Re: [PATCH 5/8] arm64: alternative: Add support for patching adrp instructions

[Suzuki K Poulose]

On 22/08/16 12:19, Will Deacon wrote:
> On Thu, Aug 18, 2016 at 02:10:29PM +0100, Suzuki K Poulose wrote:
> > adrp uses PC-relative address offset to a page (of 4K size) of
> > a symbol. If it appears in an alternative code patched in, we
> > should adjust the offset to reflect the address where it will
> > be run from. This patch adds support for fixing the offset
> > for adrp instructions.
> >
> > Cc: Will Deacon <will.deacon@xxxxxxx>
> > Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> > Cc: Andre Przywara <andre.przywara@xxxxxxx>
> > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> > ---
> >  arch/arm64/kernel/alternative.c | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> >
> > diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
> > index d2ee1b2..71c6962 100644
> > --- a/arch/arm64/kernel/alternative.c
> > +++ b/arch/arm64/kernel/alternative.c
> > @@ -80,6 +80,19 @@ static u32 get_alt_insn(struct alt_instr *alt, u32 *insnptr, u32 *altinsnptr)
> >  			offset = target - (unsigned long)insnptr;
> >  			insn = aarch64_set_branch_offset(insn, offset);
> >  		}
> > +	} else if (aarch64_insn_is_adrp(insn)) {
> > +		s32 orig_offset, new_offset;
> > +		unsigned long target;
> > +
> > +		/*
> > +		 * If we're replacing an adrp instruction, which uses PC-relative
> > +		 * immediate addressing, adjust the offset to reflect the new
> > +		 * PC. adrp operates on 4K aligned addresses.
> > +		 */
> > +		orig_offset  = aarch64_insn_adrp_get_offset(insn);
> > +		target = ((unsigned long)altinsnptr & ~0xfffUL) + orig_offset;
> > +		new_offset = target - ((unsigned long)insnptr & ~0xfffUL);
>
> The masking with ~0xfffUL might be nicer if you write it as
> align_down(ptr, SZ_4K);

Right, that definitely looks better. Will change it.

> > +		insn = aarch64_insn_adrp_set_offset(insn, new_offset);
> >  	}
> >
> >  	return insn;
>
> I wonder if we shouldn't have a catch-all for any instructions performing
> PC-relative operations here, because silent corruption of the instruction

Correct, which is what happened initially when I didn't have the adrp handling ;-).

> stream is pretty horrible. What other instructions are there? ADR, LDR
> (literal), ... ?

From a quick look, all the instructions under "Load register (literal)" :

i.e,
LDR (literal) for GPR/FP_SIMD/32bit/64bit
LDRSW (literal)
PRFM (literal)

and Data processing instructions - immediate group with PC-relative addressing:

ADR, ADRP

I will add a check to catch the unsupported instructions in the alternative code.

Thanks
Suzuki