Re: [PATCH] ppc64: Add minimal fallback unwinder.

[Mark Wielaard <mark at klomp dot org>]

On Tue, 2017-05-30 at 13:28 +0530, Naveen N. Rao wrote:
> > +  if (! readfunc(sp + LR_OFFSET, &newLr, arg))
> 
> should this be newSp + LR_OFFSET, since the LR save area is in the 
> caller's stack frame?

Of course. You are entirely right.

This also explains why the testcase didn't seem to work correctly.
It actually does with this change. I should have been more suspicious
about that then just assuming it was because the link register couldn't
always be retrieved correctly. It actually can if we pick it from the
right location!

Attached the fixed patch plus a testcase (actual binaries removed from
patch, but testcase documents how they were generated - precisely like
for the other arches).

Thanks,

Mark

From 1b2cd3f4dc581eed0fc1ee98f97aa492a19873b0 Mon Sep 17 00:00:00 2001
From: Mark Wielaard <mark@klomp.org>
Date: Sun, 21 May 2017 23:33:15 +0200
Subject: [PATCH] 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>
---
 backends/ChangeLog                     |   6 +++
 backends/Makefile.am                   |   2 +-
 backends/ppc64_init.c                  |   1 +
 backends/ppc64_unwind.c                |  76 +++++++++++++++++++++++++++++++++
 tests/ChangeLog                        |  10 +++++
 tests/Makefile.am                      |   3 ++
 tests/backtrace-subr.sh                |   2 +-
 tests/backtrace.ppc64le.fp.core.bz2    | Bin 0 -> 37786 bytes
 tests/backtrace.ppc64le.fp.exec.bz2    | Bin 0 -> 383808 bytes
 tests/run-backtrace-fp-core-ppc64le.sh |  29 +++++++++++++
 10 files changed, 127 insertions(+), 2 deletions(-)
 create mode 100644 backends/ppc64_unwind.c
 create mode 100644 tests/backtrace.ppc64le.fp.core.bz2
 create mode 100755 tests/backtrace.ppc64le.fp.exec.bz2
 create mode 100755 tests/run-backtrace-fp-core-ppc64le.sh

diff --git a/backends/ChangeLog b/backends/ChangeLog
index 1ac5bab..f0d29f6 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 ff80a82..ac45a45 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 11d3a77..e567033 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 0000000..4fa0b5a
--- /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;
+}
diff --git a/tests/ChangeLog b/tests/ChangeLog
index 5b0d486..43b5bd8 100644
--- a/tests/ChangeLog
+++ b/tests/ChangeLog
@@ -1,3 +1,13 @@
+2017-05-30  Mark Wielaard  <mark@klomp.org>
+
+	* run-backtrace-fp-core-ppc64le.sh: New test.
+	* backtrace.ppc64le.fp.core.bz2: New test file.
+	* backtrace.ppc64le.fp.exec.bz2: New testfile.
+	* backtrace-subr.sh (check_backtracegen): Accept '(null)'.
+	* Makefile.am (TESTS): Add run-backtrace-fp-core-ppc64le.sh.
+	(EXTRA_DIST): Add run-backtrace-fp-core-ppc64le.sh,
+	backtrace.ppc64le.fp.core.bz2 and backtrace.ppc64le.fp.exec.bz2.
+
 2017-02-13  Ulf Hermann  <ulf.hermann@qt.io>
 	    Mark Wielaard  <mark@klomp.org>
 
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 3a12fe3..50648db 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -117,6 +117,7 @@ TESTS = run-arextract.sh run-arsymtest.sh newfile test-nlist \
 	run-backtrace-native-core-biarch.sh run-backtrace-core-x86_64.sh \
 	run-backtrace-fp-core-x86_64.sh \
 	run-backtrace-fp-core-aarch64.sh \
+	run-backtrace-fp-core-ppc64le.sh \
 	run-backtrace-core-x32.sh \
 	run-backtrace-core-i386.sh run-backtrace-fp-core-i386.sh \
 	run-backtrace-core-ppc.sh \
@@ -303,6 +304,8 @@ EXTRA_DIST = run-arextract.sh run-arsymtest.sh \
 	     backtrace-subr.sh backtrace.i386.core.bz2 backtrace.i386.exec.bz2 \
 	     run-backtrace-fp-core-i386.sh \
 	     backtrace.i386.fp.core.bz2 backtrace.i386.fp.exec.bz2 \
+	     run-backtrace-fp-core-ppc64le.sh \
+	     backtrace.ppc64le.fp.core.bz2 backtrace.ppc64le.fp.exec.bz2 \
 	     backtrace.x86_64.core.bz2 backtrace.x86_64.exec.bz2 \
 	     backtrace.x86_64.fp.core.bz2 backtrace.x86_64.fp.exec.bz2 \
 	     backtrace.ppc.core.bz2 backtrace.ppc.exec.bz2 \
diff --git a/tests/backtrace-subr.sh b/tests/backtrace-subr.sh
index 9731c43..c1f3156 100644
--- a/tests/backtrace-subr.sh
+++ b/tests/backtrace-subr.sh
@@ -59,7 +59,7 @@ check_backtracegen()
 # Ignore it here as it is a bug of OS, not a bug of elfutils.
 check_err()
 {
-  if [ $(egrep -v <$1 'dwfl_thread_getframes: (No DWARF information found|no matching address range|address out of range|Invalid register)$' \
+  if [ $(egrep -v <$1 'dwfl_thread_getframes: (No DWARF information found|no matching address range|address out of range|Invalid register|\(null\))$' \
          | wc -c) \
        -eq 0 ]
   then
diff --git a/tests/run-backtrace-fp-core-ppc64le.sh b/tests/run-backtrace-fp-core-ppc64le.sh
new file mode 100755
index 0000000..326ca34
--- /dev/null
+++ b/tests/run-backtrace-fp-core-ppc64le.sh
@@ -0,0 +1,29 @@
+#! /bin/bash
+# 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 the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# 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 a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+. $srcdir/backtrace-subr.sh
+
+# The binary is generated by compiling backtrace-child without unwind
+# information, but with -fno-omit-frame-pointer.
+#
+# gcc -static -O2 -fno-omit-frame-pointer -fno-asynchronous-unwind-tables \
+#     -D_GNU_SOURCE -pthread -o tests/backtrace.ppc64le.fp.exec -I. -Ilib \
+#     tests/backtrace-child.c
+#
+# The core is generated by calling tests/backtrace.ppc64le.fp.exec --gencore
+
+check_core ppc64le.fp
-- 
1.8.3.1