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

[Jens Remus <jremus@linux.ibm.com>]

Add support for unwinding of user space using back chain to the
unwind user interface.  Use it as secondary fallback for unwinding
using SFrame, if that fails and the primary fallback using frame
pointer is not available.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>
---
 arch/Kconfig                          |  6 ++++++
 include/linux/unwind_user_backchain.h | 17 +++++++++++++++++
 include/linux/unwind_user_types.h     |  1 +
 kernel/unwind/Makefile                |  1 +
 kernel/unwind/user.c                  | 24 +++++++++++++++++++++---
 kernel/unwind/user_backchain.c        | 13 +++++++++++++
 6 files changed, 59 insertions(+), 3 deletions(-)
 create mode 100644 include/linux/unwind_user_backchain.h
 create mode 100644 kernel/unwind/user_backchain.c

diff --git a/arch/Kconfig b/arch/Kconfig
index 9e28dffe42cb..4fe16ad6f053 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -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
diff --git a/include/linux/unwind_user_backchain.h b/include/linux/unwind_user_backchain.h
new file mode 100644
index 000000000000..daae74c97c54
--- /dev/null
+++ b/include/linux/unwind_user_backchain.h
@@ -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 */
diff --git a/include/linux/unwind_user_types.h b/include/linux/unwind_user_types.h
index 57fd16e314cf..41b1bc082cb1 100644
--- a/include/linux/unwind_user_types.h
+++ b/include/linux/unwind_user_types.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 {
diff --git a/kernel/unwind/Makefile b/kernel/unwind/Makefile
index 146038165865..38cef261abcb 100644
--- a/kernel/unwind/Makefile
+++ b/kernel/unwind/Makefile
@@ -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
diff --git a/kernel/unwind/user.c b/kernel/unwind/user.c
index ee00d39d2a8e..3c3f75bc146b 100644
--- a/kernel/unwind/user.c
+++ b/kernel/unwind/user.c
@@ -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;
 
diff --git a/kernel/unwind/user_backchain.c b/kernel/unwind/user_backchain.c
new file mode 100644
index 000000000000..5b60a3d4f34f
--- /dev/null
+++ b/kernel/unwind/user_backchain.c
@@ -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);
+}
-- 
2.48.1