[PATCH v6 02/18] unwind_user: Add frame pointer support

Steven Rostedt
2025-04-25T14:56:14Z

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>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
arch/Kconfig | 4 +++
include/asm-generic/unwind_user.h | 9 ++++++
include/linux/unwind_user_types.h | 1 +
kernel/unwind/user.c | 51 +++++++++++++++++++++++++++++--
4 files changed, 63 insertions(+), 2 deletions(-)
create mode 100644 include/asm-generic/unwind_user.h

diff --git a/arch/Kconfig b/arch/Kconfig
index ccbcead9fac0..0e3844c0e200 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 HAVE_PERF_REGS
bool
help
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 ddf68750b3cb..73fd4e150dfd 100644
--- a/kernel/unwind/user.c
+++ b/kernel/unwind/user.c
@@ -6,10 +6,54 @@
#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)
{
- /* no implementation yet */
+ struct unwind_user_frame _frame;
+ struct unwind_user_frame *frame = &_frame;
+ unsigned long cfa = 0, fp, ra = 0;
+
+ 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;
}

@@ -24,7 +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);
--
2.47.2