[PATCH v3 2/4] arm64: kgdb: disable interrupts while a software step is enabled

[AKASHI Takahiro]

After entering kgdb mode, 'stepi' may unexpectedly breaks the execution
somewhere in el1_irq.

This happens because a debug exception is always enabled in el1_irq
due to the following commit merged in v3.16:
  commit 2a2830703a23 ("arm64: debug: avoid accessing mdscr_el1 on fault
			paths where possible")
A pending interrupt can be taken after kgdb has enabled a software step,
but before a debug exception is actually taken.

This patch enforces interrupts to be masked while single stepping.

Signed-off-by: AKASHI Takahiro <takahiro.akashi at linaro.org>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Will Deacon <will.deacon at arm.com>
Cc: Jason Wessel <jason.wessel at windriver.com>
---
 arch/arm64/kernel/kgdb.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
index b9176b324e5a..fddbc6be3780 100644
--- a/arch/arm64/kernel/kgdb.c
+++ b/arch/arm64/kernel/kgdb.c
@@ -28,6 +28,7 @@
 
 #include <asm/debug-monitors.h>
 #include <asm/insn.h>
+#include <asm/ptrace.h>
 #include <asm/traps.h>
 
 struct dbg_reg_def_t dbg_reg_def[DBG_MAX_REG_NUM] = {
@@ -111,6 +112,8 @@ struct dbg_reg_def_t dbg_reg_def[DBG_MAX_REG_NUM] = {
 	{ "fpcr", 4, -1 },
 };
 
+static DEFINE_PER_CPU(unsigned int, kgdb_pstate);
+
 char *dbg_get_reg(int regno, void *mem, struct pt_regs *regs)
 {
 	if (regno >= DBG_MAX_REG_NUM || regno < 0)
@@ -200,6 +203,10 @@ int kgdb_arch_handle_exception(int exception_vector, int signo,
 		err = 0;
 		break;
 	case 's':
+		/* mask interrupts while single stepping */
+		__this_cpu_write(kgdb_pstate, linux_regs->pstate);
+		linux_regs->pstate |= PSR_I_BIT;
+
 		/*
 		 * Update step address value with address passed
 		 * with step packet.
@@ -242,11 +249,20 @@ NOKPROBE_SYMBOL(kgdb_compiled_brk_fn);
 
 static int kgdb_step_brk_fn(struct pt_regs *regs, unsigned int esr)
 {
+	unsigned int pstate;
+
 	if (!kgdb_single_step)
 		return DBG_HOOK_ERROR;
 
 	kernel_disable_single_step();
 
+	/* restore interrupt mask status */
+	pstate = __this_cpu_read(kgdb_pstate);
+	if (pstate & PSR_I_BIT)
+		regs->pstate |= PSR_I_BIT;
+	else
+		regs->pstate &= ~PSR_I_BIT;
+
 	kgdb_handle_exception(1, SIGTRAP, 0, regs);
 	return 0;
 }
-- 
2.11.1