Re: [RFC PATCH 05/11] x86: Makefile: Add build and config option for CONFIG_FG_KASLR

[Kristen Carlson Accardi]

On Tue, 2020-02-25 at 12:55 -0500, Arvind Sankar wrote:
> On Wed, Feb 05, 2020 at 02:39:44PM -0800, Kristen Carlson Accardi
> wrote:
> > Allow user to select CONFIG_FG_KASLR if dependencies are met.
> > Change
> > the make file to build with -ffunction-sections if CONFIG_FG_KASLR
> > 
> > Signed-off-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
> > ---
> >  Makefile         |  4 ++++
> >  arch/x86/Kconfig | 13 +++++++++++++
> >  2 files changed, 17 insertions(+)
> > 
> > diff --git a/Makefile b/Makefile
> > index c50ef91f6136..41438a921666 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -846,6 +846,10 @@ ifdef CONFIG_LIVEPATCH
> >  KBUILD_CFLAGS += $(call cc-option, -flive-patching=inline-clone)
> >  endif
> >  
> > +ifdef CONFIG_FG_KASLR
> > +KBUILD_CFLAGS += -ffunction-sections
> > +endif
> > +
> 
> With -ffunction-sections I get a few unreachable code warnings from
> objtool.
> 
> arch/x86/kernel/dumpstack.o: warning: objtool: show_iret_regs()+0x10:
> unreachable instruction
> fs/sysfs/dir.o: warning: objtool: sysfs_create_mount_point()+0x4f:
> unreachable instruction
> kernel/time/clocksource.o: warning: objtool:
> __clocksource_register_scale()+0x21: unreachable instruction
> drivers/tty/sysrq.o: warning: objtool: sysrq_filter()+0x2ef:
> unreachable instruction
> arch/x86/mm/fault.o: warning: objtool: pgtable_bad()+0x3f:
> unreachable instruction
> drivers/acpi/pci_root.o: warning: objtool:
> acpi_pci_osc_control_set()+0x123: unreachable instruction
> drivers/rtc/class.o: warning: objtool:
> devm_rtc_device_register()+0x40: unreachable instruction
> kernel/power/process.o: warning: objtool:
> freeze_processes.cold()+0x0: unreachable instruction
> drivers/pnp/quirks.o: warning: objtool: quirk_awe32_resources()+0x42:
> unreachable instruction
> drivers/acpi/utils.o: warning: objtool: acpi_evaluate_dsm()+0xf1:
> unreachable instruction
> kernel/reboot.o: warning: objtool: __do_sys_reboot()+0x1b6:
> unreachable instruction
> kernel/power/swap.o: warning: objtool: swsusp_read()+0x185:
> unreachable instruction
> drivers/hid/hid-core.o: warning: objtool: hid_hw_start()+0x38:
> unreachable instruction
> drivers/acpi/battery.o: warning: objtool:
> sysfs_add_battery.cold()+0x1a: unreachable instruction
> arch/x86/kernel/cpu/mce/core.o: warning: objtool:
> do_machine_check.cold()+0x33: unreachable instruction
> drivers/pcmcia/cistpl.o: warning: objtool: pccard_store_cis()+0x4e:
> unreachable instruction
> drivers/gpu/vga/vgaarb.o: warning: objtool: pci_notify()+0x35:
> unreachable instruction
> arch/x86/kernel/tsc.o: warning: objtool:
> determine_cpu_tsc_frequencies()+0x45: unreachable instruction
> drivers/pcmcia/yenta_socket.o: warning: objtool:
> ti1250_override()+0x50: unreachable instruction
> fs/proc/proc_sysctl.o: warning: objtool:
> sysctl_print_dir.isra.0()+0x19: unreachable instruction
> drivers/iommu/intel-iommu.o: warning: objtool:
> intel_iommu_init()+0x4f4: unreachable instruction
> net/mac80211/ibss.o: warning: objtool:
> ieee80211_ibss_work.cold()+0x157: unreachable instruction
> drivers/net/ethernet/intel/e1000/e1000_main.o: warning: objtool:
> e1000_clean.cold()+0x0: unreachable instruction
> net/core/skbuff.o: warning: objtool: skb_dump.cold()+0x3fd:
> unreachable instruction

I'm still working on a solution, but the issue here is that any .cold
function is going to be in a different section than the related
function, and when objtool is searching for instructions in
find_insn(), it assumes that it must be in the same section as the
caller.