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

[Kees Cook]

On Fri, Jul 31, 2020 at 10:53:25PM -0400, Arvind Sankar wrote:
> 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@xxxxxxxxxxxx>
> > > ---
> > >  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.

ld.bfd (if I'm remembering correctly) was extraordinarily upset about it
being at the end. I will retest and report back.

-- 
Kees Cook