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 <keesc...@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.
> 
> 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.

Btw, you should move .got.plt also to the end anyway for readability,
it's unused even if non-empty. And with the ASSERT being placed
immediately after it, it's even more distracting from the actual section
layout.

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