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 v4:
    - Remove flawed heuristic to detect if topmost IP in early prologue.
      While it may resolve the caller getting skipped it may erroneously
      inject a callee as caller.
    - Fix outermost frame indication.
    - Adjust to flexible CFA and FP/RA rules.
    - Remove superfluous unwind_user_fp_get_frame define.
    
    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 | 62 +++++++++++++++++++++++++++++
 2 files changed, 63 insertions(+)

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 063f0c857600..5f7e83ba54b2 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -250,6 +250,7 @@ config S390
 	select HAVE_SOFTIRQ_ON_OWN_STACK
 	select HAVE_STACKPROTECTOR if CC_HAS_STACKPROTECTOR_GLOBAL
 	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 941aa3f0f70f..a7b97ea01c26 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.h>
+#include <asm/stacktrace.h>
+#include <linux/unwind_user_types.h>
 
 #ifdef CONFIG_UNWIND_USER
 
@@ -65,6 +69,64 @@ static inline int arch_unwind_user_get_reg(unsigned long *val,
 
 #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;
+	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 = true;
+		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];
+	}
+
+	frame->cfa.rule = UNWIND_USER_CFA_RULE_SP_OFFSET;
+	frame->cfa.offset = sp - state->sp + 160;
+	frame->sp_off = -160;
+	frame->fp.rule = UNWIND_USER_RULE_ZERO;	/* Cannot unwind FP. */
+	frame->ra.rule = UNWIND_USER_RULE_CFA_OFFSET_DEREF;
+	frame->ra.offset = (unsigned long)ra_addr - (state->sp + frame->cfa.offset);
+	frame->outermost = false;
+
+	return 0;
+}
+
+#endif /* CONFIG_HAVE_UNWIND_USER_FP */
+
 #include <asm-generic/unwind_user.h>
 
 #endif /* _ASM_S390_UNWIND_USER_H */
-- 
2.51.0