Xen Compilation Error on Ubuntu 20.04

Xen Compilation Error on Ubuntu 20.04

[Ayush Dosaj]

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Hi Xen development team,

I am Ayush. I compiled Xen Hypervisor from source on Ubuntu 20.04 machine
running on an intel-i9 CPU.
I am getting compilation error due to the following two flags.
Error: error: ‘-mindirect-branch’ and ‘-fcf-protection’ are not compatible.

Complete Error logs can be found at https://paste.ubuntu.com/p/xvvyPnhW5c/

And when I compiled Xen commenting the two flags in Rules.mk file, it
compiles and installs properly but on boot-up i see a blank black screen
and i am stuck there.


Best,
Ayush Dosaj

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Re: Xen Compilation Error on Ubuntu 20.04

[Andrew Cooper]

On 29/04/2020 18:17, Ayush Dosaj wrote:
> Hi Xen development team,
> 
> I am Ayush. I compiled Xen Hypervisor from source on Ubuntu 20.04
> machine running on an intel-i9 CPU.
> I am getting compilation error due to the following two flags.
> Error: error: ‘-mindirect-branch’ and ‘-fcf-protection’ are not compatible.
> 
> Complete Error logs can be found at https://paste.ubuntu.com/p/xvvyPnhW5c/
> 
> And when I compiled Xen commenting the two flags in Rules.mk file, it
> compiles and installs properly but on boot-up i see a blank black screen
> and i am stuck there.

That is a GCC bug (these options are actually fine in combination).  It
got fixed earlier today in master, and backported for GCC 9.4

You can work around it by appending -fcf-protection=none to CFLAGS

I wouldn't try editing the logic around -mindirect-branch, as that is
related to retpoline safety for Spectre v2, and probably relies on the
build matching the code.

~Andrew

Re: Xen Compilation Error on Ubuntu 20.04

[Ayush Dosaj]

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Awesome, thanks!

On Wed, Apr 29, 2020 at 10:55 PM Andrew Cooper <andrew.cooper3@citrix.com>
wrote:

> On 29/04/2020 18:17, Ayush Dosaj wrote:
> > Hi Xen development team,
> >
> > I am Ayush. I compiled Xen Hypervisor from source on Ubuntu 20.04
> > machine running on an intel-i9 CPU.
> > I am getting compilation error due to the following two flags.
> > Error: error: ‘-mindirect-branch’ and ‘-fcf-protection’ are not
> compatible.
> >
> > Complete Error logs can be found at
> https://paste.ubuntu.com/p/xvvyPnhW5c/
> >
> > And when I compiled Xen commenting the two flags in Rules.mk file, it
> > compiles and installs properly but on boot-up i see a blank black screen
> > and i am stuck there.
>
> That is a GCC bug (these options are actually fine in combination).  It
> got fixed earlier today in master, and backported for GCC 9.4
>
> You can work around it by appending -fcf-protection=none to CFLAGS
>
> I wouldn't try editing the logic around -mindirect-branch, as that is
> related to retpoline safety for Spectre v2, and probably relies on the
> build matching the code.
>
> ~Andrew
>


-- 
Ayush Dosaj
VIT Vellore

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Re: Xen Compilation Error on Ubuntu 20.04

[Ayush Dosaj]

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Hi Andrew, Xen Development team.

I compiled and installed Xen by appending -fcf-protection=none to CFLAGS on
Ubuntu 20.04 but it still crashes on startup.

