On Thu, Jan 31, 2019 at 11:24:20AM -0800, Thomas Garnier wrote: > The __startup_64 function assumes all symbols have relocated addresses > instead of the current boot virtual address. PIE generated code favor > relative addresses making all virtual and physical address math incorrect. > If PIE is enabled, build head64.c as mcmodel large instead to ensure absolute > references on all memory access. Add a global __force_order variable required > when using a large model with read_cr* functions. > > To build head64.c as mcmodel=large, disable the retpoline gcc flags. > This code is used at early boot and removed later, it doesn't need > retpoline mitigation. > > Position Independent Executable (PIE) support will allow to extend the > KASLR randomization range below 0xffffffff80000000. > > Signed-off-by: Thomas Garnier <thgar...@chromium.org> > --- > arch/x86/kernel/Makefile | 6 ++++++ > arch/x86/kernel/head64.c | 3 +++ > 2 files changed, 9 insertions(+) > > diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile > index 00b7e27bc2b7..1f98f52eab9f 100644 > --- a/arch/x86/kernel/Makefile > +++ b/arch/x86/kernel/Makefile > @@ -22,6 +22,12 @@ CFLAGS_REMOVE_early_printk.o = -pg > CFLAGS_REMOVE_head64.o = -pg > endif > > +ifdef CONFIG_X86_PIE > +# Remove PIE and retpoline flags that are incompatible with mcmodel=large > +CFLAGS_REMOVE_head64.o += -fPIE -mindirect-branch=thunk-extern > -mindirect-branch-register > +CFLAGS_head64.o = -mcmodel=large > +endif > + > KASAN_SANITIZE_head$(BITS).o := n > KASAN_SANITIZE_dumpstack.o := n > KASAN_SANITIZE_dumpstack_$(BITS).o := n > diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c > index 16b1cbd3a61e..22e81275495b 100644 > --- a/arch/x86/kernel/head64.c > +++ b/arch/x86/kernel/head64.c > @@ -63,6 +63,9 @@ EXPORT_SYMBOL(vmemmap_base); > > #define __head __section(.head.text) > > +/* Required for read_cr3 when building as PIE */ > +unsigned long __force_order; > +
I believe it only needed for GCC < 5. Newer GCC can eliminate the reference. See my comment in arch/x86/boot/compressed/pgtable_64.c. Maybe we should expand the comment here too? -- Kirill A. Shutemov