about summary refs log tree commit diff stats
path: root/subx/036global_variables.cc
blob: e34c5b7398eca13d9ea7ff2ebeb2224beffe3ef6 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
//: Global variables.
//:
//: Global variables are just labels in the data segment.
//: However, they can only be used in imm32 and not disp32 operands. And they
//: can't be used with jump and call instructions.
//:
//: This layer has much the same structure as rewriting labels.

:(scenario global_variable)
== code
b9/copy x/imm32  # copy to ECX
== data
x:
00 00 00 00
+transform: global variable 'x' is at address 0x08049079

:(before "End Level-2 Transforms")
Transform.push_back(rewrite_global_variables);
:(code)
void rewrite_global_variables(program& p) {
  trace(99, "transform") << "-- rewrite global variables" << end();
  map<string, uint32_t> address;
  compute_addresses_for_global_variables(p, address);
  if (trace_contains_errors()) return;
  drop_global_variables(p);
  replace_global_variables_with_addresses(p, address);
}

void compute_addresses_for_global_variables(const program& p, map<string, uint32_t>& address) {
  for (int i = /*skip code segment*/1;  i < SIZE(p.segments);  ++i)
    compute_addresses_for_global_variables(p.segments.at(i), address);
}

void compute_addresses_for_global_variables(const segment& s, map<string, uint32_t>& address) {
  int current_address = s.start;
  for (int i = 0;  i < SIZE(s.lines);  ++i) {
    const line& inst = s.lines.at(i);
    for (int j = 0;  j < SIZE(inst.words);  ++j) {
      const word& curr = inst.words.at(j);
      if (*curr.data.rbegin() != ':') {
        ++current_address;
      }
      else {
        string variable = drop_last(curr.data);
        // ensure variables look sufficiently different from raw hex
        check_valid_name(variable);
        if (trace_contains_errors()) return;
        if (j > 0)
          raise << "'" << to_string(inst) << "': global variable names can only be the first word in a line.\n" << end();
        put(address, variable, current_address);
        trace(99, "transform") << "global variable '" << variable << "' is at address 0x" << HEXWORD << current_address << end();
        // no modifying current_address; global variable definitions won't be in the final binary
      }
    }
  }
}

void drop_global_variables(program& p) {
  for (int i = /*skip code segment*/1;  i < SIZE(p.segments);  ++i)
    drop_labels(p.segments.at(i));
}

void replace_global_variables_with_addresses(program& p, const map<string, uint32_t>& address) {
  if (p.segments.empty()) return;
  segment& code = p.segments.at(0);
  for (int i = 0;  i < SIZE(code.lines);  ++i) {
    line& inst = code.lines.at(i);
    line new_inst;
    for (int j = 0;  j < SIZE(inst.words);  ++j) {
      const word& curr = inst.words.at(j);
      if (!contains_key(address, curr.data)) {
        if (!looks_like_hex_int(curr.data))
          raise << "missing reference to global '" << curr.data << "'\n" << end();
        new_inst.words.push_back(curr);
        continue;
      }
      if (!valid_use_of_global_variable(curr)) {
        raise << "'" << to_string(inst) << "': can't refer to global variable '" << curr.data << "'\n" << end();
        return;
      }
      emit_hex_bytes(new_inst, get(address, curr.data), 4);
    }
    inst.words.swap(new_inst.words);
    trace(99, "transform") << "instruction after transform: '" << data_to_string(inst) << "'" << end();
  }
}

bool valid_use_of_global_variable(const word& curr) {
  if (has_operand_metadata(curr, "imm32")) return true;
  // End Valid Uses Of Global Variable(curr)
  return false;
}

//:: a more complex sanity check for how we use global variables
//: requires first saving some data early before we pack operands

:(after "Begin Level-2 Transforms")
Transform.push_back(correlate_disp32_with_mod);
:(code)
void correlate_disp32_with_mod(program& p) {
  if (p.segments.empty()) return;
  segment& code = p.segments.at(0);
  for (int i = 0;  i < SIZE(code.lines);  ++i) {
    line& inst = code.lines.at(i);
    for (int j = 0;  j < SIZE(inst.words);  ++j) {
      word& curr = inst.words.at(j);
      if (has_operand_metadata(curr, "disp32")
          && has_operand_metadata(inst, "mod"))
        curr.metadata.push_back("has_mod");
    }
  }
}

:(before "End Valid Uses Of Global Variable(curr)")
if (has_operand_metadata(curr, "disp32"))
  return has_metadata(curr, "has_mod");
// todo: more sophisticated check, to ensure we don't use global variable
// addresses as a real displacement added to other operands.

:(code)
bool has_metadata(const word& w, const string& m) {
  for (int i = 0;  i < SIZE(w.metadata);  ++i)
    if (w.metadata.at(i) == m) return true;
  return false;
}

:(scenario global_variable_disallowed_in_jump)
% Hide_errors = true;
== code
eb/jump x/disp8
== data
x:
00 00 00 00
+error: 'eb/jump x/disp8': can't refer to global variable 'x'
# sub-optimal error message; should be
#? +error: can't jump to data (variable 'x')

:(scenario global_variable_disallowed_in_call)
% Hide_errors = true;
== code
e8/call x/disp32
== data
x:
00 00 00 00
+error: 'e8/call x/disp32': can't refer to global variable 'x'
# sub-optimal error message; should be
#? +error: can't call to the data segment ('x')

:(scenario disp32_data_with_modrm)
== code
8b/copy 0/mod/indirect 5/rm32/.disp32 2/r32/EDX x/disp32
== data
x:
00 00 00 00
$error: 0

:(scenarios transform)
:(scenario disp32_data_with_call)
== code
foo:
e8/call bar/disp32
bar:
$error: 0

:(code)
string to_full_string(const line& in) {
  ostringstream out;
  for (int i = 0;  i < SIZE(in.words);  ++i) {
    if (i > 0) out << ' ';
    out << in.words.at(i).data;
    for (int j = 0;  j < SIZE(in.words.at(i).metadata);  ++j)
      out << '/' << in.words.at(i).metadata.at(j);
  }
  return out.str();
}