Subject: [PATCH v13 09/10] arm64: Add kernel return probes support (kretprobes)

From: [email-masked] (David Long)
Date: Wed, 22 Jun 2016 14:16:25 -0400

On 06/07/2016 06:28 AM, Masami Hiramatsu wrote:
> On Thu,  2 Jun 2016 23:26:23 -0400
> David Long <[email-masked]> wrote:
>
>> From: Sandeepa Prabhu <[email-masked]>
>>
>> The pre-handler of this special 'trampoline' kprobe executes the return
>> probe handler functions and restores original return address in ELR_EL1.
>> This way the saved pt_regs still hold the original register context to be
>> carried back to the probed kernel function.
>>
>> Signed-off-by: Sandeepa Prabhu <[email-masked]>
>> Signed-off-by: David A. Long <[email-masked]>
>> ---
>>   arch/arm64/Kconfig          |  1 +
>>   arch/arm64/kernel/kprobes.c | 75 ++++++++++++++++++++++++++++++++++++++++++++-
>>   2 files changed, 75 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 5496b75..47989eb 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -89,6 +89,7 @@ config ARM64
>>   	select HAVE_RCU_TABLE_FREE
>>   	select HAVE_SYSCALL_TRACEPOINTS
>>   	select HAVE_KPROBES
>> +	select HAVE_KRETPROBES if HAVE_KPROBES
>>   	select IOMMU_DMA if IOMMU_SUPPORT
>>   	select IRQ_DOMAIN
>>   	select IRQ_FORCED_THREADING
>> diff --git a/arch/arm64/kernel/kprobes.c b/arch/arm64/kernel/kprobes.c
>> index b35f76f..c116904 100644
>> --- a/arch/arm64/kernel/kprobes.c
>> +++ b/arch/arm64/kernel/kprobes.c
>> @@ -577,7 +577,80 @@ bool arch_within_kprobe_blacklist(unsigned long addr)
>>
>>   void __kprobes __used *trampoline_probe_handler(struct pt_regs *regs)
>>   {
>> -	return NULL;
>> +	struct kretprobe_instance *ri = NULL;
>> +	struct hlist_head *head, empty_rp;
>> +	struct hlist_node *tmp;
>> +	unsigned long flags, orig_ret_addr = 0;
>> +	unsigned long trampoline_address =
>> +		(unsigned long)&kretprobe_trampoline;
>> +
>> +	INIT_HLIST_HEAD(&empty_rp);
>> +	kretprobe_hash_lock(current, &head, &flags);
>> +
>> +	/*
>> +	 * It is possible to have multiple instances associated with a given
>> +	 * task either because multiple functions in the call path have
>> +	 * a return probe installed on them, and/or more than one return
>> +	 * probe was registered for a target function.
>> +	 *
>> +	 * We can handle this because:
>> +	 *     - instances are always inserted at the head of the list
>> +	 *     - when multiple return probes are registered for the same
>> +	 *       function, the first instance's ret_addr will point to the
>> +	 *       real return address, and all the rest will point to
>> +	 *       kretprobe_trampoline
>> +	 */
>> +	hlist_for_each_entry_safe(ri, tmp, head, hlist) {
>> +		if (ri->task != current)
>> +			/* another task is sharing our hash bucket */
>> +			continue;
>> +
>> +		if (ri->rp && ri->rp->handler) {
>> +			__this_cpu_write(current_kprobe, &ri->rp->kp);
>> +			get_kprobe_ctlblk()->kprobe_status = KPROBE_HIT_ACTIVE;
>> +			ri->rp->handler(ri, regs);
>> +			__this_cpu_write(current_kprobe, NULL);
>> +		}
>> +
>> +		orig_ret_addr = (unsigned long)ri->ret_addr;
>> +		recycle_rp_inst(ri, &empty_rp);
>> +
>> +		if (orig_ret_addr != trampoline_address)
>> +			/*
>> +			 * This is the real return address. Any other
>> +			 * instances associated with this task are for
>> +			 * other calls deeper on the call stack
>> +			 */
>> +			break;
>> +	}
>
> This looks not included an improvement done by Syuhei, see
> commit 737480a0d525 ("kprobes/x86: Fix the return address of multiple kretprobes")
>
> Thank you!
>

I've updated the relevant part of this function with the current x86 
version  of the code.

>> +
>> +	kretprobe_assert(ri, orig_ret_addr, trampoline_address);
>> +	/* restore the original return address */
>> +	instruction_pointer(regs) = orig_ret_addr;
>> +	reset_current_kprobe();
>> +	kretprobe_hash_unlock(current, &flags);
>> +
>> +	hlist_for_each_entry_safe(ri, tmp, &empty_rp, hlist) {
>> +		hlist_del(&ri->hlist);
>> +		kfree(ri);
>> +	}
>> +
>> +	/* return 1 so that post handlers not called */
>> +	return (void *) orig_ret_addr;
>> +}
>> +
>> +void __kprobes arch_prepare_kretprobe(struct kretprobe_instance *ri,
>> +				      struct pt_regs *regs)
>> +{
>> +	ri->ret_addr = (kprobe_opcode_t *)regs->regs[30];
>> +
>> +	/* replace return addr (x30) with trampoline */
>> +	regs->regs[30] = (long)&kretprobe_trampoline;
>> +}
>> +
>> +int __kprobes arch_trampoline_kprobe(struct kprobe *p)
>> +{
>> +	return 0;
>>   }
>>
>>   int __init arch_init_kprobes(void)
>> --
>> 2.5.0
>>
>
>

Thanks,
-dl