Introduce a user space unwinder API which provides a generic way to
unwind user stacks.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 arch/Kconfig                |  7 +++
 include/linux/unwind_user.h | 41 +++++++++++++++
 kernel/Makefile             |  1 +
 kernel/unwind/Makefile      |  1 +
 kernel/unwind/user.c        | 99 +++++++++++++++++++++++++++++++++++++
 5 files changed, 149 insertions(+)
 create mode 100644 include/linux/unwind_user.h
 create mode 100644 kernel/unwind/Makefile
 create mode 100644 kernel/unwind/user.c

diff --git a/arch/Kconfig b/arch/Kconfig
index 7a95c1052cd5..ee8ec97ea0ef 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -435,6 +435,13 @@ config HAVE_HARDLOCKUP_DETECTOR_ARCH
 	  It uses the same command line parameters, and sysctl interface,
 	  as the generic hardlockup detectors.
 
+config UNWIND_USER
+	bool
+
+config HAVE_UNWIND_USER_FP
+	bool
+	select UNWIND_USER
+
 config HAVE_PERF_REGS
 	bool
 	help
diff --git a/include/linux/unwind_user.h b/include/linux/unwind_user.h
new file mode 100644
index 000000000000..9d28db06f33f
--- /dev/null
+++ b/include/linux/unwind_user.h
@@ -0,0 +1,41 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_UNWIND_USER_H
+#define _LINUX_UNWIND_USER_H
+
+#include <linux/types.h>
+
+enum unwind_user_type {
+	UNWIND_USER_TYPE_FP,
+};
+
+struct unwind_stacktrace {
+	unsigned int	nr;
+	unsigned long	*entries;
+};
+
+struct unwind_user_frame {
+	s32 cfa_off;
+	s32 ra_off;
+	s32 fp_off;
+	bool use_fp;
+};
+
+struct unwind_user_state {
+	unsigned long ip;
+	unsigned long sp;
+	unsigned long fp;
+	enum unwind_user_type type;
+	bool done;
+};
+
+/* Synchronous interfaces: */
+
+int unwind_user_start(struct unwind_user_state *state);
+int unwind_user_next(struct unwind_user_state *state);
+
+int unwind_user(struct unwind_stacktrace *trace, unsigned int max_entries);
+
+#define for_each_user_frame(state) \
+	for (unwind_user_start((state)); !(state)->done; unwind_user_next((state)))
+
+#endif /* _LINUX_UNWIND_USER_H */
diff --git a/kernel/Makefile b/kernel/Makefile
index 87866b037fbe..6cb4b0e02a34 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -50,6 +50,7 @@ obj-y += rcu/
 obj-y += livepatch/
 obj-y += dma/
 obj-y += entry/
+obj-y += unwind/
 obj-$(CONFIG_MODULES) += module/
 
 obj-$(CONFIG_KCMP) += kcmp.o
diff --git a/kernel/unwind/Makefile b/kernel/unwind/Makefile
new file mode 100644
index 000000000000..349ce3677526
--- /dev/null
+++ b/kernel/unwind/Makefile
@@ -0,0 +1 @@
+ obj-$(CONFIG_UNWIND_USER) += user.o
diff --git a/kernel/unwind/user.c b/kernel/unwind/user.c
new file mode 100644
index 000000000000..54b989810a0e
--- /dev/null
+++ b/kernel/unwind/user.c
@@ -0,0 +1,99 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+* Generic interfaces for unwinding user space
+*
+* Copyright (C) 2024 Josh Poimboeuf <jpoimboe@kernel.org>
+*/
+#include <linux/kernel.h>
+#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
+};
+
+int unwind_user_next(struct unwind_user_state *state)
+{
+	struct unwind_user_frame _frame;
+	struct unwind_user_frame *frame = &_frame;
+	unsigned long prev_ip, cfa, fp, ra = 0;
+
+	if (state->done)
+		return -EINVAL;
+
+	prev_ip = state->ip;
+
+	switch (state->type) {
+	case UNWIND_USER_TYPE_FP:
+		frame = &fp_frame;
+		break;
+	default:
+		BUG();
+	}
+
+	cfa = (frame->use_fp ? state->fp : state->sp) + frame->cfa_off;
+
+	if (frame->ra_off && get_user(ra, (unsigned long __user *)(cfa + frame->ra_off)))
+		goto the_end;
+
+	if (ra == prev_ip)
+		goto the_end;
+
+	if (frame->fp_off && get_user(fp, (unsigned long __user *)(cfa + frame->fp_off)))
+		goto the_end;
+
+	state->sp = cfa;
+	state->ip = ra;
+	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)
+{
+	struct pt_regs *regs = task_pt_regs(current);
+
+	memset(state, 0, sizeof(*state));
+
+	if (!current->mm) {
+		state->done = true;
+		return -EINVAL;
+	}
+
+	state->type = UNWIND_USER_TYPE_FP;
+
+	state->sp = user_stack_pointer(regs);
+	state->ip = instruction_pointer(regs);
+	state->fp = frame_pointer(regs);
+
+	return 0;
+}
+
+int unwind_user(struct unwind_stacktrace *trace, unsigned int max_entries)
+{
+	struct unwind_user_state state;
+
+	trace->nr = 0;
+
+	if (!max_entries)
+		return -EINVAL;
+
+	if (!current->mm)
+		return 0;
+
+	for_each_user_frame(&state) {
+		trace->entries[trace->nr++] = state.ip;
+		if (trace->nr >= max_entries)
+			break;
+	}
+
+	return 0;
+}
-- 
2.47.0