Re: [PATCH 4/4] livepatch: differentiate between old and new build systems

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Mar 08, 2022 at 02:38:47PM +0000, Andrew Cooper wrote:
> On 02/03/2022 14:27, Roger Pau Monne wrote:
> > diff --git a/livepatch-build b/livepatch-build
> > index 38a92be..656cdac 100755
> > --- a/livepatch-build
> > +++ b/livepatch-build
> > @@ -98,14 +98,20 @@ function build_special()
> >  
> >      # Build with special GCC flags
> >      cd "${SRCDIR}/xen" || die
> > -    sed -i 's/CFLAGS += -nostdinc/CFLAGS += -nostdinc -ffunction-sections 
> > -fdata-sections/' Rules.mk
> > -    cp -p arch/x86/Makefile arch/x86/Makefile.bak
> > -    sed -i 's/--section-alignment=0x200000/--section-alignment=0x1000/' 
> > arch/x86/Makefile
> > -    # Restore timestamps to prevent spurious rebuilding
> > -    touch --reference=arch/x86/Makefile.bak arch/x86/Makefile
> > -    make "-j$CPUS" $XEN_DEBUG &> "${OUTPUT}/build_${name}_compile.log" || 
> > die
> > -    sed -i 's/CFLAGS += -nostdinc -ffunction-sections 
> > -fdata-sections/CFLAGS += -nostdinc/' Rules.mk
> > -    mv -f arch/x86/Makefile.bak arch/x86/Makefile
> > +    if grep -q 'nostdinc' Rules.mk; then
> > +         # Support for old build system, attempt to set 
> > -f{function,data}-sections and rebuild
> > +        sed -i 's/CFLAGS += -nostdinc/CFLAGS += -nostdinc 
> > -ffunction-sections -fdata-sections/' Rules.mk
> > +        cp -p arch/x86/Makefile arch/x86/Makefile.bak
> > +        sed -i 
> > 's/--section-alignment=0x200000/--section-alignment=0x1000/' 
> > arch/x86/Makefile
> > +        # Restore timestamps to prevent spurious rebuilding
> > +        touch --reference=arch/x86/Makefile.bak arch/x86/Makefile
> > +        make "-j$CPUS" $XEN_DEBUG &> "${OUTPUT}/build_${name}_compile.log" 
> > || die
> > +        sed -i 's/CFLAGS += -nostdinc -ffunction-sections 
> > -fdata-sections/CFLAGS += -nostdinc/' Rules.mk
> > +        mv -f arch/x86/Makefile.bak arch/x86/Makefile
> > +    else
> > +        # -f{function,data}-sections set by CONFIG_LIVEPATCH
> > +        make "-j$CPUS" $XEN_DEBUG &> "${OUTPUT}/build_${name}_compile.log" 
> > || die
> > +    fi
> 
> This really ought to be the other way around, by spotting the thing we
> know is good, and then falling back to the heuristics.  In light of the
> updates to the Xen side, something like:

I'm not sure I agree. I do prefer to spot the 'bad' one, and just
fallback to expecting Xen to correctly set -f{function,data}-sections
otherwise.

> if grep -q CC_SPLIT_SECTIONS Kconfig; then

Because this logic ties us to not moving CC_SPLIT_SECTIONS from being
defined in xen/Kconfig (or even changing it's name), and gain ties the
livepatch tools to internal details about the Xen build system.

Thanks, Roger.