Re: [PATCH v5 32/36] x86/boot/compressed: Reorganize zero-size section asserts

Kees Cook
2020-08-01T05:35:14Z

On Fri, Jul 31, 2020 at 09:47:55PM -0400, Arvind Sankar wrote:
> On Fri, Jul 31, 2020 at 04:08:16PM -0700, Kees Cook wrote:
> > For readability, move the zero-sized sections to the end after DISCARDS
> > and mark them NOLOAD for good measure.
> >
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> > arch/x86/boot/compressed/vmlinux.lds.S | 42 +++++++++++++++-----------
> > 1 file changed, 25 insertions(+), 17 deletions(-)
> >
> > diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> > index 3c2ee9a5bf43..42dea70a5091 100644
> > --- a/arch/x86/boot/compressed/vmlinux.lds.S
> > +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> > @@ -42,18 +42,16 @@ SECTIONS
> > *(.rodata.*)
> > _erodata = . ;
> > }
> > - .rel.dyn : {
> > - *(.rel.*)
> > - }
> > - .rela.dyn : {
> > - *(.rela.*)
> > - }
> > - .got : {
> > - *(.got)
> > - }
> > .got.plt : {
> > *(.got.plt)
> > }
> > + ASSERT(SIZEOF(.got.plt) == 0 ||
> > +#ifdef CONFIG_X86_64
> > + SIZEOF(.got.plt) == 0x18,
> > +#else
> > + SIZEOF(.got.plt) == 0xc,
> > +#endif
> > + "Unexpected GOT/PLT entries detected!")
> >
> > .data : {
> > _data = . ;
> > @@ -85,13 +83,23 @@ SECTIONS
> > ELF_DETAILS
> >
> > DISCARDS
> > -}
> >
> > -ASSERT(SIZEOF(.got) == 0, "Unexpected GOT entries detected!")
> > -#ifdef CONFIG_X86_64
> > -ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, "Unexpected GOT/PLT entries detected!")
> > -#else
> > -ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0xc, "Unexpected GOT/PLT entries detected!")
> > -#endif
> > + /*
> > + * Sections that should stay zero sized, which is safer to
> > + * explicitly check instead of blindly discarding.
> > + */
> > + .got (NOLOAD) : {
> > + *(.got)
> > + }
> > + ASSERT(SIZEOF(.got) == 0, "Unexpected GOT entries detected!")
> >
> > -ASSERT(SIZEOF(.rel.dyn) == 0 && SIZEOF(.rela.dyn) == 0, "Unexpected run-time relocations detected!")
> > + /* ld.lld does not like .rel* sections being made "NOLOAD". */
> > + .rel.dyn : {
> > + *(.rel.*)
> > + }
> > + ASSERT(SIZEOF(.rel.dyn) == 0, "Unexpected run-time relocations (.rel) detected!")
> > + .rela.dyn : {
> > + *(.rela.*)
> > + }
> > + ASSERT(SIZEOF(.rela.dyn) == 0, "Unexpected run-time relocations (.rela) detected!")
> > +}
> > --
> > 2.25.1
> >
>
> There's no point in marking zero-size sections NOLOAD -- if the ASSERT's
> passed, they won't be present in the file at all anyway.

I did not find that universally true. I found some sections be written
out with a 0 size. Some I could remove from disk with NOLOAD, others did
not like that so much.

> The only section for which there might be a point is .got.plt, which is
> non-empty on 32-bit, and only if it is first moved to the end. That
> saves a few bytes.

What do you mean about "only if it is first moved to the end"? Would it
be zero-sized if it was closer to .text?

--
Kees Cook