[PATCH 7/8] arm64: Add kernel return probes support (kretprobes)

[David Long <dave.long@linaro.org>]

From: Sandeepa Prabhu <sandeepa.s.prabhu@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@gmail.com>
Signed-off-by: David A. Long <dave.long@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 c395386..72412de 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -82,6 +82,7 @@ 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 0ce4637..a699d03 100644
--- a/arch/arm64/kernel/kprobes.c
+++ b/arch/arm64/kernel/kprobes.c
@@ -533,7 +533,80 @@ int __kprobes longjmp_break_handler(struct kprobe *p, struct pt_regs *regs)
 
 void __kprobes __used *trampoline_probe_handler(struct pt_regs *regs)
 {
-	return (void *) 0;
+	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;
+	}
+
+	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