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//: 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 much the same structure as rewriting labels.
:(scenario global_variable)
% Mem_offset = CODE_START;
% Mem.resize(0x2000);
== 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)) {
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)
% Mem_offset = CODE_START;
% Mem.resize(0x2000);
== code
8b/copy 0/mod/indirect 5/rm32/.disp32 2/r32/EDX x/disp32
==
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();
}
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