Add frame pointer unwinding as fallback on x86_64

If we don't find any debug information for a given frame, we usually
cannot unwind any further. However, the binary in question might have
been compiled with frame pointers, in which case we can look up the
well known frame pointer locations in the stack snapshot and use them
to bridge the frames without debug information.

The "unwind" hook is the right place for this as it is so far only
used on s390 and called only after trying to unwind with debug
information.

Signed-off-by: Ulf Hermann <ulf.hermann@qt.io>

4 files changed, 94 insertions, 1 deletions

diff --git a/backends/ChangeLog b/backends/ChangeLog
index c6e0e08b..04f579da 100644
--- a/backends/ChangeLog
+++ b/backends/ChangeLog
@@ -1,3 +1,9 @@
+2017-02-09  Ulf Hermann  <ulf.hermann@qt.io>
+
+	* x86_64_unwind.c: New file
+	* Makefile.am (x86_64_SRCS): Add x86_64_unwind.c
+	* x86_64_init.c (x86_64_init): Hook x86_64_unwind
+
 2017-04-20  Ulf Hermann <ulf.hermann@qt.io>
 
 	* aarch64_initreg.c: Compile register initialization only on linux.
diff --git a/backends/Makefile.am b/backends/Makefile.am
index b553ec34..60917b92 100644
--- a/backends/Makefile.am
+++ b/backends/Makefile.am
@@ -59,7 +59,7 @@ am_libebl_sh_pic_a_OBJECTS = $(sh_SRCS:.c=.os)
 
 x86_64_SRCS = x86_64_init.c x86_64_symbol.c x86_64_corenote.c x86_64_cfi.c \
 	      x86_64_retval.c x86_64_regs.c i386_auxv.c x86_64_syscall.c \
-	      x86_64_initreg.c x32_corenote.c
+	      x86_64_initreg.c x86_64_unwind.c x32_corenote.c
 cpu_x86_64 = ../libcpu/libcpu_x86_64.a
 libebl_x86_64_pic_a_SOURCES = $(x86_64_SRCS)
 am_libebl_x86_64_pic_a_OBJECTS = $(x86_64_SRCS:.c=.os)
diff --git a/backends/x86_64_init.c b/backends/x86_64_init.c
index cfd0158c..adfa4791 100644
--- a/backends/x86_64_init.c
+++ b/backends/x86_64_init.c
@@ -68,6 +68,7 @@ x86_64_init (Elf *elf __attribute__ ((unused)),
   /* gcc/config/ #define DWARF_FRAME_REGISTERS.  */
   eh->frame_nregs = 17;
   HOOK (eh, set_initial_registers_tid);
+  HOOK (eh, unwind);
 
   return MODVERSION;
 }
diff --git a/backends/x86_64_unwind.c b/backends/x86_64_unwind.c
new file mode 100644
index 00000000..ade64c01
--- /dev/null
+++ b/backends/x86_64_unwind.c
@@ -0,0 +1,86 @@
+/* Get previous frame state for an existing frame state.
+   Copyright (C) 2016 The Qt Company Ltd.
+   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
+
+#include <stdlib.h>
+#include <assert.h>
+
+#define BACKEND x86_64_
+#include "libebl_CPU.h"
+
+/* There was no CFI. Maybe we happen to have a frame pointer and can unwind from that?  */
+
+bool
+x86_64_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)))
+{
+  // Register 6 is supposed to be rbp, thus the conventional frame pointer
+  const int fpReg = 6;
+  const int spReg = 7;
+
+  Dwarf_Word fp;
+  if (!getfunc(fpReg, 1, &fp, arg) || fp == 0)
+    return false;
+
+  // Try to read old sp, so that we can avoid infinite loops below
+  Dwarf_Word sp;
+  if (!getfunc(spReg, 1, &sp, arg))
+    sp = 0;
+
+  Dwarf_Word prev_fp;
+  if (!readfunc(fp, &prev_fp, arg))
+    prev_fp = 0;
+
+  Dwarf_Word ret;
+  if (!readfunc(fp + 8, &ret, arg))
+    return false;
+
+  if (!setfunc(fpReg, 1, &prev_fp, arg))
+    return false;
+
+  fp += 16; // Pop fp and return address and write result to sp
+  if (!setfunc(spReg, 1, &fp, arg))
+    return false;
+
+  if (!setfunc(-1, 1, &ret, arg))
+    return false;
+
+  // If the sp didn't move up we don't actually have a new stack
+  // frame but rather some random data that doesn't include frame
+  // pointers. Break the unwinding then.
+  if (sp >= fp)
+    return false;
+
+  return true;
+}