Add optional support for user space frame pointer unwinding.  If
supported, the arch needs to enable CONFIG_HAVE_UNWIND_USER_FP and
define ARCH_INIT_USER_FP_FRAME.

By encoding the frame offsets in struct unwind_user_frame, much of this
code can also be reused for future unwinder implementations like sframe.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 arch/Kconfig                      |  4 +++
 include/asm-generic/unwind_user.h |  9 ++++++
 include/linux/unwind_user_types.h |  1 +
 kernel/unwind/user.c              | 49 +++++++++++++++++++++++++++++--
 4 files changed, 60 insertions(+), 3 deletions(-)
 create mode 100644 include/asm-generic/unwind_user.h

diff --git a/arch/Kconfig b/arch/Kconfig
index c6fa2b3ecbc6..cf996cbb8142 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -438,6 +438,10 @@ config HAVE_HARDLOCKUP_DETECTOR_ARCH
 config UNWIND_USER
 	bool
 
+config HAVE_UNWIND_USER_FP
+	bool
+	select UNWIND_USER
+
 config AS_SFRAME
 	def_bool $(as-instr,.cfi_sections .sframe\n.cfi_startproc\n.cfi_endproc)
 
diff --git a/include/asm-generic/unwind_user.h b/include/asm-generic/unwind_user.h
new file mode 100644
index 000000000000..832425502fb3
--- /dev/null
+++ b/include/asm-generic/unwind_user.h
@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_GENERIC_UNWIND_USER_H
+#define _ASM_GENERIC_UNWIND_USER_H
+
+#ifndef ARCH_INIT_USER_FP_FRAME
+ #define ARCH_INIT_USER_FP_FRAME
+#endif
+
+#endif /* _ASM_GENERIC_UNWIND_USER_H */
diff --git a/include/linux/unwind_user_types.h b/include/linux/unwind_user_types.h
index 6ed1b4ae74e1..65bd070eb6b0 100644
--- a/include/linux/unwind_user_types.h
+++ b/include/linux/unwind_user_types.h
@@ -6,6 +6,7 @@
 
 enum unwind_user_type {
 	UNWIND_USER_TYPE_NONE,
+	UNWIND_USER_TYPE_FP,
 };
 
 struct unwind_stacktrace {
diff --git a/kernel/unwind/user.c b/kernel/unwind/user.c
index 456539635e49..73fd4e150dfd 100644
--- a/kernel/unwind/user.c
+++ b/kernel/unwind/user.c
@@ -6,6 +6,18 @@
 #include <linux/sched.h>
 #include <linux/sched/task_stack.h>
 #include <linux/unwind_user.h>
+#include <linux/uaccess.h>
+#include <asm/unwind_user.h>
+
+static struct unwind_user_frame fp_frame = {
+	ARCH_INIT_USER_FP_FRAME
+};
+
+static inline bool fp_state(struct unwind_user_state *state)
+{
+	return IS_ENABLED(CONFIG_HAVE_UNWIND_USER_FP) &&
+	       state->type == UNWIND_USER_TYPE_FP;
+}
 
 int unwind_user_next(struct unwind_user_state *state)
 {
@@ -13,8 +25,36 @@ int unwind_user_next(struct unwind_user_state *state)
 	struct unwind_user_frame *frame = &_frame;
 	unsigned long cfa = 0, fp, ra = 0;
 
-	/* no implementation yet */
-	-EINVAL;
+	if (state->done)
+		return -EINVAL;
+
+	if (fp_state(state))
+		frame = &fp_frame;
+	else
+		goto the_end;
+
+	cfa = (frame->use_fp ? state->fp : state->sp) + frame->cfa_off;
+
+	/* stack going in wrong direction? */
+	if (cfa <= state->sp)
+		goto the_end;
+
+	if (get_user(ra, (unsigned long *)(cfa + frame->ra_off)))
+		goto the_end;
+
+	if (frame->fp_off && get_user(fp, (unsigned long __user *)(cfa + frame->fp_off)))
+		goto the_end;
+
+	state->ip = ra;
+	state->sp = cfa;
+	if (frame->fp_off)
+		state->fp = fp;
+
+	return 0;
+
+the_end:
+	state->done = true;
+	return -EINVAL;
 }
 
 int unwind_user_start(struct unwind_user_state *state)
@@ -28,7 +68,10 @@ int unwind_user_start(struct unwind_user_state *state)
 		return -EINVAL;
 	}
 
-	state->type = UNWIND_USER_TYPE_NONE;
+	if (IS_ENABLED(CONFIG_HAVE_UNWIND_USER_FP))
+		state->type = UNWIND_USER_TYPE_FP;
+	else
+		state->type = UNWIND_USER_TYPE_NONE;
 
 	state->ip = instruction_pointer(regs);
 	state->sp = user_stack_pointer(regs);
-- 
2.48.1