[PATCH v2 10/11] arm64: debug: split bkpt32 exception entry

[Ada Couprie Diaz]

Currently all debug exceptions share common entry code and are routed
to `do_debug_exception()`, which calls dynamically-registered
handlers for each specific debug exception. This is unfortunate as
different debug exceptions have different entry handling requirements,
and it would be better to handle these distinct requirements earlier.

The BKPT32 exception can only be triggered by a BKPT instruction. Thus,
we know that the PC is a legitimate address and isn't being used to train
a branch predictor with a bogus address : we don't need to call
`arm64_apply_bp_hardening()`.

The handler for this exception only pends a signal and doesn't depend
on any per-CPU state : we don't need to inhibit preemption, nor do we
need to keep the DAIF exceptions masked, so we can unmask them earlier.

Split the BKPT32 exception entry and adjust function signatures and its
behaviour to match its relaxed constraints compared to other
debug exceptions.

This replaces the last usage of `el0_dbg()`, so remove it.

Signed-off-by: Ada Couprie Diaz <ada.coupriediaz at arm.com>
---
 arch/arm64/include/asm/exception.h |  1 +
 arch/arm64/kernel/debug-monitors.c |  9 +++++++++
 arch/arm64/kernel/entry-common.c   | 21 +++++++++------------
 3 files changed, 19 insertions(+), 12 deletions(-)

diff --git a/arch/arm64/include/asm/exception.h b/arch/arm64/include/asm/exception.h
index af9b521e35f9..4ca43d4eaf98 100644
--- a/arch/arm64/include/asm/exception.h
+++ b/arch/arm64/include/asm/exception.h
@@ -66,6 +66,7 @@ void do_softstep(unsigned long esr, struct pt_regs *regs);
 void do_watchpoint(unsigned long addr, unsigned long esr,
 			struct pt_regs *regs);
 void do_brk64(unsigned long esr, struct pt_regs *regs);
+void do_bkpt32(unsigned long esr, struct pt_regs *regs);
 void do_fpsimd_acc(unsigned long esr, struct pt_regs *regs);
 void do_sve_acc(unsigned long esr, struct pt_regs *regs);
 void do_sme_acc(unsigned long esr, struct pt_regs *regs);
diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
index 94779c957b70..53f0b2281f14 100644
--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -298,6 +298,15 @@ void do_brk64(unsigned long esr, struct pt_regs *regs)
 }
 NOKPROBE_SYMBOL(do_brk64);
 
+#ifdef CONFIG_COMPAT
+/* BRKPT exception always traps to be handled. */
+void do_bkpt32(unsigned long esr, struct pt_regs *regs)
+{
+	arm64_notify_die("aarch32 BKPT", regs, SIGTRAP, TRAP_BRKPT, regs->pc, esr);
+}
+NOKPROBE_SYMBOL(do_bkpt32);
+#endif /* CONFIG_COMPAT */
+
 int aarch32_break_handler(struct pt_regs *regs)
 {
 	u32 arm_instr;
diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index 424d072659b5..d7bc0b731424 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -845,17 +845,6 @@ static void noinstr el0_brk64(struct pt_regs *regs, unsigned long esr)
 	exit_to_user_mode(regs);
 }
 
-static void noinstr el0_dbg(struct pt_regs *regs, unsigned long esr)
-{
-	/* Only watchpoints write FAR_EL1, otherwise its UNKNOWN */
-	unsigned long far = read_sysreg(far_el1);
-
-	enter_from_user_mode(regs);
-	do_debug_exception(far, esr, regs);
-	local_daif_restore(DAIF_PROCCTX);
-	exit_to_user_mode(regs);
-}
-
 static void noinstr el0_svc(struct pt_regs *regs)
 {
 	enter_from_user_mode(regs);
@@ -1015,6 +1004,14 @@ static void noinstr el0_svc_compat(struct pt_regs *regs)
 	exit_to_user_mode(regs);
 }
 
+static void noinstr el0_bkpt32(struct pt_regs *regs, unsigned long esr)
+{
+	enter_from_user_mode(regs);
+	local_daif_restore(DAIF_PROCCTX);
+	do_bkpt32(esr, regs);
+	exit_to_user_mode(regs);
+}
+
 asmlinkage void noinstr el0t_32_sync_handler(struct pt_regs *regs)
 {
 	unsigned long esr = read_sysreg(esr_el1);
@@ -1058,7 +1055,7 @@ asmlinkage void noinstr el0t_32_sync_handler(struct pt_regs *regs)
 		el0_watchpt(regs, esr);
 		break;
 	case ESR_ELx_EC_BKPT32:
-		el0_dbg(regs, esr);
+		el0_bkpt32(regs, esr);
 		break;
 	default:
 		el0_inv(regs, esr);
-- 
2.43.0