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

[Andrey Ryabinin]

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.