s390/unwind: avoid duplicated unwinding entries for kretprobes

Currently when unwinding starts from pt_regs or encounters pt_regs along
the way unwinder tries to yield 2 unwinding entries:
1. (reliable)     ip1: pt_regs->psw.addr,  sp1: regs->gprs[15]
2. (non-reliable) ip2: sp1->gprs[8] (r14), sp2: regs->gprs[15]

In case of kretprobes those are identical and serves no other purpose
than causing confusion over duplicated entries and cause kprobes tests
to fail. So, skip a duplicate non-reliable entry in this case.

With that kretprobes and unwinder implementation now comply with
ARCH_CORRECT_STACKTRACE_ON_KRETPROBE.
Reviewed-by: Tobias Huschle <huschle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/Kconfig

@@ -58,6 +58,7 @@ config S390
 	select ALTERNATE_USER_ADDRESS_SPACE
 	select ARCH_32BIT_USTAT_F_TINODE
 	select ARCH_BINFMT_ELF_STATE
+	select ARCH_CORRECT_STACKTRACE_ON_KRETPROBE
 	select ARCH_ENABLE_MEMORY_HOTPLUG if SPARSEMEM
 	select ARCH_ENABLE_MEMORY_HOTREMOVE
 	select ARCH_ENABLE_SPLIT_PMD_PTLOCK if PGTABLE_LEVELS > 2

arch/s390/kernel/unwind_bc.c

@@ -64,8 +64,8 @@ bool unwind_next_frame(struct unwind_state *state)
 		ip = READ_ONCE_NOCHECK(sf->gprs[8]);
 		reliable = false;
 		regs = NULL;
-		if (!__kernel_text_address(ip)) {
-			/* skip bogus %r14 */
+		/* skip bogus %r14 or if is the same as regs->psw.addr */
+		if (!__kernel_text_address(ip) || state->ip == unwind_recover_ret_addr(state, ip)) {
 			state->regs = NULL;
 			return unwind_next_frame(state);
 		}