dwarf: Push correct CFA onto stack for dwarf expression evaluation. (#93)

dwarf: Push correct CFA onto stack for dwarf expression evaluation.

This change fixes a bug where stale CFAs were pushed onto the dwarf
expression stack before expression evaluation. Some optimising compilers
emit CFI which relies on this being correct.

2 files changed, 21 insertions, 10 deletions

diff --git a/src/dwarf/Gexpr.c b/src/dwarf/Gexpr.c
index 709c0c8f..2af45433 100644
--- a/src/dwarf/Gexpr.c
+++ b/src/dwarf/Gexpr.c
@@ -237,8 +237,8 @@ dwarf_stack_aligned(struct dwarf_cursor *c, unw_word_t cfa_addr,
 }
 
 HIDDEN int
-dwarf_eval_expr (struct dwarf_cursor *c, unw_word_t *addr, unw_word_t len,
-                 unw_word_t *valp, int *is_register)
+dwarf_eval_expr (struct dwarf_cursor *c, unw_word_t stack_val, unw_word_t *addr,
+                 unw_word_t len, unw_word_t *valp, int *is_register)
 {
   unw_word_t operand1 = 0, operand2 = 0, tmp1, tmp2 = 0, tmp3, end_addr;
   uint8_t opcode, operands_signature, u8;
@@ -287,10 +287,14 @@ do {                                            \
   end_addr = *addr + len;
   *is_register = 0;
 
-  Debug (14, "len=%lu, pushing cfa=0x%lx\n",
-         (unsigned long) len, (unsigned long) c->cfa);
+  Debug (14, "len=%lu, pushing initial value=0x%lx\n",
+         (unsigned long) len, (unsigned long) stack_val);
 
-  push (c->cfa);        /* push current CFA as required by DWARF spec */
+  /* The DWARF standard requires the current CFA to be pushed onto the stack */
+  /* before evaluating DW_CFA_expression and DW_CFA_val_expression programs. */
+  /* DW_CFA_def_cfa_expressions do not take an initial value, but we push on */
+  /* a dummy value to keep this logic consistent. */
+  push (stack_val);
 
   while (*addr < end_addr)
     {
diff --git a/src/dwarf/Gparser.c b/src/dwarf/Gparser.c
index 1ce862c2..fe7c5817 100644
--- a/src/dwarf/Gparser.c
+++ b/src/dwarf/Gparser.c
@@ -734,7 +734,7 @@ create_state_record_for (struct dwarf_cursor *c, dwarf_state_record_t *sr,
 }
 
 static inline int
-eval_location_expr (struct dwarf_cursor *c, unw_addr_space_t as,
+eval_location_expr (struct dwarf_cursor *c, unw_word_t stack_val, unw_addr_space_t as,
                     unw_accessors_t *a, unw_word_t addr,
                     dwarf_loc_t *locp, void *arg)
 {
@@ -746,7 +746,7 @@ eval_location_expr (struct dwarf_cursor *c, unw_addr_space_t as,
     return ret;
 
   /* evaluate the expression: */
-  if ((ret = dwarf_eval_expr (c, &addr, len, &val, &is_register)) < 0)
+  if ((ret = dwarf_eval_expr (c, stack_val, &addr, len, &val, &is_register)) < 0)
     return ret;
 
   if (is_register)
@@ -804,7 +804,10 @@ apply_reg_state (struct dwarf_cursor *c, struct dwarf_reg_state *rs)
       assert (rs->reg.where[DWARF_CFA_REG_COLUMN] == DWARF_WHERE_EXPR);
 
       addr = rs->reg.val[DWARF_CFA_REG_COLUMN];
-      if ((ret = eval_location_expr (c, as, a, addr, &cfa_loc, arg)) < 0)
+      /* The dwarf standard doesn't specify an initial value to be pushed on */
+      /* the stack before DW_CFA_def_cfa_expression evaluation. We push on a */
+      /* dummy value (0) to keep the eval_location_expr function consistent. */
+      if ((ret = eval_location_expr (c, 0, as, a, addr, &cfa_loc, arg)) < 0)
         return ret;
       /* the returned location better be a memory location... */
       if (DWARF_IS_REG_LOC (cfa_loc))
@@ -844,13 +847,17 @@ apply_reg_state (struct dwarf_cursor *c, struct dwarf_reg_state *rs)
 
         case DWARF_WHERE_EXPR:
           addr = rs->reg.val[i];
-          if ((ret = eval_location_expr (c, as, a, addr, new_loc + i, arg)) < 0)
+          /* The dwarf standard requires the current CFA to be pushed on the */
+          /* stack before DW_CFA_expression evaluation. */
+          if ((ret = eval_location_expr (c, cfa, as, a, addr, new_loc + i, arg)) < 0)
             return ret;
           break;
 
         case DWARF_WHERE_VAL_EXPR:
           addr = rs->reg.val[i];
-          if ((ret = eval_location_expr (c, as, a, addr, new_loc + i, arg)) < 0)
+          /* The dwarf standard requires the current CFA to be pushed on the */
+          /* stack before DW_CFA_val_expression evaluation. */
+          if ((ret = eval_location_expr (c, cfa, as, a, addr, new_loc + i, arg)) < 0)
             return ret;
           new_loc[i] = DWARF_VAL_LOC (c, DWARF_GET_LOC (new_loc[i]));
           break;