Add an experimental and optional frame chain unwinding for ARM

The environment variable UNW_ARM_UNWIND_METHOD controls the unwind method.

1 - debug_frame unwinding
2 - frame chain unwinding,
3 - 1 & 2 (default))

Signed-off-by: Andris Zeila <andris.zeila@accenture.com>

include/tdep-arm/libunwind_i.h

@@ -255,4 +255,14 @@ extern int tdep_access_reg (struct cursor *c, unw_regnum_t reg,
 extern int tdep_access_fpreg (struct cursor *c, unw_regnum_t reg,
 			      unw_fpreg_t *valp, int write);
 
+/* unwinding method selection support */
+#define UNW_ARM_METHOD_ALL          0xFF
+#define UNW_ARM_METHOD_DWARF        0x01
+#define UNW_ARM_METHOD_FRAME        0x02
+
+#define unwi_unwind_method   UNWI_ARCH_OBJ(unwind_method)
+extern int unwi_unwind_method;
+
+#define UNW_TRY_METHOD(x)   (unwi_unwind_method & x)
+
 #endif /* ARM_LIBUNWIND_I_H */

src/arm/Gglobal.c

@@ -29,6 +29,9 @@ WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.  */
 HIDDEN pthread_mutex_t arm_lock = PTHREAD_MUTEX_INITIALIZER;
 HIDDEN int tdep_needs_initialization = 1;
 
+/* Unwinding methods to use. See UNW_METHOD_ enums */
+HIDDEN int unwi_unwind_method = UNW_ARM_METHOD_ALL;
+
 /* FIXME: I'm pretty sure we don't need this at all for ARM, but "generic"
    code (include/dwarf_i.h) seems to expect it to be here at present.  */
 
@@ -50,6 +53,13 @@ tdep_init (void)
       /* another thread else beat us to it... */
       goto out;
 
+    /* read ARM unwind method setting */
+    const char* str = getenv ("UNW_ARM_UNWIND_METHOD");
+    if (str)
+      {
+        unwi_unwind_method = atoi (str);
+      }
+
     mi_init ();
 
     dwarf_init ();

src/arm/Gstep.c

@@ -29,20 +29,83 @@ PROTECTED int
 unw_step (unw_cursor_t *cursor)
 {
   struct cursor *c = (struct cursor *) cursor;
-  int ret;
+  int ret = -UNW_EUNSPEC;
 
   Debug (1, "(cursor=%p)\n", c);
 
   /* Try DWARF-based unwinding...  this is the only method likely to work for
      ARM.  */
-  ret = dwarf_step (&c->dwarf);
+  if (UNW_TRY_METHOD(UNW_ARM_METHOD_DWARF))
+    {
+      ret = dwarf_step (&c->dwarf);
+      Debug(1, "dwarf_step()=%d\n", ret);
 
-  if (unlikely (ret == -UNW_ESTOPUNWIND))
-    return ret;
+      if (unlikely (ret == -UNW_ESTOPUNWIND))
+        return ret;
+
+    if (ret < 0 && ret != -UNW_ENOINFO)
+      {
+        Debug (2, "returning %d\n", ret);
+        return ret;
+      }
+    }
 
-  /* Dwarf unwinding didn't work, stop.  */
   if (unlikely (ret < 0))
-    return 0;
-
-  return (c->dwarf.ip == 0) ? 0 : 1;
+    {
+      if (UNW_TRY_METHOD(UNW_ARM_METHOD_FRAME))
+        {
+          ret = UNW_ESUCCESS;
+          /* DWARF unwinding failed, try to follow APCS/optimized APCS frame chain */
+          unw_word_t instr, i;
+          Debug (13, "dwarf_step() failed (ret=%d), trying frame-chain\n", ret);
+          dwarf_loc_t ip_loc, fp_loc;
+          unw_word_t frame;
+          /* Mark all registers unsaved, since we don't know where
+             they are saved (if at all), except for the EBP and
+             EIP.  */
+          if (dwarf_get(&c->dwarf, c->dwarf.loc[UNW_ARM_R11], &frame) < 0)
+            {
+              return 0;
+            }
+          for (i = 0; i < DWARF_NUM_PRESERVED_REGS; ++i) {
+            c->dwarf.loc[i] = DWARF_NULL_LOC;
+          }
+          if (frame)
+            {
+              if (dwarf_get(&c->dwarf, DWARF_LOC(frame, 0), &instr) < 0)
+                {
+                  return 0;
+                }
+              instr -= 8;
+              if (dwarf_get(&c->dwarf, DWARF_LOC(instr, 0), &instr) < 0)
+                {
+                  return 0;
+                }
+              if ((instr & 0xFFFFD800) == 0xE92DD800)
+                {
+                  /* Standard APCS frame. */
+                  ip_loc = DWARF_LOC(frame - 4, 0);
+                  fp_loc = DWARF_LOC(frame - 12, 0);
+                }
+              else
+                {
+                  /* Codesourcery optimized normal frame. */
+                  ip_loc = DWARF_LOC(frame, 0);
+                  fp_loc = DWARF_LOC(frame - 4, 0);
+                }
+              if (dwarf_get(&c->dwarf, ip_loc, &c->dwarf.ip) < 0)
+                {
+                  return 0;
+                }
+              c->dwarf.loc[UNW_ARM_R12] = ip_loc;
+              c->dwarf.loc[UNW_ARM_R11] = fp_loc;
+              Debug(15, "ip=%lx\n", c->dwarf.ip);
+            }
+          else
+            {
+              ret = -UNW_ENOINFO;
+            }
+        }
+    }
+  return ret == -UNW_ENOINFO ? 0 : 1;
 }

src/arm/getcontext.S

@@ -50,3 +50,7 @@ _Uarm_getcontext:
 	str r1, [r0, #15 * 4]
 	ldmfd sp!, {r0, r1}
 	bx lr
+#ifdef __linux__
+ /* We do not need executable stack.  */
+ .section  .note.GNU-stack,"",%progbits
+#endif

src/arm/siglongjmp.S

@@ -6,3 +6,7 @@
 _UI_siglongjmp_cont:
 _UI_longjmp_cont:
 	bx lr
+#ifdef __linux__
+ /* We do not need executable stack.  */
+ .section  .note.GNU-stack,"",%progbits
+#endif