Date: Wed, 3 Jan 2018 14:30:36 +0100
From: Lars Wendler <wendler.lars@web.de>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        Borislav Betkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Greg KH <gregkh@linuxfoundation.org>
Subject: Re: CONFIG_PAGE_TABLE_ISOLATION=y on x86_64 causes gcc to segfault
 when building x86_32 binaries
Message-ID: <20180103143036.60e592eb@abudhabi.paradoxon.rec>
In-Reply-To: <alpine.DEB.2.20.1801031259530.1957@nanos>
References: <20180103123723.1dd26828@abudhabi.paradoxon.rec>
 <alpine.DEB.2.20.1801031256440.1957@nanos>
 <alpine.DEB.2.20.1801031259530.1957@nanos>
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Am Wed, 3 Jan 2018 13:05:38 +0100 (CET)
schrieb Thomas Gleixner <tglx@linutronix.de>:

> On Wed, 3 Jan 2018, Thomas Gleixner wrote:
> > On Wed, 3 Jan 2018, Lars Wendler wrote:
> > Thanks for the report.
> >  =20
> > > I already had a conversation with Thomas Gleixner about this
> > > issue and he asked me to post it here.
> > >=20
> > > After upgrading my system to vanilla 4.14.11 kernel with
> > > CONFIG_PAGE_TABLE_ISOLATION=3Dy I could no longer build any 32bit
> > > binaries because gcc (7.2.0) segfaults. =20
> >=20
> > That's a 64bit binary, right?

Yes it is.

> > > This only happens with CONFIG_PAGE_TABLE_ISOLATION=3Dy
> > > I can get rid of this issue by setting
> > > CONFIG_PAGE_TABLE_ISOLATION=3Dn _or_ adding pti=3Doff to the kernel
> > > command line (the latter was suggested by Thomas).
> > >=20
> > > Please find attached the kernel's config together with the dmesg
> > > output of the kernel that produces segfaults and the one with
> > > pti=3Doff. I also attached a config.log file from a build where gcc
> > > segfaulted.
> > >=20
> > > If you need more information, please don't hesitate to ask. =20
> >=20
> > will do. =20
>=20
> Here you go.
>=20
> Is this anything special you are trying to build or is a simple
>=20
>    gcc -m32 -o hello hello.c

A simple hello world compiles as 32bit binary but running it causes a
segfault. Compiling the same hello world as 64bit does not cause any
segfaults. Switching off pti resolves the issue and I can run the 32bit
hello world.

I first found this issue while trying to emerge gdbm-1.14 as 32bit.
Running configure failed and the config.log file said that gcc produced
a segfault while compiling a test binary called "conftest" (see the
gdbm-1.14-abi_x86_32.x86_config.log attachment in my initial report).

> 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.


> Thanks,
>=20
> 	tglx

Kind regards

Lars

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