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Date:   Sun, 24 May 2020 17:14:26 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     linux-efi <linux-efi@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Arvind Sankar <nivedita@alum.mit.edu>
Subject: Re: [PATCH v2 1/3] x86/boot/compressed: move .got.plt entries out of
 the .got section
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* Ard Biesheuvel <ardb@kernel.org> wrote:

> On Sun, 24 May 2020 at 17:08, Ingo Molnar <mingo@kernel.org> wrote:
> >
> >
> > * Ard Biesheuvel <ardb@kernel.org> wrote:
> >
> > > The .got.plt section contains the part of the GOT which is used by PLT
> > > entries, and which gets updated lazily by the dynamic loader when
> > > function calls are dispatched through those PLT entries.
> > >
> > > On fully linked binaries such as the kernel proper or the decompressor,
> > > this never happens, and so in practice, the .got.plt section consists
> > > only of the first 3 magic entries that are meant to point at the _DYNAMIC
> > > section and at the fixup routine in the loader. However, since we don't
> > > use a dynamic loader, those entries are never populated or used.
> > >
> > > This means that treating those entries like ordinary GOT entries, and
> > > updating their values based on the actual placement of the executable in
> > > memory is completely pointless, and we can just ignore the .got.plt
> > > section entirely, provided that it has no additional entries beyond
> > > the first 3 ones.
> > >
> > > So add an assertion in the linker script to ensure that this assumption
> > > holds, and move the contents out of the [_got, _egot) memory range that
> > > is modified by the GOT fixup routines.
> > >
> > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > > ---
> > >  arch/x86/boot/compressed/vmlinux.lds.S | 13 ++++++++++++-
> > >  1 file changed, 12 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> > > index 0dc5c2b9614b..ce3fdfb93b57 100644
> > > --- a/arch/x86/boot/compressed/vmlinux.lds.S
> > > +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> > > @@ -44,10 +44,13 @@ SECTIONS
> > >       }
> > >       .got : {
> > >               _got = .;
> > > -             KEEP(*(.got.plt))
> > >               KEEP(*(.got))
> > >               _egot = .;
> > >       }
> > > +     .got.plt : {
> > > +             KEEP(*(.got.plt))
> > > +     }
> > > +
> > >       .data : {
> > >               _data = . ;
> > >               *(.data)
> > > @@ -75,3 +78,11 @@ SECTIONS
> > >       . = ALIGN(PAGE_SIZE);   /* keep ZO size page aligned */
> > >       _end = .;
> > >  }
> > > +
> > > +#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
> >
> > Cool debugging check!
> >
> > Minor consistent-style nit:
> >
> > s/ASSERT (
> >  /ASSERT(
> >
> 
> ok
> 
> > Optional: maybe even merge these on a single line, as a special exception to the col80 rule?
> >
> >  #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
> >
> > But fine either way.
> >
> 
> SIZEOF(.got.plt) <= 0x18
> 
> could be used as well, given that the section is either empty, or has
> at least the 3 magic entries.

This this has bitten us before, so I think the precise assert is better.

Two-line version is OK too.

Thanks,

	Ingo