On Sun, 24 May 2020 at 17:12, Ingo Molnar <mi...@kernel.org> wrote: > > > * Ard Biesheuvel <a...@kernel.org> wrote: > > > Eliminate all GOT entries in the decompressor binary, by forcing hidden > > visibility for all symbol references, which informs the compiler that > > such references will be resolved at link time without the need for > > allocating GOT entries. > > > > To ensure that no GOT entries will creep back in, add an assertion to > > the decompressor linker script that will fire if the .got section has > > a non-zero size. > > > > Signed-off-by: Ard Biesheuvel <a...@kernel.org> > > --- > > arch/x86/boot/compressed/Makefile | 1 + > > arch/x86/boot/compressed/hidden.h | 19 +++++++++++++++++++ > > arch/x86/boot/compressed/vmlinux.lds.S | 1 + > > 3 files changed, 21 insertions(+) > > > > diff --git a/arch/x86/boot/compressed/Makefile > > b/arch/x86/boot/compressed/Makefile > > index 5f7c262bcc99..aa9ed814e5fa 100644 > > --- a/arch/x86/boot/compressed/Makefile > > +++ b/arch/x86/boot/compressed/Makefile > > @@ -40,6 +40,7 @@ KBUILD_CFLAGS += $(call cc-disable-warning, gnu) > > KBUILD_CFLAGS += -Wno-pointer-sign > > KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=) > > KBUILD_CFLAGS += -fno-asynchronous-unwind-tables > > +KBUILD_CFLAGS += -include hidden.h > > > + * When building position independent code with GCC using the -fPIC option, > > + * (or even the -fPIE one on older versions), it will assume that we are > > + * building a dynamic object (either a shared library or an executable) > > that > > + * may have symbol references that can only be resolved at load time. For a > > + * variety of reasons (ELF symbol preemption, the CoW footprint of the > > section > > + * that is modified by the loader), this results in all references to > > symbols > > + * with external linkage to go via entries in the Global Offset Table > > (GOT), > > + * which carries absolute addresses which need to be fixed up when the > > + * executable image is loaded at an offset which is different from its link > > + * time offset. > > + * > > + * Fortunately, there is a way to inform the compiler that such symbol > > + * references will be satisfied at link time rather than at load time, by > > + * giving them 'hidden' visibility. > > + */ > > + > > +#pragma GCC visibility push(hidden) > > BTW., how many such GOT entries did we have before this change, on a typical > kernel? >
None if you are using a recent GCC/binutils that use a relocation type that can be relaxed into an ordinary relative reference by the linker. Older GCC/binutils may emit a couple for the decompressor. > > +ASSERT (SIZEOF(.got) == 0, "Unexpected GOT entries detected!") > > s/ASSERT ( > /ASSERT( > OK > Thanks, > > Ingo
