[PATCH 2/5] arm64: alternative: Allow immediate branch as alternative instruction

[Jon Medhurst (Tixy)]

On Thu, 2015-03-26 at 22:19 +0000, Marc Zyngier wrote:
> On Thu, 26 Mar 2015 22:03:23 +0000
> Will Deacon <will.deacon at arm.com> wrote:
> 
> > On Thu, Mar 19, 2015 at 01:59:33PM +0000, Marc Zyngier wrote:
> > > Since all immediate branches are PC-relative on Aarch64, these
> > > instructions cannot be used as an alternative with the simplistic
> > > approach we currently have (the immediate has been computed from
> > > the .altinstr_replacement section, and end-up being completely off
> > > if we insert it directly).
> > > 
> > > This patch handles the b and bl instructions in a different way,
> > > using the insn framework to recompute the immediate, and generate
> > > the right displacement.
> > > 
> > > Reviewed-by: Andre Przywara <andre.przywara at arm.com>
> > > Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
> > > ---
> > >  arch/arm64/kernel/alternative.c | 55 +++++++++++++++++++++++++++++++++++++++--
> > >  1 file changed, 53 insertions(+), 2 deletions(-)
> > 
> > [...]
> > 
> > >  static int __apply_alternatives(void *alt_region)
> > >  {
> > >  	struct alt_instr *alt;
> > > @@ -40,16 +83,24 @@ static int __apply_alternatives(void *alt_region)
> > >  	u8 *origptr, *replptr;
> > >  
> > >  	for (alt = region->begin; alt < region->end; alt++) {
> > > +		u32 insn;
> > > +		int i;
> > > +
> > >  		if (!cpus_have_cap(alt->cpufeature))
> > >  			continue;
> > >  
> > > -		BUG_ON(alt->alt_len > alt->orig_len);
> > > +		BUG_ON(alt->alt_len != alt->orig_len);
> > >  
> > >  		pr_info_once("patching kernel code\n");
> > >  
> > >  		origptr = (u8 *)&alt->orig_offset + alt->orig_offset;
> > >  		replptr = (u8 *)&alt->alt_offset + alt->alt_offset;
> > > -		memcpy(origptr, replptr, alt->alt_len);
> > > +
> > > +		for (i = 0; i < alt->alt_len; i += sizeof(insn)) {
> > > +			insn = get_alt_insn(origptr + i, replptr + i);
> > > +			*(u32 *)(origptr + i) = insn;
> > 
> > My brain's not firing on all cylinders right now, but do you need a
> > cpu_to_le32 here?
> 
> I'm not 100% awake myself (probably some acute form of firmwaritis),
> but I suspect you're quite right (get_alt_insn calls aarch64_insn_read,
> which does a le32_to_cpu). Obviously, we need to revert the conversion
> when writing the instruction back.

Isn't aarch64_insn_write the inverse of aarch64_insn_read and more
correct than using cpu_to_le32?

-- 
Tixy