Re: exercising current aarch64 kprobe support with systemtap

[Pratyush Anand <panand at redhat dot com>]

On 10/06/2016:01:49:10 AM, David Long wrote:
> Attached are incremental diffs I hope will fix the latest systemtap
> failures, without abandoning atomic sequence checking.  I'm trying to avoid
> the hex constants but I don't think the insn.c functions help in this case.

It will save us from current problem by checking "stp x29,x30,[sp,...]"
instruction and returning false if matches. However, we will have to find some
way to recognize .word instructions.

* An assembly function may not start with "stp x29,x30,[sp,...]", e.g.
 __dma_map_area(), _cpu_resume etc. However, it could be least likely that a
 .word instruction exists before start of assembly function and that too
 contains a word value which could be misleading.

* But major issue is, what if someone instruments a kprobe at an address which
 contains  .word values. Instruction will never hit, so probe function will not
 be called, but when real code reads that .word value, it reads a wrong value.

Can GCC provide some compiler option where .word values are located into a
specific area?

~Pratyush

> 
> -dl
> 

> diff --git a/arch/arm64/kernel/kprobes-arm64.c b/arch/arm64/kernel/kprobes-arm64.c
> index 28b9c5b..36b4ea5 100644
> --- a/arch/arm64/kernel/kprobes-arm64.c
> +++ b/arch/arm64/kernel/kprobes-arm64.c
> @@ -127,7 +127,9 @@ is_probed_address_atomic(kprobe_opcode_t *scan_start, kprobe_opcode_t *scan_end)
>  		 * atomic region starts from exclusive load and ends with
>  		 * exclusive store.
>  		 */
> -		if (aarch64_insn_is_store_ex(le32_to_cpu(*scan_start)))
> +		if ((le32_to_cpu(*scan_start) & 0xffc07fff) == 0xa9807bfd)
> +			return false;
> +		else if (aarch64_insn_is_store_ex(le32_to_cpu(*scan_start)))
>  			return false;
>  		else if (aarch64_insn_is_load_ex(le32_to_cpu(*scan_start)))
>  			return true;