[RFC PATCH v1 12/16] unwind_user/backchain: Introduce back chain user space unwinding

@@ -438,6 +438,12 @@ config HAVE_HARDLOCKUP_DETECTOR_ARCH
 config UNWIND_USER
 	bool
 
+config HAVE_UNWIND_USER_BACKCHAIN
+	bool
+	select UNWIND_USER
+	help
+	  The arch supports unwinding of user space using back chain.
+
 config HAVE_UNWIND_USER_FP
 	bool
 	select UNWIND_USER

@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_UNWIND_USER_BACKCHAIN_H
+#define _LINUX_UNWIND_USER_BACKCHAIN_H
+
+struct unwind_user_state;
+
+#ifdef CONFIG_HAVE_UNWIND_USER_BACKCHAIN
+
+extern int unwind_user_backchain_next(struct unwind_user_state *state);
+
+#else /* !CONFIG_HAVE_UNWIND_USER_BACKCHAIN */
+
+static inline int unwind_user_backchain_next(struct unwind_user_state *state) { return -EINVAL; }
+
+#endif /* !CONFIG_HAVE_UNWIND_USER_BACKCHAIN */
+
+#endif /* _LINUX_UNWIND_USER_BACKCHAIN_H */

@@ -14,6 +14,7 @@ enum unwind_user_type {
 	UNWIND_USER_TYPE_FP,
 	UNWIND_USER_TYPE_COMPAT_FP,
 	UNWIND_USER_TYPE_SFRAME,
+	UNWIND_USER_TYPE_BACKCHAIN,
 };
 
 struct unwind_stacktrace {

@@ -1,2 +1,3 @@
  obj-$(CONFIG_UNWIND_USER)		+= user.o deferred.o
+ obj-$(CONFIG_HAVE_UNWIND_USER_BACKCHAIN)	+= user_backchain.o
  obj-$(CONFIG_HAVE_UNWIND_USER_SFRAME)	+= sframe.o

@@ -7,6 +7,7 @@
 #include <linux/sched.h>
 #include <linux/sched/task_stack.h>
 #include <linux/unwind_user.h>
+#include <linux/unwind_user_backchain.h>
 #include <linux/uaccess.h>
 #include <linux/sframe.h>
 

@@ -39,6 +40,12 @@ static inline bool sframe_state(struct unwind_user_state *state)
 	       state->type == UNWIND_USER_TYPE_SFRAME;
 }
 
+static inline bool backchain_state(struct unwind_user_state *state)
+{
+	return IS_ENABLED(CONFIG_HAVE_UNWIND_USER_BACKCHAIN) &&
+	       state->type == UNWIND_USER_TYPE_BACKCHAIN;
+}
+
 #define unwind_get_user_long(to, from, state)				\
 ({									\
 	int __ret;							\

@@ -66,12 +73,20 @@ static int unwind_user_next(struct unwind_user_state *state)
 		/* sframe expects the frame to be local storage */
 		frame = &_frame;
 		if (sframe_find(state->ip, frame, topmost)) {
-			if (!IS_ENABLED(CONFIG_HAVE_UNWIND_USER_FP))
-				goto done;
-			frame = &fp_frame;
+			if (IS_ENABLED(CONFIG_HAVE_UNWIND_USER_FP)) {
+				frame = &fp_frame;
+			} else if (IS_ENABLED(CONFIG_HAVE_UNWIND_USER_BACKCHAIN)) {
+				if (unwind_user_backchain_next(state))
+					goto done;
+				goto done_backchain;
+			}
 		}
 	} else if (fp_state(state)) {
 		frame = &fp_frame;
+	} else if (backchain_state(state)) {
+		if (unwind_user_backchain_next(state))
+			goto done;
+		goto done_backchain;
 	} else {
 		goto done;
 	}

@@ -153,6 +168,7 @@ static int unwind_user_next(struct unwind_user_state *state)
 
 	arch_unwind_user_next(state);
 
+done_backchain:
 	state->topmost = false;
 	return 0;
 

@@ -178,6 +194,8 @@ static int unwind_user_start(struct unwind_user_state *state)
 		state->type = UNWIND_USER_TYPE_SFRAME;
 	else if (IS_ENABLED(CONFIG_HAVE_UNWIND_USER_FP))
 		state->type = UNWIND_USER_TYPE_FP;
+	else if (IS_ENABLED(CONFIG_HAVE_UNWIND_USER_BACKCHAIN))
+		state->type = UNWIND_USER_TYPE_BACKCHAIN;
 	else
 		state->type = UNWIND_USER_TYPE_NONE;
 

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#define pr_fmt(fmt)	"backchain: " fmt
+
+#include <linux/sched.h>
+#include <linux/unwind_user.h>
+#include <linux/unwind_user_backchain.h>
+#include <asm/unwind_user_backchain.h>
+
+int unwind_user_backchain_next(struct unwind_user_state *state)
+{
+	return arch_unwind_user_backchain_next(state);
+}