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

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Date:   Mon, 15 Jun 2020 16:50:18 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     linux-kernel@...r.kernel.org
Cc:     linux-tip-commits@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86 <x86@...nel.org>
Subject: Re: [tip: x86/entry] x86/entry: Treat BUG/WARN as NMI-like entries

On Fri, Jun 12, 2020 at 07:50:08PM -0000, tip-bot2 for Andy Lutomirski wrote:
> +DEFINE_IDTENTRY_RAW(exc_invalid_op)
>  {
> +	bool rcu_exit;
> +
> +	/*
> +	 * 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.
> +	 */
> +	if (!user_mode(regs) && is_valid_bugaddr(regs->ip)) {

vmlinux.o: warning: objtool: exc_invalid_op()+0x47: call to probe_kernel_read() leaves .noinstr.text section

> +		enum bug_trap_type type;
> +
> +		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();
> +
> +		if (type == BUG_TRAP_TYPE_WARN) {
> +			/* Skip the ud2. */
> +			regs->ip += LEN_UD2;
> +			return;
> +		}
> +
> +		/*
> +		 * 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.
> +		 */
> +	}
> +
> +	rcu_exit = idtentry_enter_cond_rcu(regs);
> +	instrumentation_begin();
>  	handle_invalid_op(regs);
> +	instrumentation_end();
> +	idtentry_exit_cond_rcu(regs, rcu_exit);
>  }


For now something like so will do, but we need a DEFINE_IDTENTRY_foo()
for the whole:

	if (user_mode()) {
		rcu = idtentry_enter_cond_rcu()
		foo_user()
		idtentry_exit_cond_rcu(rcu);
	} else {
		nmi_enter();
		foo_kernel()
		nmi_exit()
	}

thing, we're repeating that far too often.


---

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index af75109485c26..a47e74923c4c8 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -218,21 +218,22 @@ static inline void handle_invalid_op(struct pt_regs *regs)
 
 DEFINE_IDTENTRY_RAW(exc_invalid_op)
 {
-	bool rcu_exit;
-
 	/*
 	 * 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.
 	 */
-	if (!user_mode(regs) && is_valid_bugaddr(regs->ip)) {
-		enum bug_trap_type type;
+	if (!user_mode(regs)) {
+		enum bug_trap_type type = BUG_TRAP_TYPE_NONE;
 
 		nmi_enter();
 		instrumentation_begin();
 		trace_hardirqs_off_finish();
-		type = report_bug(regs->ip, regs);
+
+		if (is_valid_bugaddr(regs->ip))
+			type = report_bug(regs->ip, regs);
+
 		if (regs->flags & X86_EFLAGS_IF)
 			trace_hardirqs_on_prepare();
 		instrumentation_end();
@@ -249,13 +250,16 @@ DEFINE_IDTENTRY_RAW(exc_invalid_op)
 		 * was just a normal #UD, we want to continue onward and
 		 * crash.
 		 */
-	}
+		handle_invalid_op(regs);
+	} else {
+		bool rcu_exit;
 
-	rcu_exit = idtentry_enter_cond_rcu(regs);
-	instrumentation_begin();
-	handle_invalid_op(regs);
-	instrumentation_end();
-	idtentry_exit_cond_rcu(regs, rcu_exit);
+		rcu_exit = idtentry_enter_cond_rcu(regs);
+		instrumentation_begin();
+		handle_invalid_op(regs);
+		instrumentation_end();
+		idtentry_exit_cond_rcu(regs, rcu_exit);
+	}
 }
 
 DEFINE_IDTENTRY(exc_coproc_segment_overrun)