Re: [tip: x86/entry] x86/entry: Treat BUG/WARN as NMI-like entries

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jun 15, 2020 at 03:46:00PM -0700, Andy Lutomirski wrote:

> In some sense, #UD and #PF are fundamentally different.  #PF wants to
> be able to schedule in the kernel.  #UD wants to be as minimal as
> possible in the kernel but probably still wants to do the nmi_enter()
> dance in case it's an RCU warning and the warning handler code wants
> to use RCU.
> 
> One solution would be to get rid of ud2 for warnings and replace it
> with CALL warning_thunk :)  But I guess I'm okay with your patch.

Well, the raisin we use UD2 is because it's only 2 bytes, which makes
for nice and compact code. Ideally we'd have a single byte #UD
instruction, but alas.

However, I realized that there's another analogy with #PF that does
transfer to #UD. For #PF we state that in-kernel #PF only happens when
RCU is already watching -- by virtue of us being careful in noinstr.

But similarly we can state we only have UD2 when we want to call
WARN/BUG and can forgo exception entry.

That would then result in something like this...

---
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index af75109485c2..8fe57b07a03b 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -216,40 +216,35 @@ static inline void handle_invalid_op(struct pt_regs *regs)
 		      ILL_ILLOPN, error_get_trap_addr(regs));
 }
 
-DEFINE_IDTENTRY_RAW(exc_invalid_op)
+static noinstr bool handle_bug(struct pt_regs *regs)
 {
-	bool rcu_exit;
+	bool handled = false;
 
 	/*
-	 * Handle BUG/WARN like NMIs instead of like normal idtentries:
-	 * if we bugged/warned in a bad RCU context, for example, the last
-	 * thing we want is to BUG/WARN again in the idtentry code, ad
-	 * infinitum.
+	 * All lies, just get the WARN/BUG out.
 	 */
-	if (!user_mode(regs) && is_valid_bugaddr(regs->ip)) {
-		enum bug_trap_type type;
+	instrumentation_begin();
+	if (is_valid_bugaddr(regs->ip) &&
+	    report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
+		regs->ip += LEN_UD2;
+		handled = true;
+	}
+	instrumentation_end();
 
-		nmi_enter();
-		instrumentation_begin();
-		trace_hardirqs_off_finish();
-		type = report_bug(regs->ip, regs);
-		if (regs->flags & X86_EFLAGS_IF)
-			trace_hardirqs_on_prepare();
-		instrumentation_end();
-		nmi_exit();
+	return handled;
+}
 
-		if (type == BUG_TRAP_TYPE_WARN) {
-			/* Skip the ud2. */
-			regs->ip += LEN_UD2;
-			return;
-		}
+DEFINE_IDTENTRY_RAW(exc_invalid_op)
+{
+	bool rcu_exit;
 
-		/*
-		 * Else, if this was a BUG and report_bug returns or if this
-		 * was just a normal #UD, we want to continue onward and
-		 * crash.
-		 */
-	}
+	/*
+	 * We use UD2 as a short encoding for 'CALL __WARN', as such
+	 * handle it before exception entry to avoid recursive WARN
+	 * in case exception entry is the one triggering WARNs.
+	 */
+	if (!user_mode(regs) && handle_bug(regs))
+		return;
 
 	rcu_exit = idtentry_enter_cond_rcu(regs);
 	instrumentation_begin();