[PATCH v3 04/10] x86/traps: Allow custom fixups in handle_bug()

[Peter Zijlstra <peterz@infradead.org>]

The normal fixup in handle_bug() is simply continuing at the next
instruction. However upcomming patches make this the wrong thing, so
allow handlers (specifically handle_cfi_failure()) to over-ride
regs->ip.

The callchain is such that the fixup needs to be done before it is
determined if the exception is fatal, as such, revert any changes in
that case.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/traps.c |   12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -287,11 +287,12 @@ static inline void handle_invalid_op(str
 
 static noinstr bool handle_bug(struct pt_regs *regs)
 {
+	unsigned long addr = regs->ip;
 	bool handled = false;
 	int ud_type, ud_len;
 	s32 ud_imm;
 
-	ud_type = decode_bug(regs->ip, &ud_imm, &ud_len);
+	ud_type = decode_bug(addr, &ud_imm, &ud_len);
 	if (ud_type == BUG_NONE)
 		return handled;
 
@@ -315,7 +316,8 @@ static noinstr bool handle_bug(struct pt
 	switch (ud_type) {
 	case BUG_EA:
 		if (handle_cfi_failure(regs) == BUG_TRAP_TYPE_WARN) {
-			regs->ip += ud_len;
+			if (regs->ip == addr)
+				regs->ip += ud_len;
 			handled = true;
 		}
 		break;
@@ -323,7 +325,8 @@ static noinstr bool handle_bug(struct pt
 	case BUG_UD2:
 		if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN ||
 		    handle_cfi_failure(regs) == BUG_TRAP_TYPE_WARN) {
-			regs->ip += ud_len;
+			if (regs->ip == addr)
+				regs->ip += ud_len;
 			handled = true;
 		}
 		break;
@@ -340,6 +343,9 @@ static noinstr bool handle_bug(struct pt
 		break;
 	}
 
+	if (!handled && regs->ip != addr)
+		regs->ip = addr;
+
 	if (regs->flags & X86_EFLAGS_IF)
 		raw_local_irq_disable();
 	instrumentation_end();