[RFC PATCH v3 11/17] unwind_user: Enable archs that save RA/FP in other registers

Jens Remus
2025-12-08T17:16:39Z

Enable unwinding of user space for architectures, such as s390, that
save the return address (RA) and/or frame pointer (FP) in other
registers. This is only valid in the topmost frame, for instance when
in a leaf function.

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

Notes (jremus):
Changes in RFC v3:
- Rename UNWIND_USER_LOC_NONE to UNWIND_USER_LOC_RETAIN to better
disambiguate from new UNWIND_USER_LOC_UNKNOWN to be introduced for
for back chain unwinding on s390.
Other naming options: IDENTITY, KEEP, PRESERVE, SAME, UNCHANGED.

Changes in RFC v2:
- Reword HAVE_UNWIND_USER_LOC_REG help text.
- Rename struct unwind_user_reginfo field frame_off to offset. (Josh)
- Move dummy unwind_user_get_reg() from asm-generic/unwind_user.h
to linux/unwind_user.h, drop its function comment, warn once,
return -EINVAL, and guard by !HAVE_UNWIND_USER_LOC_REG. (Josh)
- Rename generic_sframe_set_frame_reginfo() to sframe_init_reginfo()
and drop its function comment. (Josh)
- Do not check FP/RA offset for zero for UNWIND_USER_LOC_STACK. (Josh)
- Do not check for !IS_ENABLED(CONFIG_HAVE_UNWIND_USER_LOC_REG), as
the dummy implementation of unwind_user_get_reg() returns -EINVAL.
- Drop config option HAVE_UNWIND_USER_LOC_REG, as it is no longer of
any value.
- Keep checking for topmost for UNWIND_USER_LOC_REG. (Jens)
- Explicitly preserve FP if UNWIND_USER_LOC_NONE and drop later test
for frame->fp.loc != UNWIND_USER_LOC_NONE. (Josh)

Would it make sense to rename UNWIND_USER_LOC_NONE to one of the
following to clarify its meaning for the unwinder?
- UNWIND_USER_LOC_UNCHANGED
- UNWIND_USER_LOC_RETAIN
- UNWIND_USER_LOC_PRESERVED
- UNWIND_USER_LOC_IDENTITY

arch/x86/include/asm/unwind_user.h | 21 +++++++++++---
include/asm-generic/unwind_user_sframe.h | 15 ++++++++++
include/linux/unwind_user.h | 9 ++++++
include/linux/unwind_user_types.h | 18 ++++++++++--
kernel/unwind/sframe.c | 4 +--
kernel/unwind/user.c | 37 +++++++++++++++++++-----
6 files changed, 89 insertions(+), 15 deletions(-)

diff --git a/arch/x86/include/asm/unwind_user.h b/arch/x86/include/asm/unwind_user.h
index d70ffd7bbdb7..2480d86a405e 100644
--- a/arch/x86/include/asm/unwind_user.h
+++ b/arch/x86/include/asm/unwind_user.h
@@ -22,16 +22,27 @@ 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), \
+ .ra = { \
+ .loc = UNWIND_USER_LOC_STACK,\
+ .offset = -1*(ws), \
+ }, \
+ .fp = { \
+ .loc = UNWIND_USER_LOC_STACK,\
+ .offset = -2*(ws), \
+ }, \
.use_fp = true, \
.outermost = false,

#define ARCH_INIT_USER_FP_ENTRY_FRAME(ws) \
.cfa_off = 1*(ws), \
.sp_off = 0, \
- .ra_off = -1*(ws), \
- .fp_off = 0, \
+ .ra = { \
+ .loc = UNWIND_USER_LOC_STACK,\
+ .offset = -1*(ws), \
+ }, \
+ .fp = { \
+ .loc = UNWIND_USER_LOC_RETAIN,\
+ }, \
.use_fp = false, \
.outermost = false,

@@ -43,4 +54,6 @@ static inline bool unwind_user_at_function_start(struct pt_regs *regs)

#endif /* CONFIG_HAVE_UNWIND_USER_FP */

+#include <asm-generic/unwind_user.h>
+
#endif /* _ASM_X86_UNWIND_USER_H */
diff --git a/include/asm-generic/unwind_user_sframe.h b/include/asm-generic/unwind_user_sframe.h
index 8c9ac47bc8bd..fd71d6b1916b 100644
--- a/include/asm-generic/unwind_user_sframe.h
+++ b/include/asm-generic/unwind_user_sframe.h
@@ -2,6 +2,7 @@
#ifndef _ASM_GENERIC_UNWIND_USER_SFRAME_H
#define _ASM_GENERIC_UNWIND_USER_SFRAME_H

