[PATCH v2 03/11] arm64: debug: call step handlers statically

[Ada Couprie Diaz]

Software stepping checks for the correct handler by iterating over a list
of dynamically registered handlers and calling all of them until one
handles the exception.

This is the only generic way to handle software stepping handlers in arm64
as the exception does not provide an immediate that could be checked,
contrary to software breakpoints.

However, the registration mechanism is not exported and has only
two current users : the KGDB stepping handler, and the uprobe single step
handler.
Given that one comes from user mode and the other from kernel mode, call
the appropriate one by checking the source EL of the exception, if it
is enabled.

Unify the naming of the handler to XXX_singlestep_handler(), making it
clear they are related.

Signed-off-by: Ada Couprie Diaz <ada.coupriediaz at arm.com>
---
 arch/arm64/include/asm/kgdb.h      |  1 +
 arch/arm64/include/asm/uprobes.h   |  1 +
 arch/arm64/kernel/debug-monitors.c | 32 +++++++++++++-----------------
 arch/arm64/kernel/kgdb.c           | 17 +++-------------
 arch/arm64/kernel/probes/uprobes.c |  9 +--------
 5 files changed, 20 insertions(+), 40 deletions(-)

diff --git a/arch/arm64/include/asm/kgdb.h b/arch/arm64/include/asm/kgdb.h
index 82a76b2102fb..fd287ec38bb7 100644
--- a/arch/arm64/include/asm/kgdb.h
+++ b/arch/arm64/include/asm/kgdb.h
@@ -26,6 +26,7 @@ extern int kgdb_fault_expected;
 
 int kgdb_brk_handler(struct pt_regs *regs, unsigned long esr);
 int kgdb_compiled_brk_handler(struct pt_regs *regs, unsigned long esr);
+int kgdb_singlestep_handler(struct pt_regs *regs, unsigned long esr);
 
 #endif /* !__ASSEMBLY__ */
 
diff --git a/arch/arm64/include/asm/uprobes.h b/arch/arm64/include/asm/uprobes.h
index 3659a79a9f32..e44bbef40eca 100644
--- a/arch/arm64/include/asm/uprobes.h
+++ b/arch/arm64/include/asm/uprobes.h
@@ -29,5 +29,6 @@ struct arch_uprobe {
 };
 
 int uprobe_brk_handler(struct pt_regs *regs, unsigned long esr);
+int uprobe_singlestep_handler(struct pt_regs *regs, unsigned long esr);
 
 #endif
diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
index 4ece4a93b872..81b813e16842 100644
--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -200,30 +200,26 @@ void unregister_kernel_step_hook(struct step_hook *hook)
 }
 
 /*
- * Call registered single step handlers
+ * Call single step handlers
  * There is no Syndrome info to check for determining the handler.
- * So we call all the registered handlers, until the right handler is
- * found which returns zero.
+ * However, there is only one possible handler for user and kernel modes, so
+ * check and call the appropriate one if it is enabled.
  */
 static int call_step_hook(struct pt_regs *regs, unsigned long esr)
 {
-	struct step_hook *hook;
-	struct list_head *list;
-	int retval = DBG_HOOK_ERROR;
-
-	list = user_mode(regs) ? &user_step_hook : &kernel_step_hook;
-
-	/*
-	 * Since single-step exception disables interrupt, this function is
-	 * entirely not preemptible, and we can use rcu list safely here.
-	 */
-	list_for_each_entry_rcu(hook, list, node)	{
-		retval = hook->fn(regs, esr);
-		if (retval == DBG_HOOK_HANDLED)
-			break;
+	if (user_mode(regs)) {
+#if CONFIG_UPROBES
+		return uprobe_singlestep_handler(regs, esr);
+#else
+		return DBG_HOOK_ERROR;
+#endif
 	}
 
-	return retval;
+#ifdef CONFIG_KGDB
+	return kgdb_singlestep_handler(regs, esr);
+#else
+	return DBG_HOOK_ERROR;
+#endif
 }
 NOKPROBE_SYMBOL(call_step_hook);
 
diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
index b5a3b9c85a65..8f6ce2ea005c 100644
--- a/arch/arm64/kernel/kgdb.c
+++ b/arch/arm64/kernel/kgdb.c
@@ -250,7 +250,7 @@ int kgdb_compiled_brk_handler(struct pt_regs *regs, unsigned long esr)
 }
 NOKPROBE_SYMBOL(kgdb_compiled_brk_handler);
 
-static int kgdb_step_brk_fn(struct pt_regs *regs, unsigned long esr)
+int kgdb_singlestep_handler(struct pt_regs *regs, unsigned long esr)
 {
 	if (!kgdb_single_step)
 		return DBG_HOOK_ERROR;
@@ -258,11 +258,7 @@ static int kgdb_step_brk_fn(struct pt_regs *regs, unsigned long esr)
 	kgdb_handle_exception(0, SIGTRAP, 0, regs);
 	return DBG_HOOK_HANDLED;
 }
-NOKPROBE_SYMBOL(kgdb_step_brk_fn);
-
-static struct step_hook kgdb_step_hook = {
-	.fn		= kgdb_step_brk_fn
-};
+NOKPROBE_SYMBOL(kgdb_singlestep_handler);
 
 static int __kgdb_notify(struct die_args *args, unsigned long cmd)
 {
@@ -301,13 +297,7 @@ static struct notifier_block kgdb_notifier = {
  */
 int kgdb_arch_init(void)
 {
-	int ret = register_die_notifier(&kgdb_notifier);
-
-	if (ret != 0)
-		return ret;
-
-	register_kernel_step_hook(&kgdb_step_hook);
-	return 0;
+	return register_die_notifier(&kgdb_notifier);
 }
 
 /*
@@ -317,7 +307,6 @@ int kgdb_arch_init(void)
  */
 void kgdb_arch_exit(void)
 {
-	unregister_kernel_step_hook(&kgdb_step_hook);
 	unregister_die_notifier(&kgdb_notifier);
 }
 
diff --git a/arch/arm64/kernel/probes/uprobes.c b/arch/arm64/kernel/probes/uprobes.c
index ad68b4a5974d..fefc990860bc 100644
--- a/arch/arm64/kernel/probes/uprobes.c
+++ b/arch/arm64/kernel/probes/uprobes.c
@@ -182,7 +182,7 @@ int uprobe_brk_handler(struct pt_regs *regs,
 	return DBG_HOOK_ERROR;
 }
 
-static int uprobe_single_step_handler(struct pt_regs *regs,
+int uprobe_singlestep_handler(struct pt_regs *regs,
 				      unsigned long esr)
 {
 	struct uprobe_task *utask = current->utask;
@@ -194,15 +194,8 @@ static int uprobe_single_step_handler(struct pt_regs *regs,
 	return DBG_HOOK_ERROR;
 }
 
-/* uprobe single step handler hook */
-static struct step_hook uprobes_step_hook = {
-	.fn = uprobe_single_step_handler,
-};
-
 static int __init arch_init_uprobes(void)
 {
-	register_user_step_hook(&uprobes_step_hook);
-
 	return 0;
 }
 
-- 
2.43.0