9-e5d67bd2e7cf@lwfinger.net>

Larry Finger <Larry.Finger@lwfinger.net> writes:

> On 06/23/2017 03:29 PM, Al Viro wrote:
>> 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.
>
> I made a break through. If I turn off inline copy to/from users for 32-bit ppc 
> with the following patch, then the system boots:
>
> diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
> index 5c0d8a8cdae5..1e6a8723f497 100644
> --- a/arch/powerpc/include/asm/uaccess.h
> +++ b/arch/powerpc/include/asm/uaccess.h
> @@ -267,12 +267,7 @@ do { 
>         \
>   extern unsigned long __copy_tofrom_user(void __user *to,
>                  const void __user *from, unsigned long size);
>
> -#ifndef __powerpc64__
> -
> -#define INLINE_COPY_FROM_USER
> -#define INLINE_COPY_TO_USE
> -
> -#else /* __powerpc64__ */
> +#ifdef __powerpc64__
>
>   static inline unsigned long
>   raw_copy_in_user(void __user *to, const void __user *from, unsigned long n)

Thanks for debugging this.

I just sent a fix based on the above. Let me know if it doesn't work for
you.

cheers