Re: [PATCH v2 2/3] x86/boot/compressed: force hidden visibility for all symbol references

[Ard Biesheuvel]

On Sun, 24 May 2020 at 17:12, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>
> * Ard Biesheuvel <ardb@xxxxxxxxxx> 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 <ardb@xxxxxxxxxx>
> > ---
> >  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