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

[Masami Hiramatsu]

On Thu,  2 Jun 2016 23:26:23 -0400
David Long <dave.long at linaro.org> wrote:

> From: Sandeepa Prabhu <sandeepa.s.prabhu at gmail.com>
> 
> 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 <sandeepa.s.prabhu at gmail.com>
> Signed-off-by: David A. Long <dave.long at linaro.org>
> ---
>  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!

> +
> +	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
> 


-- 
Masami Hiramatsu <mhiramat at kernel.org>