ppc64: Add minimal fallback unwinder.

This adds a minimal fallback unwinder for ppc64[le] in case we cannot find
CFI for a particular address. It simply always sets the program counter to
the link register, picks the previous stack pointer from the backchain,
and the previous link register from the LR save area.

This is enough for some simple situations when we don't have CFI and
seems to work nicely in the case of perf with libdw powerpc support:
https://lkml.org/lkml/2017/5/18/998

Signed-off-by: Mark Wielaard <mark@klomp.org>

4 files changed, 84 insertions, 1 deletions

diff --git a/backends/ChangeLog b/backends/ChangeLog
index 1ac5bab3..f0d29f62 100644
--- a/backends/ChangeLog
+++ b/backends/ChangeLog
@@ -1,3 +1,9 @@
+2017-05-30  Mark Wielaard  <mark@klomp.org>
+
+	* ppc64_unwind.c: New file.
+	* ppc64_init.c (pcc64_init): Hook unwind.
+	* Makefile.am (ppc64_SRCS): Add ppc64_unwind.c
+
 2017-04-06  Mark Wielaard  <mark@klomp.org>
 
 	* i386_unwind.c: New file.
diff --git a/backends/Makefile.am b/backends/Makefile.am
index ff80a82c..ac45a452 100644
--- a/backends/Makefile.am
+++ b/backends/Makefile.am
@@ -98,7 +98,7 @@ am_libebl_ppc_pic_a_OBJECTS = $(ppc_SRCS:.c=.os)
 
 ppc64_SRCS = ppc64_init.c ppc64_symbol.c ppc64_retval.c \
 	     ppc64_corenote.c ppc_regs.c ppc_auxv.c ppc_attrs.c ppc_syscall.c \
-	     ppc_cfi.c ppc_initreg.c ppc64_resolve_sym.c
+	     ppc_cfi.c ppc_initreg.c ppc64_unwind.c ppc64_resolve_sym.c
 libebl_ppc64_pic_a_SOURCES = $(ppc64_SRCS)
 am_libebl_ppc64_pic_a_OBJECTS = $(ppc64_SRCS:.c=.os)
 
diff --git a/backends/ppc64_init.c b/backends/ppc64_init.c
index 11d3a77f..e5670338 100644
--- a/backends/ppc64_init.c
+++ b/backends/ppc64_init.c
@@ -73,6 +73,7 @@ ppc64_init (Elf *elf __attribute__ ((unused)),
   eh->frame_nregs = (114 - 1) + 32;
   HOOK (eh, set_initial_registers_tid);
   HOOK (eh, dwarf_to_regno);
+  HOOK (eh, unwind);
   HOOK (eh, resolve_sym_value);
 
   /* Find the function descriptor .opd table for resolve_sym_value.  */
diff --git a/backends/ppc64_unwind.c b/backends/ppc64_unwind.c
new file mode 100644
index 00000000..4fa0b5a9
--- /dev/null
+++ b/backends/ppc64_unwind.c
@@ -0,0 +1,76 @@
+/* Get previous frame state for an existing frame state.
+   Copyright (C) 2017 Red Hat, Inc.
+   This file is part of elfutils.
+
+   This file is free software; you can redistribute it and/or modify
+   it under the terms of either
+
+     * the GNU Lesser General Public License as published by the Free
+       Software Foundation; either version 3 of the License, or (at
+       your option) any later version
+
+   or
+
+     * the GNU General Public License as published by the Free
+       Software Foundation; either version 2 of the License, or (at
+       your option) any later version
+
+   or both in parallel, as here.
+
+   elfutils is distributed in the hope that it will be useful, but
+   WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   General Public License for more details.
+
+   You should have received copies of the GNU General Public License and
+   the GNU Lesser General Public License along with this program.  If
+   not, see <http://www.gnu.org/licenses/>.  */
+
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
+#define BACKEND ppc64_
+
+#define LR_REG 65 /* Not 108, see ppc_dwarf_to_regno.  */
+#define SP_REG  1
+
+#define LR_OFFSET 16
+
+#include "libebl_CPU.h"
+
+/* Simplistic fallback frame unwinder. SP points to the backchain (contains
+   address of previous stack pointer). At SP offset 16 is the LR save area
+   (contains the value of the previous LR).  */
+
+bool
+EBLHOOK(unwind) (Ebl *ebl __attribute__ ((unused)),
+		 Dwarf_Addr pc __attribute__ ((unused)),
+                 ebl_tid_registers_t *setfunc, ebl_tid_registers_get_t *getfunc,
+                 ebl_pid_memory_read_t *readfunc, void *arg,
+                 bool *signal_framep __attribute__ ((unused)))
+{
+  Dwarf_Word sp, newSp, lr, newLr;
+
+  /* Stack pointer points to the backchain which contains the previous sp.  */
+  if (! getfunc (SP_REG, 1, &sp, arg))
+    sp = 0;
+
+  /* Link register contains previous program counter.  */
+  if (! getfunc (LR_REG, 1, &lr, arg)
+      || lr == 0
+      || ! setfunc (-1, 1, &lr, arg))
+    return false;
+
+  if (! readfunc(sp, &newSp, arg))
+    newSp = 0;
+
+  if (! readfunc(newSp + LR_OFFSET, &newLr, arg))
+    newLr = 0;
+
+  setfunc(SP_REG, 1, &newSp, arg);
+  setfunc(LR_REG, 1, &newLr, arg);
+
+  /* Sanity check the stack grows down.  */
+  return newSp > sp;
+}