On Fri, Feb 1, 2019 at 3:15 AM Kirill A. Shutemov <kir...@shutemov.name> wrote: > > 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?
Make sense, I will add a similar comment in the next iteration. Thanks for pointing that out. > > -- > Kirill A. Shutemov
