[PATCH] x86/extable: Fix ex_handler_msr() print condition

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jul 20, 2022 at 03:48:56PM +0300, Stephane Eranian wrote:

> Tested-by: Stephane Eranian <eranian@google.com>

Thanks, I'll queue the below.

---
Subject: x86/extable: Fix ex_handler_msr() print condition
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri, 17 Jun 2022 16:52:06 +0200

On Fri, Jun 17, 2022 at 02:08:52PM +0300, Stephane Eranian wrote:
> Some changes to the way invalid MSR accesses are reported by the
> kernel is causing some problems with messages printed on the
> console.
>
> We have seen several cases of ex_handler_msr() printing invalid MSR
> accesses once but the callstack multiple times causing confusion on
> the console.

> The problem here is that another earlier commit (5.13):
>
> a358f40600b3 ("once: implement DO_ONCE_LITE for non-fast-path "do once" functionality")
>
> Modifies all the pr_*_once() calls to always return true claiming
> that no caller is ever checking the return value of the functions.
>
> This is why we are seeing the callstack printed without the
> associated printk() msg.

Extract the ONCE_IF(cond) part into __ONCE_LTE_IF() and use that to
implement DO_ONCE_LITE_IF() and fix the extable code.

Fixes: a358f40600b3 ("once: implement DO_ONCE_LITE for non-fast-path "do once" functionality")
Reported-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Stephane Eranian <eranian@google.com>
Link: https://lkml.kernel.org/r/YqyVFsbviKjVGGZ9@worktop.programming.kicks-ass.net
---
 arch/x86/mm/extable.c     |   16 +++++++++-------
 include/linux/once_lite.h |   20 ++++++++++++++++----
 2 files changed, 25 insertions(+), 11 deletions(-)

--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -94,16 +94,18 @@ static bool ex_handler_copy(const struct
 static bool ex_handler_msr(const struct exception_table_entry *fixup,
 			   struct pt_regs *regs, bool wrmsr, bool safe, int reg)
 {
-	if (!safe && wrmsr &&
-	    pr_warn_once("unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x) at rIP: 0x%lx (%pS)\n",
-			 (unsigned int)regs->cx, (unsigned int)regs->dx,
-			 (unsigned int)regs->ax,  regs->ip, (void *)regs->ip))
+	if (__ONCE_LITE_IF(!safe && wrmsr)) {
+		pr_warn("unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x) at rIP: 0x%lx (%pS)\n",
+			(unsigned int)regs->cx, (unsigned int)regs->dx,
+			(unsigned int)regs->ax,  regs->ip, (void *)regs->ip);
 		show_stack_regs(regs);
+	}
 
-	if (!safe && !wrmsr &&
-	    pr_warn_once("unchecked MSR access error: RDMSR from 0x%x at rIP: 0x%lx (%pS)\n",
-			 (unsigned int)regs->cx, regs->ip, (void *)regs->ip))
+	if (__ONCE_LITE_IF(!safe && !wrmsr)) {
+		pr_warn("unchecked MSR access error: RDMSR from 0x%x at rIP: 0x%lx (%pS)\n",
+			(unsigned int)regs->cx, regs->ip, (void *)regs->ip);
 		show_stack_regs(regs);
+	}
 
 	if (!wrmsr) {
 		/* Pretend that the read succeeded and returned 0. */
--- a/include/linux/once_lite.h
+++ b/include/linux/once_lite.h
@@ -9,15 +9,27 @@
  */
 #define DO_ONCE_LITE(func, ...)						\
 	DO_ONCE_LITE_IF(true, func, ##__VA_ARGS__)
-#define DO_ONCE_LITE_IF(condition, func, ...)				\
+
+#define __ONCE_LITE_IF(condition)					\
 	({								\
 		static bool __section(".data.once") __already_done;	\
-		bool __ret_do_once = !!(condition);			\
+		bool __ret_cond = !!(condition);			\
+		bool __ret_once = false;				\
 									\
-		if (unlikely(__ret_do_once && !__already_done)) {	\
+		if (unlikely(__ret_cond && !__already_done)) {		\
 			__already_done = true;				\
-			func(__VA_ARGS__);				\
+			__ret_once = true;				\
 		}							\
+		unlikely(__ret_once);					\
+	})
+
+#define DO_ONCE_LITE_IF(condition, func, ...)				\
+	({								\
+		bool __ret_do_once = !!(condition);			\
+									\
+		if (__ONCE_LITE_IF(__ret_do_once))			\
+			func(__VA_ARGS__);				\
+									\
 		unlikely(__ret_do_once);				\
 	})