linux-kernel - Re: CONFIG_PAGE_TABLE_ISOLATION=y on x86_64 causes gcc to segfault when building x86

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Message-ID: <20180103143036.60e592eb@abudhabi.paradoxon.rec>
Date:   Wed, 3 Jan 2018 14:30:36 +0100
From:   Lars Wendler <wendler.lars@....de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Borislav Betkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: CONFIG_PAGE_TABLE_ISOLATION=y on x86_64 causes gcc to segfault
 when building x86_32 binaries

Am Wed, 3 Jan 2018 13:05:38 +0100 (CET)
schrieb Thomas Gleixner <tglx@...utronix.de>:

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

Yes it is.

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

Kind regards

Lars

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