[PATCH 1/2] x86/stacktrace: do not fail when regs on stack for ORC

[Jiri Slaby]

save_stack_trace_reliable now returns "non reliable" when there are
kernel pt_regs on stack. This means an interrupt or exception happened.
Somewhere down the route. It is a problem for frame pointer unwinder,
because the frame might now have been set up yet when the irq happened,
so it might fail to unwind from the interrupted function.

With ORC, this is not a problem, as ORC has out-of-band data. We can
find ORC data even for the IP in interrupted function and always unwind
one level up.

So introduce `unwind_regs_reliable' which decides if this is an issue
for the currently selected unwinder at all and change the code
accordingly.

Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 arch/x86/include/asm/unwind.h | 21 +++++++++++++++++++++
 arch/x86/kernel/stacktrace.c  | 21 ++++++++-------------
 2 files changed, 29 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/unwind.h b/arch/x86/include/asm/unwind.h
index 5be2fb23825a..2e345b3ef1d4 100644
--- a/arch/x86/include/asm/unwind.h
+++ b/arch/x86/include/asm/unwind.h
@@ -73,6 +73,27 @@ static inline struct pt_regs *unwind_get_entry_regs(struct unwind_state *state)
 }
 #endif
 
+#if defined(CONFIG_UNWINDER_ORC)
+/*
+ * ORC is never afraid of stored regs -- out of band data tell him what to do
+ * at each instruction reliably.
+ */
+static inline bool unwind_regs_reliable(struct pt_regs *regs)
+{
+	return true;
+}
+#else
+/*
+ * Kernel mode registers on the stack indicate an in-kernel interrupt or
+ * exception (e.g., preemption or a page fault), which can make frame pointers
+ * unreliable.
+ */
+static inline bool unwind_regs_reliable(struct pt_regs *regs)
+{
+	return user_mode(regs);
+}
+#endif
+
 #ifdef CONFIG_UNWINDER_ORC
 void unwind_init(void);
 void unwind_module_init(struct module *mod, void *orc_ip, size_t orc_ip_size,
diff --git a/arch/x86/kernel/stacktrace.c b/arch/x86/kernel/stacktrace.c
index 77835bc021c7..221a03e251bb 100644
--- a/arch/x86/kernel/stacktrace.c
+++ b/arch/x86/kernel/stacktrace.c
@@ -103,20 +103,15 @@ __save_stack_trace_reliable(struct stack_trace *trace,
 	     unwind_next_frame(&state)) {
 
 		regs = unwind_get_entry_regs(&state);
-		if (regs) {
-			/*
-			 * Kernel mode registers on the stack indicate an
-			 * in-kernel interrupt or exception (e.g., preemption
-			 * or a page fault), which can make frame pointers
-			 * unreliable.
-			 */
-			if (!user_mode(regs))
-				return -EINVAL;
 
-			/*
-			 * The last frame contains the user mode syscall
-			 * pt_regs.  Skip it and finish the unwind.
-			 */
+		if (regs && !unwind_regs_reliable(regs))
+			return -EINVAL;
+
+		/*
+		 * The last frame contains the user mode syscall pt_regs.  Skip
+		 * it and finish the unwind.
+		 */
+		if (regs && user_mode(regs)) {
 			unwind_next_frame(&state);
 			if (!unwind_done(&state)) {
 				STACKTRACE_DUMP_ONCE(task);
-- 
2.15.0