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Re: exercising current aarch64 kprobe support with systemtap
- From: William Cohen <wcohen at redhat dot com>
- To: Pratyush Anand <panand at redhat dot com>, David Long <dave dot long at linaro dot org>
- Cc: systemtap at sourceware dot org, Mark Brown <broonie at linaro dot org>
- Date: Fri, 10 Jun 2016 10:03:29 -0400
- Subject: Re: exercising current aarch64 kprobe support with systemtap
- Authentication-results: sourceware.org; auth=none
- References: <befacf57-b8eb-2926-8f4f-742f0f055a4c at redhat dot com> <599229e0-49ad-1c8e-1055-81e38692e5ec at redhat dot com> <575A54D6 dot 2070801 at linaro dot org> <20160610134251 dot GA15590 at dhcppc6>
On 06/10/2016 09:42 AM, Pratyush Anand wrote:
> 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
Hi Dave and Pratyush,
Expecting the instruction to the stp x29, x30, [sp,...] would be pretty fragile. The compiler might not generate that for some very simple function or with certain types of optimization. If the compiler could generate a sentinel word before the start of each function that might be a more robust solution. Maybe something like a breakpoint instruction or something that clearly would not be in an atomic region.
-Will
>
>>
>> -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;
>