Re: [PATCH] mini-os: Avoid segfaults in tc{g,s}etattr

From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jason Andryuk <jandryuk@gmail.com>, Wei Liu <wl@xen.org>
Cc: minios-devel@lists.xenproject.org, samuel.thibault@ens-lyon.org,
	JBeulich@suse.com, Stefan Bader <stefan.bader@canonical.com>,
	xen-devel@lists.xenproject.org
Subject: Re: [PATCH] mini-os: Avoid segfaults in tc{g,s}etattr
Date: Wed, 29 Apr 2020 13:34:42 +0100	[thread overview]
Message-ID: <34cc563f-9e05-b55c-54f4-55104d2d42b5@citrix.com> (raw)
In-Reply-To: <c242b963-ae80-1ca0-9b4d-fe2c8f66b6a2@citrix.com>

On 28/04/2020 12:55, Andrew Cooper wrote:
>> Below is what I was preparing to submit as a patch.  So, yes it hacks around
>> it, but it isn't messy.
>>
>> ---
>> Disable fcf-protection to build working binaries
>>
>> Ubuntu gcc-9 enables -fcf-protection by default, which conflicts with
>> -mindirect-branch=extern and prevents building the hypervisor with
>> CONFIG_INDIRECT_THUNK:
>> xmalloc.h:81:1: error: ‘-mindirect-branch’ and ‘-fcf-protection’ are not
>> compatible
>>
>> Stefan Bader also noticed that build32.mk requires -fcf-protection=none
>> or else the hypervisor will not boot.
>> https://bugs.launchpad.net/ubuntu/+source/gcc-9/+bug/1863260  Similarly,
>> rombios reboots almost immediately without -fcf-protection=none.  Both
>> of those can be handled by setting it in EMBEDDED_EXTRA_CFLAGS.
>>
>> CC: Stefan Bader <stefan.bader@canonical.com>
>> Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
> Sadly, this isn't really appropriate.  We specifically do want to use
> both -fcf-protection and -mindirect-branch=thunk-extern together, when
> GCC isn't broken.
>
> Overriding -fcf-protection is ok but only when we're certain we've got a
> buggy GCC, so that when this bug is fixed, we can return to sensible
> behaviour.

GCC has been adjusted on master
(https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=9be3bb2c0a258fd6a7d3d05d232a21930c757d3c)
and the gcc-9 branch
(https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=a03efb266fcbf4a01285fff871a5bfe5caac4944). 
This should be fixed for GCC 10 and 9.4

I checked the resulting hypervisor build with both -fcf-protection and
retpolines, and it works fine.

The question now is what to do all the buggy GCCs out there.  We can
either ignore the problem and it will eventually go away, or spot the
problematic compiler and clobber -fcf-protection.

We also need to see what is wrong with RomBIOS, because that is weird. 
However, we should not be interfering with the HOSTCC settings.

~Andrew