Re: [Xen-devel] [PATCH] xen/x86: fix linker script to work with lld

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Mon, Jul 09, 2018 at 06:45:16PM +0200, Roger Pau Monné wrote:
> On Fri, Jul 06, 2018 at 03:48:39PM +0200, Daniel Kiper wrote:
> > On Thu, Jul 05, 2018 at 09:47:57AM +0200, Roger Pau Monné wrote:
> > > On Wed, Jul 04, 2018 at 12:49:00PM +0200, Daniel Kiper wrote:
> > > > On Wed, Jul 04, 2018 at 01:57:58AM -0600, Jan Beulich wrote:
> > > > > >>> On 03.07.18 at 18:02, <daniel.kiper@xxxxxxxxxx> wrote:
> > > > > > On Thu, Jun 28, 2018 at 11:35:24PM -0600, Jan Beulich wrote:
> > > > > >> >>> Roger Pau Monne <roger.pau@xxxxxxxxxx> 06/28/18 5:38 PM >>>
> > > > > >> >lld (the llvm linker) has some issues with Xen linker script. It
> > > > > >> >doesn't understand '||' in assert expressions:
> > > > > >> >
> > > > > >> >ld    -melf_x86_64_fbsd  -T xen.lds -N prelink.o --build-id=sha1 \
> > > > > >> >/root/src/xen/xen/common/symbols-dummy.o -o 
> > > > > >> >/root/src/xen/xen/.xen-syms.0
> > > > > >> >ld: error: xen.lds:260: malformed number: |
> > > > > >> >>>> ASSERT(__image_base__ > (((((((((261 >> 8) * 
> > > > > >> >>>> 0xffff000000000000) | (261 << 39)))
> > > > > > + ((1 << 39) / 2)) + (64 << 30)) + (1 << 30)) + (1 << 30))) ||
> > > > > >> >>>>
> > > > > >                                                                   ^
> > > > > >> >
> > > > > >> >And doesn't work properly with the 'DEFINED(foo) ? foo : ...'
> > > > > >> >expression:
> > > > > >> >
> > > > > >> >ld    -melf_x86_64_fbsd  -T xen.lds -N prelink.o --build-id=sha1 \
> > > > > >> >/root/src/xen/xen/common/symbols-dummy.o -o 
> > > > > >> >/root/src/xen/xen/.xen-syms.0
> > > > > >> >ld: error: xen.lds:233: symbol not found: efi
> > > > > >
> > > > > > This smells like lld bug. efi symbol is clearly undefined in 
> > > > > > prelink.o
> > > > > > (lld does not support i386pep emulation):
> > > > > >
> > > > > >  11147: 0000000000000000     0 NOTYPE  GLOBAL HIDDEN   UND efi
> > > > > >
> > > > > > However, surprisingly DEFINED() states that it is and ternary 
> > > > > > operator
> > > > > > fires address calculation from undefined symbol. So, I think that 
> > > > > > lld
> > > > > > have to be fixed instead of Xen. It should be noted that binutils ld
> > > > > > works without any issue.
> > > > >
> > > > > Right, but if we can make Xen build nevertheless, this would be 
> > > > > better.
> > > > > So what we need here is a (re-)explanation of why you've needed to
> > > > > do the very change Roger is suggesting to revert.
> > > >
> > > > After commit b199c44 both ELF and PE output need an efi symbol. So, we
> > > > cannot use simple "#ifdef EFI" for differentiation as it was earlier.
> > >
> > > I'm not sure I follow, why is that not an option?
> > >
> > > Can you detail which conditions will make the build to fail with the
> > > proposed patch?
> >
> > The problem is that your patch will change efi symbol address in ELF
> > output. The symbol is provided by Xen EFI runtime code. So, this way
> > Xen will not work on EFI platform if it is loaded from GRUB2.
> >
> > > I assume that for the ELF output the EFI code is not added to the
> > > image, and as such it requires the efi symbol to be defined because
> > > it's referenced from common code?
> >
> > EFI code is added to the ELF image because it has to run on EFI
> > platforms too. Of course via GRUB2 or to be more precise any
> > Multiboot2 compatible boot loader.
>
> So now both the ELF and the PE binaries will contain the efi symbol?
> Then which binary doesn't contain it?

Both always. The difference is where it come from. If build tools
(compiler and linker) are able to generate PE files then efi symbol
is derived from source files (xen/common/efi/runtime.c to be precise).
Otherwise it is derived from xen.lds.S.

> Sorry for asking so many questions, but I would like to try to avoid
> the DEFINED conditional in the linker script if possible.

If you wish to do that then put dummy efi symbol into xen/arch/x86/efi/stub.c.

Daniel

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