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Date:   Sun, 24 May 2020 17:08:41 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     linux-efi@vger.kernel.org, x86@kernel.org,
        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
Message-ID: <20200524150841.GA11617@gmail.com>
References: <20200523120021.34996-1-ardb@kernel.org>
 <20200523120021.34996-2-ardb@kernel.org>
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* 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(

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.

Thanks,

	Ingo