[PATCH 1/7] x86/entry: Fix #UD vs WARN more

Peter Zijlstra
2020-06-18T14:50:53Z

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

Since we use UD2 as a short-cut for 'CALL __WARN', treat it as such.
Have the bare exception handler do the report_bug() thing.

Fixes: 15a416e8aaa7 ("x86/entry: Treat BUG/WARN as NMI-like entries")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
---
arch/x86/kernel/traps.c | 50 +++++++++++++++++++++---------------------------
1 file changed, 22 insertions(+), 28 deletions(-)

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -216,40 +216,34 @@ static inline void handle_invalid_op(str
ILL_ILLOPN, error_get_trap_addr(regs));
}

+static noinstr bool handle_bug(struct pt_regs *regs)
+{
+ bool handled = false;
+
+ /*
+ * All lies, just get the WARN/BUG out.
+ */
+ instrumentation_begin();
+ if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
+ regs->ip += LEN_UD2;
+ handled = true;
+ }
+ instrumentation_end();
+
+ return handled;
+}
+
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.
+ * 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) && is_valid_bugaddr(regs->ip)) {
- 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.
- */
- }
+ if (!user_mode(regs) && handle_bug(regs))
+ return;

rcu_exit = idtentry_enter_cond_rcu(regs);
instrumentation_begin();