From 2d6a50435bb743be1e4d88eee002372344348349 Mon Sep 17 00:00:00 2001
From: Yichao Yu <yyc1992@gmail.com>
Date: Sun, 29 Aug 2021 13:43:01 -0700
Subject: [PATCH] Prefer EXTBL unwinding on ARM

It is part of the C++ ABI so a EXTBL unwind info that's not `CANT_UNWIND`
should always be reliable/correct.
Ignore `ESTOPUNWIND` so that a `CANT_UNWIND` info can fallback to unwinding
using the debug info instead.
---
 include/tdep-arm/libunwind_i.h |  4 +++
 src/arm/Gex_tables.c           | 18 ++++++++---
 src/arm/Gstep.c                | 55 ++++++++++++++++++++--------------
 3 files changed, 51 insertions(+), 26 deletions(-)

diff --git a/include/tdep-arm/libunwind_i.h b/include/tdep-arm/libunwind_i.h
index 88ebfb069..5bd28c953 100644
--- a/include/tdep-arm/libunwind_i.h
+++ b/include/tdep-arm/libunwind_i.h
@@ -256,6 +256,7 @@ dwarf_put (struct dwarf_cursor *c, dwarf_loc_t loc, unw_word_t val)
 #define tdep_init_done                  UNW_OBJ(init_done)
 #define tdep_init                       UNW_OBJ(init)
 #define arm_find_proc_info              UNW_OBJ(find_proc_info)
+#define arm_find_proc_info2             UNW_OBJ(find_proc_info2)
 #define arm_put_unwind_info             UNW_OBJ(put_unwind_info)
 /* Platforms that support UNW_INFO_FORMAT_TABLE need to define
    tdep_search_unwind_table.  */
@@ -297,6 +298,9 @@ extern void tdep_init (void);
 extern int arm_find_proc_info (unw_addr_space_t as, unw_word_t ip,
                                unw_proc_info_t *pi, int need_unwind_info,
                                void *arg);
+extern int arm_find_proc_info2 (unw_addr_space_t as, unw_word_t ip,
+                                unw_proc_info_t *pi, int need_unwind_info,
+                                void *arg, int methods);
 extern void arm_put_unwind_info (unw_addr_space_t as,
                                   unw_proc_info_t *pi, void *arg);
 extern int tdep_search_unwind_table (unw_addr_space_t as, unw_word_t ip,
diff --git a/src/arm/Gex_tables.c b/src/arm/Gex_tables.c
index efdcf2978..083d2b2f7 100644
--- a/src/arm/Gex_tables.c
+++ b/src/arm/Gex_tables.c
@@ -506,18 +506,20 @@ arm_phdr_cb (struct dl_phdr_info *info, size_t size, void *data)
 }
 
 HIDDEN int
-arm_find_proc_info (unw_addr_space_t as, unw_word_t ip,
-                    unw_proc_info_t *pi, int need_unwind_info, void *arg)
+arm_find_proc_info2 (unw_addr_space_t as, unw_word_t ip,
+                     unw_proc_info_t *pi, int need_unwind_info, void *arg,
+                     int methods)
 {
   int ret = -1;
   intrmask_t saved_mask;
 
   Debug (14, "looking for IP=0x%lx\n", (long) ip);
 
-  if (UNW_TRY_METHOD(UNW_ARM_METHOD_DWARF))
+  if (UNW_TRY_METHOD (UNW_ARM_METHOD_DWARF) && (methods & UNW_ARM_METHOD_DWARF))
     ret = dwarf_find_proc_info (as, ip, pi, need_unwind_info, arg);
 
-  if (ret < 0 && UNW_TRY_METHOD (UNW_ARM_METHOD_EXIDX))
+  if (ret < 0 && UNW_TRY_METHOD (UNW_ARM_METHOD_EXIDX) &&
+      (methods & UNW_ARM_METHOD_EXIDX))
     {
       struct arm_cb_data cb_data;
 
@@ -540,6 +542,14 @@ arm_find_proc_info (unw_addr_space_t as, unw_word_t ip,
   return ret;
 }
 
+HIDDEN int
+arm_find_proc_info (unw_addr_space_t as, unw_word_t ip,
+                    unw_proc_info_t *pi, int need_unwind_info, void *arg)
+{
+    return arm_find_proc_info2 (as, ip, pi, need_unwind_info, arg,
+                                UNW_ARM_METHOD_ALL);
+}
+
 HIDDEN void
 arm_put_unwind_info (unw_addr_space_t as, unw_proc_info_t *proc_info, void *arg)
 {
diff --git a/src/arm/Gstep.c b/src/arm/Gstep.c
index 895e8a892..e4ada651b 100644
--- a/src/arm/Gstep.c
+++ b/src/arm/Gstep.c
@@ -54,17 +54,22 @@ arm_exidx_step (struct cursor *c)
                                      c->dwarf.as_arg);
   if (ret == -UNW_ENOINFO)
     {
+#ifdef UNW_LOCAL_ONLY
+      if ((ret = arm_find_proc_info2 (c->dwarf.as, ip, &c->dwarf.pi,
+                                      1, c->dwarf.as_arg,
+                                      UNW_ARM_METHOD_EXIDX)) < 0)
+        return ret;
+#else
       if ((ret = tdep_find_proc_info (&c->dwarf, ip, 1)) < 0)
         return ret;
+#endif
     }
 
