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

[Ryan Roberts <ryan.roberts@arm.com>]

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?
Perhaps worth a WARN_ON_ONCE() though?

Thanks,
Ryan

> +	__le32 *updptr = ptr;
> +
> +	for (int i = 0; i < nr_inst; i++)
> +		updptr[i] = cpu_to_le32(AARCH64_BREAK_FAULT);
> +
> +	flush_icache_range((unsigned long)ptr, (unsigned long)ptr + size);
> +}
> +#endif
> +
>  static u64 module_direct_base __ro_after_init = 0;
>  static u64 module_plt_base __ro_after_init = 0;
>