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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 28/04/2020 12:16, Wei Liu wrote:
>>>> ---
>>>> I can't get ioemu-stubdom to start without this.  With this, the guest
>>>> just reboots immediately, but it does that with a non-stubdom
>>>> device_model_version="qemu-xen-traditional" .  The same guest disk image
>>>> (cirros 0.5.1) boots with a linux stubdom or non-stubdom Ubuntu
>>>> qemu-system-x86_64.
>> Ubuntu gcc-9 adds -fcf-protection by default.  Somehow that flag
>> caused rombios (I think) to restart.  Setting -fcf-protection=none
>> like below (probably just the EMBEDDED_EXTRA_CFLAGS part) lets rombios
>> start properly.

All it does is insert ENDBR{32,64} instructions, which are nops on older
processors.

I suspect that it is not the -fcf-protection= directly, but some change
in alignment of a critical function.

>>   The hypervisor needs it as well via
>> EXTRA_CFLAGS_XEN_CORE=-fcf-protection=none and maybe also added to
>> xen/arch/x86/boot/build32.mk .
> Are you able to turn this into a proper patch? I suspect you will need
> to test the availability of this new (?) flag.
>
> Also Cc Jan and Andrew because it affects hypervisor build too.

I need to chase this up.  It is a GCC bug breaking the hypervisor build,
and I'm moderately disinclined to hack around it, seeing as
-fcf-protection is something we want in due course.

The bug is that GCC falsely declares that -fcf-protection is
incompatible with -mindirect-thunk=extern, despite me spending a week
during the Spectre embargo period specifically arranging for the two to
be compatible, because we knew we'd want to build retpoline-safe
binaries which could also use CET on newer hardware.

~Andrew