backends: Add frame pointer unwinding for LoongArch

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.

Signed-off-by: Liwei Ge <geliwei@openanolis.org>
Signed-off-by: Youling Tang <tangyouling@loongson.cn>

4 files changed, 93 insertions, 1 deletions

diff --git a/backends/ChangeLog b/backends/ChangeLog
index df762073..f1613d80 100644
--- a/backends/ChangeLog
+++ b/backends/ChangeLog
@@ -1,5 +1,11 @@
 2023-04-07  Youling Tang  <tangyouling@loongson.cn>
 
+	* Makefile.am (loongarch_SRCS): Add loongarch_unwind.c.
+	* loongarch_init.c (loongarch_init): Hook unwind.
+	* loongarch_unwind.c: New file.
+
+2023-04-07  Youling Tang  <tangyouling@loongson.cn>
+
 	* Makefile.am (loongarch_SRCS): Add loongarch_retval.c.
 	* loongarch_init.c (loongarch_init): Hook return_value_location.
 	* loongarch_retval.c: New file.
diff --git a/backends/Makefile.am b/backends/Makefile.am
index 50e78ea5..1261f5c6 100644
--- a/backends/Makefile.am
+++ b/backends/Makefile.am
@@ -97,7 +97,8 @@ csky_SRCS = csky_attrs.c csky_init.c csky_symbol.c csky_cfi.c \
 	    csky_regs.c csky_initreg.c csky_corenote.c
 
 loongarch_SRCS = loongarch_init.c loongarch_symbol.c loongarch_cfi.c \
-	    loongarch_regs.c loongarch_retval.c
+	    loongarch_regs.c loongarch_retval.c \
+	    loongarch_unwind.c
 
 arc_SRCS = arc_init.c arc_symbol.c
 
diff --git a/backends/loongarch_init.c b/backends/loongarch_init.c
index fd63f373..1e8846a8 100644
--- a/backends/loongarch_init.c
+++ b/backends/loongarch_init.c
@@ -54,6 +54,7 @@ loongarch_init (Elf *elf __attribute__ ((unused)),
   eh->frame_nregs = 74;
   HOOK (eh, check_special_symbol);
   HOOK (eh, return_value_location);
+  HOOK (eh, unwind);
 
   return eh;
 }
diff --git a/backends/loongarch_unwind.c b/backends/loongarch_unwind.c
new file mode 100644
index 00000000..fb748083
--- /dev/null
+++ b/backends/loongarch_unwind.c
@@ -0,0 +1,84 @@
+/* Get previous frame state for an existing frame state.
+   Copyright (C) 2023 OpenAnolis community LoongArch SIG.
+   Copyright (C) 2023 Loongson Technology Corporation Limited.
+   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 loongarch_
+#define RA_REG 1
+#define SP_REG 3
+#define FP_REG 22
+
+#define RA_OFFSET 8
+#define FP_OFFSET 16
+
+#include "libebl_CPU.h"
+
+/* There was no CFI. Maybe we happen to have a frame pointer and can unwind from that?  */
+
+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 fp, ra, sp;
+
+  if (!getfunc(RA_REG, 1, &ra, arg))
+    return false;
+
+  if (ra == 0 || !setfunc(-1, 1, &ra, arg))
+    return false;
+
+  if (!getfunc(FP_REG, 1, &fp, arg))
+    fp = 0;
+
+  if (!getfunc(SP_REG, 1, &sp, arg))
+    sp = 0;
+
+  Dwarf_Word newRa, newFp, newSp;
+
+  if (!readfunc(fp - RA_OFFSET, &newRa, arg))
+    newRa = 0;
+
+  if (!readfunc(fp - FP_OFFSET, &newFp, arg))
+    newFp = 0;
+
+  newSp = fp;
+
+  // These are not fatal if they don't work. They will just prevent unwinding at the next frame.
+  setfunc(RA_REG, 1, &newRa, arg);
+  setfunc(FP_REG, 1, &newFp, arg);
+  setfunc(SP_REG, 1, &newSp, arg);
+
+  // If the fp is invalid, we might still have a valid ra.
+  // But if the fp is valid, then the stack should be moving in the right direction.
+  return fp == 0 || newSp > sp;
+}