+#include <linux/unwind_user_types.h>
#include <linux/types.h>

#ifndef SFRAME_SP_OFFSET
@@ -9,4 +10,18 @@
#define SFRAME_SP_OFFSET 0
#endif

+#ifndef sframe_init_reginfo
+static inline void
+sframe_init_reginfo(struct unwind_user_reginfo *reginfo, s32 offset)
+{
+ if (offset) {
+ reginfo->loc = UNWIND_USER_LOC_STACK;
+ reginfo->offset = offset;
+ } else {
+ reginfo->loc = UNWIND_USER_LOC_RETAIN;
+ }
+}
+#define sframe_init_reginfo sframe_init_reginfo
+#endif
+
#endif /* _ASM_GENERIC_UNWIND_USER_SFRAME_H */
diff --git a/include/linux/unwind_user.h b/include/linux/unwind_user.h
index bc2edae39955..61fd5c05d0f0 100644
--- a/include/linux/unwind_user.h
+++ b/include/linux/unwind_user.h
@@ -32,6 +32,15 @@ static inline int unwind_user_get_ra_reg(unsigned long *val)
#define unwind_user_get_ra_reg unwind_user_get_ra_reg
#endif

+#ifndef unwind_user_get_reg
+static inline int unwind_user_get_reg(unsigned long *val, int regnum)
+{
+ WARN_ON_ONCE(1);
+ return -EINVAL;
+}
+#define unwind_user_get_reg unwind_user_get_reg
+#endif
+
int unwind_user(struct unwind_stacktrace *trace, unsigned int max_entries);

#endif /* _LINUX_UNWIND_USER_H */
diff --git a/include/linux/unwind_user_types.h b/include/linux/unwind_user_types.h
index 4656aa08a7db..4f78999a0750 100644
--- a/include/linux/unwind_user_types.h
+++ b/include/linux/unwind_user_types.h
@@ -27,11 +27,25 @@ struct unwind_stacktrace {
unsigned long *entries;
};

+enum unwind_user_loc {
+ UNWIND_USER_LOC_RETAIN,
+ UNWIND_USER_LOC_STACK,
+ UNWIND_USER_LOC_REG,
+};
+
+struct unwind_user_reginfo {
+ enum unwind_user_loc loc;
+ union {
+ s32 offset;
+ int regnum;
+ };
+};
+
struct unwind_user_frame {
s32 cfa_off;
s32 sp_off;
- s32 ra_off;
- s32 fp_off;
+ struct unwind_user_reginfo ra;
+ struct unwind_user_reginfo fp;
bool use_fp;
bool outermost;
};
diff --git a/kernel/unwind/sframe.c b/kernel/unwind/sframe.c
index 38b3577f5253..45cd7380ac38 100644
--- a/kernel/unwind/sframe.c
+++ b/kernel/unwind/sframe.c
@@ -307,8 +307,8 @@ static __always_inline int __find_fre(struct sframe_section *sec,

frame->cfa_off = fre->cfa_off;
frame->sp_off = SFRAME_SP_OFFSET;
- frame->ra_off = fre->ra_off;
- frame->fp_off = fre->fp_off;
+ sframe_init_reginfo(&frame->ra, fre->ra_off);
+ sframe_init_reginfo(&frame->fp, fre->fp_off);
frame->use_fp = SFRAME_FRE_CFA_BASE_REG_ID(fre->info) == SFRAME_BASE_REG_FP;
frame->outermost = fre->ra_undefined;

diff --git a/kernel/unwind/user.c b/kernel/unwind/user.c
index 58e1549cd9f4..45f82ed28fcb 100644
--- a/kernel/unwind/user.c
+++ b/kernel/unwind/user.c
@@ -62,22 +62,45 @@ static int unwind_user_next_common(struct unwind_user_state *state,
return -EINVAL;

/* Get the Return Address (RA) */
- if (frame->ra_off) {
- if (get_user_word(&ra, cfa, frame->ra_off, state->ws))
- return -EINVAL;
- } else {
+ switch (frame->ra.loc) {
+ case UNWIND_USER_LOC_RETAIN:
if (!state->topmost || unwind_user_get_ra_reg(&ra))
return -EINVAL;
+ break;
+ case UNWIND_USER_LOC_STACK:
+ if (get_user_word(&ra, cfa, frame->ra.offset, state->ws))
+ return -EINVAL;
+ break;
+ case UNWIND_USER_LOC_REG:
+ if (!state->topmost || unwind_user_get_reg(&ra, frame->ra.regnum))
+ return -EINVAL;
+ break;
+ default:
+ WARN_ON_ONCE(1);
+ return -EINVAL;
}

/* Get the Frame Pointer (FP) */
- if (frame->fp_off && get_user_word(&fp, cfa, frame->fp_off, state->ws))
+ switch (frame->fp.loc) {
+ case UNWIND_USER_LOC_RETAIN:
+ fp = state->fp;
+ break;
+ case UNWIND_USER_LOC_STACK:
+ if (get_user_word(&fp, cfa, frame->fp.offset, state->ws))
+ return -EINVAL;
+ break;
+ case UNWIND_USER_LOC_REG:
+ if (!state->topmost || unwind_user_get_reg(&fp, frame->fp.regnum))
+ return -EINVAL;
+ break;
+ default:
+ WARN_ON_ONCE(1);
return -EINVAL;
+ }

state->ip = ra;
state->sp = sp;
- if (frame->fp_off)
- state->fp = fp;
+ state->fp = fp;
state->topmost = false;
return 0;
}
--
2.51.0