[RFC PATCH v3 09/17] unwind_user: Enable archs that define CFA = SP_callsite + offset

Jens Remus
2025-12-08T17:16:53Z

Most architectures define their CFA as the value of the stack pointer
(SP) at the call site in the previous frame, as suggested by the DWARF
standard. Therefore the SP at call site can be unwound using an
implicitly assumed value offset from CFA rule with an offset of zero:

.cfi_val_offset <SP>, 0

As a result the SP at call site computes as follows:

SP = CFA

Enable unwinding of user space for architectures, such as s390, which
define their CFA as the value of the SP at the call site in the previous
frame with an offset. Do so by enabling architectures to override the
default SP value offset from CFA of zero with an architecture-specific
one:

.cfi_val_offset <SP>, offset

So that the SP at call site computes as follows:

SP = CFA + offset

Signed-off-by: Jens Remus <jremus@linux.ibm.com>
---

Notes (jremus):
Changes in RFC v2:
- Reword commit message. (Josh)
- Use term "sp_off" instead of "sp_val_off". (Josh)
- Move definition, initialization, and setting of sp_off field to
happen right after the cfa_off field.
- Use SFRAME_SP_OFFSET macro instead of sframe_sp_off() function,
which can be overridden by an architecture, such as s390.
- Drop lengthy sframe_sp_[val_]off() comment.

arch/x86/include/asm/unwind_user.h | 2 ++
include/asm-generic/Kbuild | 1 +
include/asm-generic/unwind_user_sframe.h | 12 ++++++++++++
include/linux/unwind_user_types.h | 1 +
kernel/unwind/sframe.c | 2 ++
kernel/unwind/user.c | 11 ++++++-----
6 files changed, 24 insertions(+), 5 deletions(-)
create mode 100644 include/asm-generic/unwind_user_sframe.h

diff --git a/arch/x86/include/asm/unwind_user.h b/arch/x86/include/asm/unwind_user.h
index 2dfb5ef11e36..d70ffd7bbdb7 100644
--- a/arch/x86/include/asm/unwind_user.h
+++ b/arch/x86/include/asm/unwind_user.h
@@ -21,6 +21,7 @@ static inline int unwind_user_word_size(struct pt_regs *regs)

#define ARCH_INIT_USER_FP_FRAME(ws) \
.cfa_off = 2*(ws), \
+ .sp_off = 0, \
.ra_off = -1*(ws), \
.fp_off = -2*(ws), \
.use_fp = true, \
@@ -28,6 +29,7 @@ static inline int unwind_user_word_size(struct pt_regs *regs)

#define ARCH_INIT_USER_FP_ENTRY_FRAME(ws) \
.cfa_off = 1*(ws), \
+ .sp_off = 0, \
.ra_off = -1*(ws), \
.fp_off = 0, \
.use_fp = false, \
diff --git a/include/asm-generic/Kbuild b/include/asm-generic/Kbuild
index 295c94a3ccc1..b1d448ef4a50 100644
--- a/include/asm-generic/Kbuild
+++ b/include/asm-generic/Kbuild
@@ -60,6 +60,7 @@ mandatory-y += topology.h
mandatory-y += trace_clock.h
mandatory-y += uaccess.h
mandatory-y += unwind_user.h
+mandatory-y += unwind_user_sframe.h
mandatory-y += vermagic.h
mandatory-y += vga.h
mandatory-y += video.h
diff --git a/include/asm-generic/unwind_user_sframe.h b/include/asm-generic/unwind_user_sframe.h
new file mode 100644
index 000000000000..8c9ac47bc8bd
--- /dev/null
+++ b/include/asm-generic/unwind_user_sframe.h
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_GENERIC_UNWIND_USER_SFRAME_H
+#define _ASM_GENERIC_UNWIND_USER_SFRAME_H
+
+#include <linux/types.h>
+
+#ifndef SFRAME_SP_OFFSET
+/* Most archs/ABIs define CFA as SP at call site, so that SP = CFA + 0. */
+#define SFRAME_SP_OFFSET 0
+#endif
+
+#endif /* _ASM_GENERIC_UNWIND_USER_SFRAME_H */
diff --git a/include/linux/unwind_user_types.h b/include/linux/unwind_user_types.h
index 616cc5ee4586..4656aa08a7db 100644
--- a/include/linux/unwind_user_types.h
+++ b/include/linux/unwind_user_types.h
@@ -29,6 +29,7 @@ struct unwind_stacktrace {

struct unwind_user_frame {
s32 cfa_off;
+ s32 sp_off;
s32 ra_off;
s32 fp_off;
bool use_fp;
diff --git a/kernel/unwind/sframe.c b/kernel/unwind/sframe.c
index 6465e7a315bc..7952b041dd23 100644
--- a/kernel/unwind/sframe.c
+++ b/kernel/unwind/sframe.c
@@ -12,6 +12,7 @@
#include <linux/mm.h>
#include <linux/string_helpers.h>
#include <linux/sframe.h>
+#include <asm/unwind_user_sframe.h>
#include <linux/unwind_user_types.h>

#include "sframe.h"
@@ -307,6 +308,7 @@ static __always_inline int __find_fre(struct sframe_section *sec,
fre = prev_fre;

frame->cfa_off = fre->cfa_off;
+ frame->sp_off = SFRAME_SP_OFFSET;
frame->ra_off = fre->ra_off;
frame->fp_off = fre->fp_off;
frame->use_fp = SFRAME_FRE_CFA_BASE_REG_ID(fre->info) == SFRAME_BASE_REG_FP;
diff --git a/kernel/unwind/user.c b/kernel/unwind/user.c
index fdb1001e3750..6c75a7411871 100644
--- a/kernel/unwind/user.c
+++ b/kernel/unwind/user.c
@@ -30,7 +30,7 @@ get_user_word(unsigned long *word, unsigned long base, int off, unsigned int ws)
static int unwind_user_next_common(struct unwind_user_state *state,
const struct unwind_user_frame *frame)
{
- unsigned long cfa, fp, ra;
+ unsigned long cfa, sp, fp, ra;

/* Stop unwinding when reaching an outermost frame. */
if (frame->outermost) {
@@ -48,12 +48,13 @@ static int unwind_user_next_common(struct unwind_user_state *state,
}
cfa += frame->cfa_off;

+ /* Get the Stack Pointer (SP) */
+ sp = cfa + frame->sp_off;
/* Make sure that stack is not going in wrong direction */
- if (cfa <= state->sp)
+ if (sp <= state->sp)
return -EINVAL;
-
/* Make sure that the address is word aligned */
- if (cfa & (state->ws - 1))
+ if (sp & (state->ws - 1))
return -EINVAL;

/* Get the Return Address (RA) */
@@ -65,7 +66,7 @@ static int unwind_user_next_common(struct unwind_user_state *state,
return -EINVAL;

state->ip = ra;
- state->sp = cfa;
+ state->sp = sp;
if (frame->fp_off)
state->fp = fp;
state->topmost = false;
--
2.51.0