   if (c->dwarf.pi.format != UNW_INFO_FORMAT_ARM_EXIDX)
     return -UNW_ENOINFO;
 
   ret = arm_exidx_extract (&c->dwarf, buf);
-  if (ret == -UNW_ESTOPUNWIND)
-    return 0;
-  else if (ret < 0)
+  if (ret < 0)
     return ret;
 
   ret = arm_exidx_decode (buf, ret, &c->dwarf);
@@ -88,6 +93,7 @@ unw_step (unw_cursor_t *cursor)
 {
   struct cursor *c = (struct cursor *) cursor;
   int ret = -UNW_EUNSPEC;
+  int has_stopunwind = 0;
 
   Debug (1, "(cursor=%p)\n", c);
 
@@ -95,17 +101,31 @@ unw_step (unw_cursor_t *cursor)
   if (unw_is_signal_frame (cursor) > 0)
      return arm_handle_signal_frame (cursor);
 
+  /* First, try extbl-based unwinding. */
+  if (UNW_TRY_METHOD (UNW_ARM_METHOD_EXIDX))
+    {
+      ret = arm_exidx_step (c);
+      Debug(1, "arm_exidx_step()=%d\n", ret);
+      if (ret > 0)
+        return 1;
+      if (ret == 0)
+        return ret;
+      if (ret == -UNW_ESTOPUNWIND)
+        has_stopunwind = 1;
+    }
+
 #ifdef CONFIG_DEBUG_FRAME
-  /* First, try DWARF-based unwinding. */
+  /* Second, try DWARF-based unwinding. */
   if (UNW_TRY_METHOD(UNW_ARM_METHOD_DWARF))
     {
+      Debug (13, "%s(ret=%d), trying extbl\n",
+             UNW_TRY_METHOD(UNW_ARM_METHOD_EXIDX) ? "arm_exidx_step() failed " : "",
+             ret);
       ret = dwarf_step (&c->dwarf);
       Debug(1, "dwarf_step()=%d\n", ret);
 
       if (likely (ret > 0))
         return 1;
-      else if (unlikely (ret == -UNW_ESTOPUNWIND))
-        return ret;
 
       if (ret < 0 && ret != -UNW_ENOINFO)
         {
@@ -115,18 +135,9 @@ unw_step (unw_cursor_t *cursor)
     }
 #endif /* CONFIG_DEBUG_FRAME */
 
-  /* Next, try extbl-based unwinding. */
-  if (UNW_TRY_METHOD (UNW_ARM_METHOD_EXIDX))
-    {
-      Debug (13, "%s(ret=%d), trying extbl\n",
-             UNW_TRY_METHOD(UNW_ARM_METHOD_DWARF) ? "dwarf_step() failed " : "",
-             ret);
-      ret = arm_exidx_step (c);
-      if (ret > 0)
-        return 1;
-      if (ret == -UNW_ESTOPUNWIND || ret == 0)
-        return ret;
-    }
+  // Before trying the fallback, if any unwind info tell us to stop, do that.
+  if (has_stopunwind)
+    return -UNW_ESTOPUNWIND;
 
   /* Fall back on APCS frame parsing.
      Note: This won't work in case the ARM EABI is used. */
@@ -139,13 +150,13 @@ unw_step (unw_cursor_t *cursor)
       if (UNW_TRY_METHOD(UNW_ARM_METHOD_FRAME))
         {
           Debug (13, "%s%s%s%s(ret=%d), trying frame-chain\n",
-                 UNW_TRY_METHOD(UNW_ARM_METHOD_DWARF) ? "dwarf_step() " : "",
-                 (UNW_TRY_METHOD(UNW_ARM_METHOD_DWARF) && UNW_TRY_METHOD(UNW_ARM_METHOD_EXIDX)) ? "and " : "",
                  UNW_TRY_METHOD(UNW_ARM_METHOD_EXIDX) ? "arm_exidx_step() " : "",
-                 (UNW_TRY_METHOD(UNW_ARM_METHOD_DWARF) || UNW_TRY_METHOD(UNW_ARM_METHOD_EXIDX)) ? "failed " : "",
+                 (UNW_TRY_METHOD(UNW_ARM_METHOD_EXIDX) && UNW_TRY_METHOD(UNW_ARM_METHOD_DWARF)) ? "and " : "",
+                 UNW_TRY_METHOD(UNW_ARM_METHOD_DWARF) ? "dwarf_step() " : "",
+                 (UNW_TRY_METHOD(UNW_ARM_METHOD_EXIDX) || UNW_TRY_METHOD(UNW_ARM_METHOD_DWARF)) ? "failed " : "",
                  ret);
           ret = UNW_ESUCCESS;
-          /* DWARF unwinding failed, try to follow APCS/optimized APCS frame chain */
+          /* EXIDX and/or DWARF unwinding failed, try to follow APCS/optimized APCS frame chain */
           unw_word_t instr, i;
           dwarf_loc_t ip_loc, fp_loc;
           unw_word_t frame;