[RFC PATCH v3 17/17] s390/unwind_user/fp: Enable back chain unwinding of user space

Jens Remus
2025-12-08T17:16:46Z

Unwinding of user space using frame pointer (FP) is virtually impossible
on s390 for the following reasons: The s390 64-bit (s390x) ELF ABI [1]
does only designate a "preferred" FP register and does not mandate fixed
FP and return address (RA) stack save slots. Therefore neither the FP
register nor the FP/RA stack save slot offsets from CFA are known.
Compilers, such as GCC and Clang, do not necessarily setup a FP register
early in the function prologue, even not with compiler option
-fno-omit-frame-pointer. Therefore the CFA offset from FP register is
not known.

This could be resolved by having compiler option -no-omit-frame-pointer
enforce all of the following: Use the preferred FP register 11 as frame
pointer, use fixed FP/RA stack slot offsets from CFA (e.g. -72 for FP
and -48 for RA), and setup the FP register immediately after saving the
call saved registers.

Fortunately s390 provides an alternative to frame pointer: back chain,
which can be enabled using s390-specific compiler option -mbackchain.
The back chain is very similar to a frame pointer on the stack.

Leverage the unwind user fp infrastructure to enable unwinding of user
space using back chain. Enable HAVE_UNWIND_USER_FP and provide a s390-
specific implementation of unwind_user_fp_get_frame(), which uses the
back chain.

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

Notes (jremus):
Changes in RFC v3:
- New patch. Implement unwind user fp using back chain on s390. Reuses
logic from RFC v2 patch "unwind_user/backchain: Introduce back chain
user space unwinding". (Josh)

arch/s390/Kconfig | 1 +
arch/s390/include/asm/unwind_user.h | 83 +++++++++++++++++++++++++++++
2 files changed, 84 insertions(+)

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 52d3f3b3e086..eb6a0fe895bc 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -246,6 +246,7 @@ config S390
select HAVE_SETUP_PER_CPU_AREA
select HAVE_SOFTIRQ_ON_OWN_STACK
select HAVE_SYSCALL_TRACEPOINTS
+ select HAVE_UNWIND_USER_FP
select HAVE_UNWIND_USER_SFRAME
select HAVE_VIRT_CPU_ACCOUNTING
select HAVE_VIRT_CPU_ACCOUNTING_IDLE
diff --git a/arch/s390/include/asm/unwind_user.h b/arch/s390/include/asm/unwind_user.h
index 3a95be1eb886..99cbb83dd248 100644
--- a/arch/s390/include/asm/unwind_user.h
+++ b/arch/s390/include/asm/unwind_user.h
@@ -3,8 +3,12 @@
#define _ASM_S390_UNWIND_USER_H

#include <linux/sched/task_stack.h>
+#include <linux/security.h>
#include <linux/types.h>
+#include <asm/asm-offsets.h>
#include <asm/fpu-insn.h>
+#include <asm/stacktrace.h>
+#include <linux/unwind_user_types.h>

#ifdef CONFIG_UNWIND_USER

@@ -95,6 +99,85 @@ static inline int arch_unwind_user_get_reg(unsigned long *val, int regnum)

#endif /* CONFIG_UNWIND_USER */

+#ifdef CONFIG_HAVE_UNWIND_USER_FP
+
+static inline bool ip_within_vdso(unsigned long ip)
+{
+ return in_range(ip, current->mm->context.vdso_base, vdso_text_size());
+}
+
+static inline int unwind_user_fp_get_frame(struct unwind_user_state *state,
+ struct unwind_user_frame *frame)
+{
+ struct stack_frame_user __user *sf;
+ unsigned long __user *ra_addr;
+ unsigned long sp;
+
+ sf = (void __user *)state->sp;
+
+ /*
+ * In topmost frame check whether IP in early prologue, RA and SP
+ * registers saved, and no new stack frame allocated.
+ */
+ if (state->topmost) {
+ unsigned long ra, ra_reg;
+
+ ra_addr = (unsigned long __user *)&sf->gprs[8];
+ if (__get_user(ra, ra_addr))
+ return -EINVAL;
+ if (__get_user(sp, (unsigned long __user *)&sf->gprs[9]))
+ return -EINVAL;
+ if (unwind_user_get_ra_reg(&ra_reg))
+ return -EINVAL;
+ if (ra == ra_reg && sp == state->sp)
+ goto done;
+ }
+
+ if (__get_user(sp, (unsigned long __user *)&sf->back_chain))
+ return -EINVAL;
+ if (!sp && ip_within_vdso(state->ip)) {
+ /*
+ * Assume non-standard vDSO user wrapper stack frame.
+ * See vDSO user wrapper code for details.
+ */
+ struct stack_frame_vdso_wrapper *sf_vdso = (void __user *)sf;
+
+ ra_addr = (unsigned long __user *)&sf_vdso->return_address;
+ sf = (void __user *)((unsigned long)sf + STACK_FRAME_VDSO_OVERHEAD);
+ if (__get_user(sp, (unsigned long __user *)&sf->back_chain))
+ return -EINVAL;
+ } else if (!sp) {
+ /*
+ * Assume outermost frame reached. unwind_user_next_common()
+ * disregards all other fields in outermost frame.
+ */
+ frame->outermost = false;
+ return 0;
+ } else {
+ /*
+ * Assume IP past prologue and new stack frame allocated.
+ * Follow back chain, which then equals the SP at entry.
+ * Skips caller if wrong in topmost frame.
+ */
+ sf = (void __user *)sp;
+ ra_addr = (unsigned long __user *)&sf->gprs[8];
+ }
+
+done:
+ frame->cfa_off = sp - state->sp + 160;
+ frame->sp_off = -160;
+ frame->fp.loc = UNWIND_USER_LOC_UNKNOWN; /* Cannot unwind FP. */
+ frame->use_fp = false;
+ frame->ra.loc = UNWIND_USER_LOC_STACK;
+ frame->ra.offset = (unsigned long)ra_addr - (state->sp + frame->cfa_off);
+ frame->outermost = false;
+
+ return 0;
+}
+#define unwind_user_fp_get_frame unwind_user_fp_get_frame
+
+#endif /* CONFIG_HAVE_UNWIND_USER_FP */
+
#include <asm-generic/unwind_user.h>

#endif /* _ASM_S390_UNWIND_USER_H */
--
2.51.0