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SubjectRe: [PATCH v2 1/3] x86/boot/compressed: move .got.plt entries out of the .got section

* 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

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