Re: [Xen-devel] Inheriting CFLAGS

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> > I am attempting to install Xen-2.0 using the Gentoo Ebuilds written by 
> > Philip
> > Taylor, and have run into the same problem as A Streecar Named with needing 
> > to
> > implement the -nopie flag.
> 
> strange, I didn't have to do it (using ebuilds from 
> http://bugs.gentoo.org/show_bug.cgi?id=70161)

Yes, I did wonder about this.

Can you take a look in xen/arch/x86/Rules.mk and see if you have the
lines:
 # Disable PIE/SSP if GCC supports them. They can break us.
 CFLAGS  += $(call test-gcc-flag,-nopie)
 CFLAGS  += $(call test-gcc-flag,-fno-stack-protector)
 CFLAGS  += $(call test-gcc-flag,-fno-stack-protector-all)

If so then you have a *very* up-to-date tree. :-) I recently checked
in a build fix that would disable PIE/SSP iff they are supported by
GCC. 

If not then I'm confused!

> > Is it possible to have Xen inherit these CFLAGS as a base, and then modify
> > them as required? Part of the enthusiasm over Gentoo is the ability to
> > customise and optimise the applications.
> 
> If you're talking about kernel, I don't think it's a good idea. None of 
> the packages under sys-kernel inherits CFLAGS from Portage's 
> /etc/make.conf.

Yeah, it's a bad idea! Kernels are very sensitive to compile flags
-- both Xen and Linux use a wide range of GCC features and frequently
use inline assembler, so the code is fragile when flags are changed
from what we developers use. Allowing CFLAGS to be modified would be
giving users a loaded gun. :-)

 -- Keir


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