Implement DWARF DW_CFA_val_expression for x86_64

Ubuntu's libc-bin (2.15-0ubuntu20.2) on x86_64 uses DW_CFA_val_expression
in describing the pthread spinlock operations __lll_unlock_wake() and
__lll_lock_wait().  libunwind 1.1 doesn't understand that opcode and
so backtraces from those operations are truncated.

This changeset adds basic support for it, by adding a new type to
dwarf_loc_t that describes the register's actual contents rather than
its location.  I've only implemented the new type for x86_64, and
stubbed it out for all other architectures -- it looks like a lot
of that code is duplicated so oughtn't to be that hard, but I don't
have test cases for them.

Tested that DW_CFA_val_expression works on x86_64 (by using
https://code.google.com/p/gperftools/ on a lock-heavy program).
Build-tested on x86, x86_64 and arm.  The unit tests don't pass for me
on any of those archs, but this cset doesn't break anything that was
passing before.

Signed-off-by: Tim Deegan <tjd@phlegethon.org>

1 files changed, 23 insertions, 1 deletions

diff --git a/src/dwarf/Gparser.c b/src/dwarf/Gparser.c
index b251e311..fefd8093 100644
--- a/src/dwarf/Gparser.c
+++ b/src/dwarf/Gparser.c
@@ -334,7 +334,22 @@ run_cfi_program (struct dwarf_cursor *c, dwarf_state_record_t *sr,
 	  *addr += len;
 	  break;
 
-	case DW_CFA_GNU_args_size:
+        case DW_CFA_val_expression:
+          if ((ret = read_regnum (as, a, addr, &regnum, arg)) < 0)
+            goto fail;
+
+          /* Save the address of the DW_FORM_block for later evaluation. */
+          set_reg (sr, regnum, DWARF_WHERE_VAL_EXPR, *addr);
+
+          if ((ret = dwarf_read_uleb128 (as, a, addr, &len, arg)) < 0)
+            goto fail;
+
+          Debug (15, "CFA_val_expression r%lu @ 0x%lx [%lu bytes]\n",
+                 (long) regnum, (long) addr, (long) len);
+          *addr += len;
+          break;
+
+        case DW_CFA_GNU_args_size:
 	  if ((ret = dwarf_read_uleb128 (as, a, addr, &val, arg)) < 0)
 	    goto fail;
 	  sr->args_size = val;
@@ -785,6 +800,13 @@ apply_reg_state (struct dwarf_cursor *c, struct dwarf_reg_state *rs)
 	  if ((ret = eval_location_expr (c, as, a, addr, c->loc + i, arg)) < 0)
 	    return ret;
 	  break;
+
+        case DWARF_WHERE_VAL_EXPR:
+          addr = rs->reg[i].val;
+          if ((ret = eval_location_expr (c, as, a, addr, c->loc + i, arg)) < 0)
+            return ret;
+          c->loc[i] = DWARF_VAL_LOC (c, DWARF_GET_LOC (c->loc[i]));
+          break;
 	}
     }