Re: [RFC PATCH 4/6] arm64: mm: add helper to fill execmem with trapping instructions

[Mike Rapoport <rppt@kernel.org>]

On Fri, Jun 19, 2026 at 11:54:25AM +0100, Ryan Roberts wrote:
> On 11/06/2026 14:01, Adrian Barnaś wrote:
> > Implement the architecture-specific execmem_fill_trapping_insns() helper
> > to poison executable memory regions.
> > 
> > When CONFIG_ARCH_HAS_EXECMEM_ROX is enabled, the execmem subsystem
> > requires a way to fill unused or freed executable memory with
> > architecture-specific trapping instructions. This implementation fills
> > the specified region with AARCH64_BREAK_FAULT instructions and flushes
> > the icache to ensure the traps are immediately visible to execution.
> > 
> > Signed-off-by: Adrian Barnaś <abarnas@google.com>
> > ---
> >  arch/arm64/mm/init.c | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> > 
> > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> > index c673a9a839dd..71aa745e0bef 100644
> > --- a/arch/arm64/mm/init.c
> > +++ b/arch/arm64/mm/init.c
> > @@ -408,6 +408,20 @@ void dump_mem_limit(void)
> >  }
> >  
> >  #ifdef CONFIG_EXECMEM
> > +
> > +#ifdef CONFIG_ARCH_HAS_EXECMEM_ROX
> > +void execmem_fill_trapping_insns(void *ptr, size_t size)
> > +{
> > +	int nr_inst = size / AARCH64_INSN_SIZE;
> 
> The x86 instruction is 1 byte, so it can exactly fill any provided buffer. For
> arm64, the instruction is 4 bytes so we can only exactly fill the buffer if it's
> size is 4 byte aligned.
> 
> I'm guessing that in practice, size will always be page aligned so we are good?

The size is always page aligned:

void *execmem_alloc(enum execmem_type type, size_t size)
{
	...

	size = PAGE_ALIGN(size);

-- 
Sincerely yours,
Mike.