On Mon, Jun 15, 2020 at 7:50 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> 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.
>
>
Hmm. IMO you're making two changes here, and this is fiddly enough
that it might be worth separating them for bisection purposes.
> ---
>
> 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);
> +
Sigh, this is indeed necessary.
> 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);
But this is really a separate change. This makes handle_invalid_op()
be NMI-like even for non-BUG/WARN kernel #UD entries. One might argue
that this doesn't matter, and that's probably right, but I think it
should be its own change with its own justification. With just my
patch, I intentionally call handle_invalid_op() via the normal
idtentry_enter_cond_rcu() path.
--Andy
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