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();
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