Re: [Xen-devel] [PATCH v1 00/27] x86: PIE support and option to extend KASLR randomization

[Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx>]

On 2017.10.12 at 08:34 -0700, Thomas Garnier wrote:
> On Wed, Oct 11, 2017 at 2:34 PM, Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
> > On 10/11/2017 3:30 PM, Thomas Garnier wrote:
> >> Changes:
> >>   - patch v1:
> >>     - Simplify ftrace implementation.
> >>     - Use gcc mstack-protector-guard-reg=%gs with PIE when possible.
> >>   - rfc v3:
> >>     - Use --emit-relocs instead of -pie to reduce dynamic relocation space 
> >> on
> >>       mapped memory. It also simplifies the relocation process.
> >>     - Move the start the module section next to the kernel. Remove the 
> >> need for
> >>       -mcmodel=large on modules. Extends module space from 1 to 2G maximum.
> >>     - Support for XEN PVH as 32-bit relocations can be ignored with
> >>       --emit-relocs.
> >>     - Support for GOT relocations previously done automatically with -pie.
> >>     - Remove need for dynamic PLT in modules.
> >>     - Support dymamic GOT for modules.
> >>   - rfc v2:
> >>     - Add support for global stack cookie while compiler default to fs 
> >> without
> >>       mcmodel=kernel
> >>     - Change patch 7 to correctly jump out of the identity mapping on 
> >> kexec load
> >>       preserve.
> >>
> >> These patches make the changes necessary to build the kernel as Position
> >> Independent Executable (PIE) on x86_64. A PIE kernel can be relocated below
> >> the top 2G of the virtual address space. It allows to optionally extend the
> >> KASLR randomization range from 1G to 3G.
> >
> > Hi Thomas,
> >
> > I've applied your patches so that I can verify that SME works with PIE.
> > Unfortunately, I'm running into build warnings and errors when I enable
> > PIE.
> >
> > With CONFIG_STACK_VALIDATION=y I receive lots of messages like this:
> >
> >   drivers/scsi/libfc/fc_exch.o: warning: objtool: 
> > fc_destroy_exch_mgr()+0x0: call without frame pointer save/setup
> >
> > Disabling CONFIG_STACK_VALIDATION suppresses those.
> 
> I ran into that, I plan to fix it in the next iteration.
> 
> >
> > But near the end of the build, I receive errors like this:
> >
> >   arch/x86/kernel/setup.o: In function `dump_kernel_offset':
> >   .../arch/x86/kernel/setup.c:801:(.text+0x32): relocation truncated to 
> > fit: R_X86_64_32S against symbol `_text' defined in .text section in 
> > .tmp_vmlinux1
> >   .
> >   . about 10 more of the above type messages
> >   .
> >   make: *** [vmlinux] Error 1
> >   Error building kernel, exiting
> >
> > Are there any config options that should or should not be enabled when
> > building with PIE enabled?  Is there a compiler requirement for PIE (I'm
> > using gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.5))?
> 
> I never ran into these ones and I tested compilers older and newer.
> What was your exact configuration?

I get with gcc trunk and CONFIG_RANDOMIZE_BASE_LARGE=y:

...
  MODPOST vmlinux.o                         
  ld: failed to convert GOTPCREL relocation; relink with --no-relax

and after adding --no-relax to vmlinux_link() in scripts/link-vmlinux.sh:

  MODPOST vmlinux.o
virt/kvm/vfio.o: In function `kvm_vfio_update_coherency.isra.4':
vfio.c:(.text+0x63): relocation truncated to fit: R_X86_64_PLT32 against 
undefined symbol `vfio_external_check_extension'
virt/kvm/vfio.o: In function `kvm_vfio_destroy':
vfio.c:(.text+0xf7): relocation truncated to fit: R_X86_64_PLT32 against 
undefined symbol `vfio_group_set_kvm'
vfio.c:(.text+0x10a): relocation truncated to fit: R_X86_64_PLT32 against 
undefined symbol `vfio_group_put_external_user'
virt/kvm/vfio.o: In function `kvm_vfio_set_attr':
vfio.c:(.text+0x2bc): relocation truncated to fit: R_X86_64_PLT32 against 
undefined symbol `vfio_external_group_match_file'
vfio.c:(.text+0x307): relocation truncated to fit: R_X86_64_PLT32 against 
undefined symbol `vfio_group_set_kvm'
vfio.c:(.text+0x31a): relocation truncated to fit: R_X86_64_PLT32 against 
undefined symbol `vfio_group_put_external_user'
vfio.c:(.text+0x3b9): relocation truncated to fit: R_X86_64_PLT32 against 
undefined symbol `vfio_group_get_external_user'
vfio.c:(.text+0x462): relocation truncated to fit: R_X86_64_PLT32 against 
undefined symbol `vfio_group_set_kvm'
vfio.c:(.text+0x4bd): relocation truncated to fit: R_X86_64_PLT32 against 
undefined symbol `vfio_group_put_external_user'
make: *** [Makefile:1000: vmlinux] Error 1

Works fine with CONFIG_RANDOMIZE_BASE_LARGE unset.

-- 
Markus

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