On Wed, Apr 29, 2020 at 10:58 PM Ayush Dosaj <ayushdosaj2313@gmail.com>
wrote:

> Awesome, thanks!
>
> On Wed, Apr 29, 2020 at 10:55 PM Andrew Cooper <andrew.cooper3@citrix.com>
> wrote:
>
>> On 29/04/2020 18:17, Ayush Dosaj wrote:
>> > Hi Xen development team,
>> >
>> > I am Ayush. I compiled Xen Hypervisor from source on Ubuntu 20.04
>> > machine running on an intel-i9 CPU.
>> > I am getting compilation error due to the following two flags.
>> > Error: error: ‘-mindirect-branch’ and ‘-fcf-protection’ are not
>> compatible.
>> >
>> > Complete Error logs can be found at
>> https://paste.ubuntu.com/p/xvvyPnhW5c/
>> >
>> > And when I compiled Xen commenting the two flags in Rules.mk file, it
>> > compiles and installs properly but on boot-up i see a blank black screen
>> > and i am stuck there.
>>
>> That is a GCC bug (these options are actually fine in combination).  It
>> got fixed earlier today in master, and backported for GCC 9.4
>>
>> You can work around it by appending -fcf-protection=none to CFLAGS
>>
>> I wouldn't try editing the logic around -mindirect-branch, as that is
>> related to retpoline safety for Spectre v2, and probably relies on the
>> build matching the code.
>>
>> ~Andrew
>>
>
>
> --
> Ayush Dosaj
> VIT Vellore
>
>

-- 
Ayush Dosaj

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Re: Xen Compilation Error on Ubuntu 20.04

[Jason Andryuk]

On Thu, Apr 30, 2020 at 4:18 AM Ayush Dosaj <ayushdosaj2313@gmail.com> wrote:
>
> Hi Andrew, Xen Development team.
>
> I compiled and installed Xen by appending -fcf-protection=none to CFLAGS on Ubuntu 20.04 but it still crashes on startup.

Ayush, try the patch below.  EMBEDDED_EXTRA_CFLAGS ensures it is set
down in xen/arch/x86/boot/build32.mk

-Jason

diff --git a/Config.mk b/Config.mk
index 0f303c79b2..efb3d42bc4 100644
--- a/Config.mk
+++ b/Config.mk
@@ -205,6 +205,7 @@ APPEND_CFLAGS += $(foreach i, $(APPEND_INCLUDES), -I$(i))

 EMBEDDED_EXTRA_CFLAGS := -nopie -fno-stack-protector -fno-stack-protector-all
 EMBEDDED_EXTRA_CFLAGS += -fno-exceptions
+EMBEDDED_EXTRA_CFLAGS += -fcf-protection=none

 XEN_EXTFILES_URL ?= http://xenbits.xen.org/xen-extfiles
 # All the files at that location were downloaded from elsewhere on
diff --git a/xen/arch/x86/Rules.mk b/xen/arch/x86/Rules.mk
index 4b7ab78467..c3cbae69d2 100644
--- a/xen/arch/x86/Rules.mk
+++ b/xen/arch/x86/Rules.mk
@@ -69,6 +69,7 @@ CFLAGS += -mno-sse $(call cc-option,$(CC),-mskip-rax-setup)
 ifeq ($(CONFIG_INDIRECT_THUNK),y)
 CFLAGS += -mindirect-branch=thunk-extern -mindirect-branch-register
 CFLAGS += -fno-jump-tables
+$(call cc-option-add,CFLAGS,CC,-fcf-protection=none)
 endif

 # If supported by the compiler, reduce stack alignment to 8 bytes. But allow

Re: Xen Compilation Error on Ubuntu 20.04

[Pry Mar]

Hello Ayush Dosaj,

https://bugs.launchpad.net/ubuntu/+source/xen/+bug/1851091

see the bug report above ^.

As soon as Ubuntu 19.10 went to LZ4 kernel compression this bug
appeared. As Stefan Bader reports in his testing, the hypervisor not
only crashes, but resets the machine.

The same trouble continued with 20.04. Their solution was to forward
package the Disco hypervisor. You could also hybridize with the Buster
hypervisor - I've done both with perfect results.

Whatever this bug ends up to be, it goes back to 2015 when the lz4
decompression code was first introduced:
[xen.git] / xen / common / lz4 /

I reported trouble too back in Oct 2019:
https://lists.xenproject.org/archives/html/xen-devel/2019-10/msg01637.html

cheers,
PryMar56
##xen-packaging on Freenode IRC