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>

include/dwarf.h

@@ -165,6 +165,7 @@ typedef enum
     DW_CFA_offset_extended_sf	= 0x11,
     DW_CFA_def_cfa_sf		= 0x12,
     DW_CFA_def_cfa_offset_sf	= 0x13,
+    DW_CFA_val_expression	= 0x16,
     DW_CFA_lo_user		= 0x1c,
     DW_CFA_MIPS_advance_loc8	= 0x1d,
     DW_CFA_GNU_window_save	= 0x2d,
@@ -224,6 +225,7 @@ typedef enum
     DWARF_WHERE_CFAREL,		/* register saved at CFA-relative address */
     DWARF_WHERE_REG,		/* register saved in another register */
     DWARF_WHERE_EXPR,		/* register saved */
+    DWARF_WHERE_VAL_EXPR,	/* register has computed value */
   }
 dwarf_where_t;
 

include/libunwind_i.h

@@ -351,6 +351,11 @@ static inline void invalidate_edi (struct elf_dyn_info *edi)
 # define tdep_get_func_addr(as,addr,v)		(*(v) = addr, 0)
 #endif
 
+#ifndef DWARF_VAL_LOC
+# define DWARF_IS_VAL_LOC(l)	0
+# define DWARF_VAL_LOC(c,v)	DWARF_NULL_LOC
+#endif
+
 #define UNW_ALIGN(x,a) (((x)+(a)-1UL)&~((a)-1UL))
 
 #endif /* libunwind_i_h */

include/tdep-x86_64/libunwind_i.h

@@ -109,25 +109,33 @@ dwarf_get_uc(const struct dwarf_cursor *cursor)
 # define DWARF_IS_NULL_LOC(l)	(DWARF_GET_LOC (l) == 0)
 # define DWARF_LOC(r, t)	((dwarf_loc_t) { .val = (r) })
 # define DWARF_IS_REG_LOC(l)	0
+# define DWARF_IS_MEM_LOC(l)	1
+# define DWARF_IS_VAL_LOC(l)	0
 # define DWARF_REG_LOC(c,r)	(DWARF_LOC((unw_word_t)			     \
 				 x86_64_r_uc_addr(dwarf_get_uc(c), (r)), 0))
 # define DWARF_MEM_LOC(c,m)	DWARF_LOC ((m), 0)
 # define DWARF_FPREG_LOC(c,r)	(DWARF_LOC((unw_word_t)			     \
 				 x86_64_r_uc_addr(dwarf_get_uc(c), (r)), 0))
+# define DWARF_VAL_LOC(c,v)	DWARF_NULL_LOC
+
 #else /* !UNW_LOCAL_ONLY */
 
 # define DWARF_LOC_TYPE_FP	(1 << 0)
 # define DWARF_LOC_TYPE_REG	(1 << 1)
+# define DWARF_LOC_TYPE_VAL	(1 << 2)
 # define DWARF_NULL_LOC		DWARF_LOC (0, 0)
 # define DWARF_IS_NULL_LOC(l)						\
 		({ dwarf_loc_t _l = (l); _l.val == 0 && _l.type == 0; })
 # define DWARF_LOC(r, t)	((dwarf_loc_t) { .val = (r), .type = (t) })
 # define DWARF_IS_REG_LOC(l)	(((l).type & DWARF_LOC_TYPE_REG) != 0)
 # define DWARF_IS_FP_LOC(l)	(((l).type & DWARF_LOC_TYPE_FP) != 0)
+# define DWARF_IS_MEM_LOC(l)	((l).type == 0)
+# define DWARF_IS_VAL_LOC(l)	(((l).type & DWARF_LOC_TYPE_VAL) != 0)
 # define DWARF_REG_LOC(c,r)	DWARF_LOC((r), DWARF_LOC_TYPE_REG)
 # define DWARF_MEM_LOC(c,m)	DWARF_LOC ((m), 0)
 # define DWARF_FPREG_LOC(c,r)	DWARF_LOC((r), (DWARF_LOC_TYPE_REG	\
 						| DWARF_LOC_TYPE_FP))
+# define DWARF_VAL_LOC(c,v)	DWARF_LOC ((v), DWARF_LOC_TYPE_VAL)
 
 #endif /* !UNW_LOCAL_ONLY */
 
@@ -158,14 +166,19 @@ dwarf_get (struct dwarf_cursor *c, dwarf_loc_t loc, unw_word_t *val)
   if (DWARF_IS_REG_LOC (loc))
     return (*c->as->acc.access_reg) (c->as, DWARF_GET_LOC (loc), val,
 				     0, c->as_arg);
-  else
+  if (DWARF_IS_MEM_LOC (loc))
     return (*c->as->acc.access_mem) (c->as, DWARF_GET_LOC (loc), val,
 				     0, c->as_arg);
+  assert(DWARF_IS_VAL_LOC (loc));
+  *val = DWARF_GET_LOC (loc);
+  return 0;
 }
 
 static inline int
 dwarf_put (struct dwarf_cursor *c, dwarf_loc_t loc, unw_word_t val)
 {
+  assert(!DWARF_IS_VAL_LOC (loc));
+
   if (DWARF_IS_NULL_LOC (loc))
     return -UNW_EBADREG;
 

src/dwarf/Gparser.c

@@ -334,6 +334,21 @@ run_cfi_program (struct dwarf_cursor *c, dwarf_state_record_t *sr,
 	  *addr += len;
 	  break;
 
+        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;
@@ -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;
 	}
     }