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Subject: Re: CONFIG_PAGE_TABLE_ISOLATION=y on x86_64 causes gcc to segfault
 when building x86_32 binaries
To: Thomas Gleixner <tglx@linutronix.de>,
        Lars Wendler <wendler.lars@web.de>
References: <20180103123723.1dd26828@abudhabi.paradoxon.rec>
 <alpine.DEB.2.20.1801031256440.1957@nanos>
 <alpine.DEB.2.20.1801031259530.1957@nanos>
 <20180103143036.60e592eb@abudhabi.paradoxon.rec>
 <alpine.DEB.2.20.1801031620020.1957@nanos>
Cc: LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        Borislav Betkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Greg KH <gregkh@linuxfoundation.org>
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <a5bdc44d-6c65-563f-a9cc-aee1d264d8f1@linux.intel.com>
Date: Wed, 3 Jan 2018 07:42:08 -0800
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On 01/03/2018 07:21 AM, Thomas Gleixner wrote:
> On Wed, 3 Jan 2018, Lars Wendler wrote:
>> Am Wed, 3 Jan 2018 13:05:38 +0100 (CET)
>> schrieb Thomas Gleixner <tglx@linutronix.de>:
>>> Also can you please try Linus v4.15-rc6 with PTI enabled so we can see
>>> whether that's a backport issue or a general one?
>> Same problem with 4.15-rc6. So I suppose that means it's a general
>> issue.
> Just a shot in the dark as I just decoded another issue on a AMD CPU. Can
> you please try the patch below?

Hi Lars,

I see the double-faults in your dmesg.  Those are usually indicative of
a failed user->kernel CR3 switch on entering the kernel.

Could you run a copy of the x86 selftests?  From the kernel tree I
usually do:

	cd tools/testing/selftests/x86/
	make
	for i in *_64 *_32; do ./$i || break; done

See if any of those fail.

I wonder if we screwed up something in the SYSCALL code in
entry_64_compat.S.  It's probably been much more lightly tested than all
the other entry code.

I'll see if I can reproduce this.