[PATCH v4 3/3] arm64/efi: isolate EFI stub from the kernel proper

[Andrey Ryabinin]

2015-10-09 12:43 GMT+03:00 Will Deacon <will.deacon at arm.com>:
> On Fri, Oct 09, 2015 at 12:40:21PM +0300, Andrey Ryabinin wrote:
>> 2015-10-09 12:10 GMT+03:00 Will Deacon <will.deacon at arm.com>:
>> > On Fri, Oct 09, 2015 at 11:12:24AM +0300, Andrey Ryabinin wrote:
>> >> 2015-10-08 22:02 GMT+03:00 Ard Biesheuvel <ard.biesheuvel at linaro.org>:
>> >> > --- a/arch/arm64/kernel/image.h
>> >> > +++ b/arch/arm64/kernel/image.h
>> >> > @@ -59,4 +59,31 @@
>> >> >         _kernel_offset_le       = DATA_LE64(TEXT_OFFSET);       \
>> >> >         _kernel_flags_le        = DATA_LE64(__HEAD_FLAGS);
>> >> >
>> >> > +#ifdef CONFIG_EFI
>> >> > +
>> >> > +/*
>> >> > + * The EFI stub has its own symbol namespace prefixed by __efistub_, to
>> >> > + * isolate it from the kernel proper. The following symbols are legally
>> >> > + * accessed by the stub, so provide some aliases to make them accessible.
>> >> > + * Only include data symbols here, or text symbols of functions that are
>> >> > + * guaranteed to be safe when executed at another offset than they were
>> >> > + * linked at. The routines below are all implemented in assembler in a
>> >> > + * position independent manner
>> >> > + */
>> >> > +__efistub_memcmp               = __pi_memcmp;
>> >> > +__efistub_memchr               = __pi_memchr;
>> >> > +__efistub_memcpy               = __pi_memcpy;
>> >> > +__efistub_memmove              = __pi_memmove;
>> >> > +__efistub_memset               = __pi_memset;
>> >> > +__efistub_strlen               = __pi_strlen;
>> >> > +__efistub_strcmp               = __pi_strcmp;
>> >> > +__efistub_strncmp              = __pi_strncmp;
>> >> > +__efistub___flush_dcache_area  = __pi___flush_dcache_area;
>> >>
>> >> So why we need these __pi_* aliases?
>> >> We could just do __efistub_memcmp = memcmp; Right?
>> >
>> > We *could*, but that defeats the whole purpose of tagging
>> > position-independent functions explicitly in the kernel text.
>> >
>>
>> I just don't get that "the whole purpose of tagging".
>>
>> Yes, the EFI stub is allowed to call only PI kernel functions.
>> But the EFI stub already protected by __efistub_ namespace.
>> If we want to use some new PI function in the stub, we would need add
>> it into this list at first.
>> So that __pi_ namespace doesn't bring any protection or isolation.
>
> What it does it force people making future changes to the __pi_* functions
> to think about the stub, otherwise if the function happens to be
> position-independent when efistub starts using it, it could easily break
> due to some future patch where the author didn't realise that it was
> being used in that manner.
>

That makes sense, thanks.

> Will