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

From: [email-masked] (Will Deacon)
Date: Fri, 9 Oct 2015 10:10:37 +0100

On Fri, Oct 09, 2015 at 11:12:24AM +0300, Andrey Ryabinin wrote:
> 2015-10-08 22:02 GMT+03:00 Ard Biesheuvel <[email-masked]>:
> > --- 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.

Will