arm64: support single-step and breakpoint handler hooks

AArch64 Single Steping and Breakpoint debug exceptions will be
used by multiple debug framworks like kprobes & kgdb.

This patch implements the hooks for those frameworks to register
their own handlers for handling breakpoint and single step events.

Reworked the debug exception handler in entry.S: do_dbg to route
software breakpoint (BRK64) exception to do_debug_exception()

Signed-off-by: Sandeepa Prabhu <sandeepa.prabhu@linaro.org>
Signed-off-by: Deepak Saxena <dsaxena@linaro.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

3 files changed, 110 insertions, 1 deletions

diff --git a/arch/arm64/include/asm/debug-monitors.h b/arch/arm64/include/asm/debug-monitors.h
index a2232d07be9d..62314791570c 100644
--- a/arch/arm64/include/asm/debug-monitors.h
+++ b/arch/arm64/include/asm/debug-monitors.h
@@ -62,6 +62,27 @@ struct task_struct;
 
 #define DBG_ARCH_ID_RESERVED    0       /* In case of ptrace ABI updates. */
 
+#define DBG_HOOK_HANDLED        0
+#define DBG_HOOK_ERROR          1
+
+struct step_hook {
+        struct list_head node;
+        int (*fn)(struct pt_regs *regs, unsigned int esr);
+};
+
+void register_step_hook(struct step_hook *hook);
+void unregister_step_hook(struct step_hook *hook);
+
+struct break_hook {
+        struct list_head node;
+        u32 esr_val;
+        u32 esr_mask;
+        int (*fn)(struct pt_regs *regs, unsigned int esr);
+};
+
+void register_break_hook(struct break_hook *hook);
+void unregister_break_hook(struct break_hook *hook);
+
 u8 debug_monitors_arch(void);
 
 void enable_debug_monitors(enum debug_el el);
diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
index 4ae68579031d..636ba8b6240b 100644
--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -187,6 +187,48 @@ static void clear_regs_spsr_ss(struct pt_regs *regs)
         regs->pstate = spsr;
 }
 
+/* EL1 Single Step Handler hooks */
+static LIST_HEAD(step_hook);
+DEFINE_RWLOCK(step_hook_lock);
+
+void register_step_hook(struct step_hook *hook)
+{
+        write_lock(&step_hook_lock);
+        list_add(&hook->node, &step_hook);
+        write_unlock(&step_hook_lock);
+}
+
+void unregister_step_hook(struct step_hook *hook)
+{
+        write_lock(&step_hook_lock);
+        list_del(&hook->node);
+        write_unlock(&step_hook_lock);
+}
+
+/*
+ * Call registered single step handers
+ * 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.
+ */
+static int call_step_hook(struct pt_regs *regs, unsigned int esr)
+{
+        struct step_hook *hook;
+        int retval = DBG_HOOK_ERROR;
+
+        read_lock(&step_hook_lock);
+
+        list_for_each_entry(hook, &step_hook, node)     {
+                retval = hook->fn(regs, esr);
+                if (retval == DBG_HOOK_HANDLED)
+                        break;
+        }
+
+        read_unlock(&step_hook_lock);
+
+        return retval;
+}
+
 static int single_step_handler(unsigned long addr, unsigned int esr,
                                struct pt_regs *regs)
 {
@@ -214,7 +256,9 @@ static int single_step_handler(unsigned long addr, unsigned int esr,
                  */
                 user_rewind_single_step(current);
         } else {
-                /* TODO: route to KGDB */
+                if (call_step_hook(regs, esr) == DBG_HOOK_HANDLED)
+                        return 0;
+
                 pr_warning("Unexpected kernel single-step exception at EL1\n");
                 /*
                  * Re-enable stepping since we know that we will be
@@ -226,11 +270,53 @@ static int single_step_handler(unsigned long addr, unsigned int esr,
         return 0;
 }
 
+/*
+ * Breakpoint handler is re-entrant as another breakpoint can
+ * hit within breakpoint handler, especically in kprobes.
+ * Use reader/writer locks instead of plain spinlock.
+ */
+static LIST_HEAD(break_hook);
+DEFINE_RWLOCK(break_hook_lock);
+
+void register_break_hook(struct break_hook *hook)
+{
+        write_lock(&break_hook_lock);
+        list_add(&hook->node, &break_hook);
+        write_unlock(&break_hook_lock);
+}
+
+void unregister_break_hook(struct break_hook *hook)
+{
+        write_lock(&break_hook_lock);
+        list_del(&hook->node);
+        write_unlock(&break_hook_lock);
+}
+
+static int call_break_hook(struct pt_regs *regs, unsigned int esr)
+{
+        struct break_hook *hook;
+        int (*fn)(struct pt_regs *regs, unsigned int esr) = NULL;
+
+        read_lock(&break_hook_lock);
+        list_for_each_entry(hook, &break_hook, node)
+                if ((esr & hook->esr_mask) == hook->esr_val)
+                        fn = hook->fn;
+        read_unlock(&break_hook_lock);
+
+        return fn ? fn(regs, esr) : DBG_HOOK_ERROR;
+}
+
 static int brk_handler(unsigned long addr, unsigned int esr,
                        struct pt_regs *regs)
 {
         siginfo_t info;
 
+        if (call_break_hook(regs, esr) == DBG_HOOK_HANDLED)
+                return 0;
+
+        pr_warn("unexpected brk exception at %lx, esr=0x%x\n",
+                        (long)instruction_pointer(regs), esr);
+
         if (!user_mode(regs))
                 return -EFAULT;
 
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 4d2c6f3f0c41..32d7fe6c3d6a 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -288,6 +288,8 @@ el1_dbg:
         /*
          * Debug exception handling
          */
+        cmp     x24, #ESR_EL1_EC_BRK64          // if BRK64
+        cinc    x24, x24, eq                    // set bit '0'
         tbz     x24, #0, el1_inv                // EL1 only
         mrs     x0, far_el1
         mov     x2, sp                          // struct pt_regs