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//: The bedrock level 1 of abstraction is now done, and we're going to start
//: building levels above it that make programming in x86 machine code a
//: little more ergonomic.
//:
//: All levels will be "pass through by default". Whatever they don't
//: understand they will silently pass through to lower levels.
//:
//: Since raw hex bytes of machine code are always possible to inject, SubX is
//: not a language, and we aren't building a compiler. This is something
//: deliberately leakier. Levels are more for improving auditing, checks and
//: error messages rather than for hiding low-level details.
//: Translator workflow: read 'source' file. Run a series of transforms on it,
//: each passing through what it doesn't understand. The final program should
//: be just machine code, suitable to write to an ELF binary.
//:
//: Higher levels usually transform code on the basis of metadata.
:(before "End Main")
if (is_equal(argv[1], "translate")) {
START_TRACING_UNTIL_END_OF_SCOPE;
reset();
program p;
string output_filename;
for (int i = /*skip 'subx translate'*/2; i < argc; ++i) {
if (is_equal(argv[i], "-o")) {
++i;
if (i >= argc) {
print_translate_usage();
cerr << "'-o' must be followed by a filename to write results to\n";
exit(1);
}
output_filename = argv[i];
}
else {
ifstream fin(argv[i]);
if (!fin) {
cerr << "could not open " << argv[i] << '\n';
return 1;
}
parse(fin, p);
if (trace_contains_errors()) return 1;
}
}
if (p.segments.empty()) {
print_translate_usage();
cerr << "nothing to do; must provide at least one file to read\n";
exit(1);
}
if (output_filename.empty()) {
print_translate_usage();
cerr << "must provide a filename to write to using '-o'\n";
exit(1);
}
transform(p);
if (trace_contains_errors()) return 1;
save_elf(p, output_filename);
if (trace_contains_errors()) {
unlink(output_filename.c_str());
return 1;
}
return 0;
}
:(code)
void print_translate_usage() {
cerr << "Usage: subx translate file1 file2 ... -o output\n";
}
// write out a program to a bare-bones ELF file
void save_elf(const program& p, const string& filename) {
ofstream out(filename.c_str(), ios::binary);
write_elf_header(out, p);
for (size_t i = 0; i < p.segments.size(); ++i)
write_segment(p.segments.at(i), out);
out.close();
}
void write_elf_header(ostream& out, const program& p) {
char c = '\0';
#define O(X) c = (X); out.write(&c, sizeof(c))
// host is required to be little-endian
#define emit(X) out.write(reinterpret_cast<const char*>(&X), sizeof(X))
//// ehdr
// e_ident
O(0x7f); O(/*E*/0x45); O(/*L*/0x4c); O(/*F*/0x46);
O(0x1); // 32-bit format
O(0x1); // little-endian
O(0x1); O(0x0);
for (size_t i = 0; i < 8; ++i) { O(0x0); }
// e_type
O(0x02); O(0x00);
// e_machine
O(0x03); O(0x00);
// e_version
O(0x01); O(0x00); O(0x00); O(0x00);
// e_entry
int e_entry = p.segments.at(0).start; // convention
emit(e_entry);
// e_phoff -- immediately after ELF header
int e_phoff = 0x34;
emit(e_phoff);
// e_shoff; unused
int dummy32 = 0;
emit(dummy32);
// e_flags; unused
emit(dummy32);
// e_ehsize
uint16_t e_ehsize = 0x34;
emit(e_ehsize);
// e_phentsize
uint16_t e_phentsize = 0x20;
emit(e_phentsize);
// e_phnum
uint16_t e_phnum = SIZE(p.segments);
emit(e_phnum);
// e_shentsize
uint16_t dummy16 = 0x0;
emit(dummy16);
// e_shnum
emit(dummy16);
// e_shstrndx
emit(dummy16);
uint32_t p_offset = /*size of ehdr*/0x34 + SIZE(p.segments)*0x20/*size of each phdr*/;
for (int i = 0; i < SIZE(p.segments); ++i) {
//// phdr
// p_type
uint32_t p_type = 0x1;
emit(p_type);
// p_offset
emit(p_offset);
// p_vaddr
uint32_t p_start = p.segments.at(i).start;
emit(p_start);
// p_paddr
emit(p_start);
// p_filesz
uint32_t size = num_words(p.segments.at(i));
assert(p_offset + size < SEGMENT_SIZE);
emit(size);
// p_memsz
emit(size);
// p_flags
uint32_t p_flags = (i == 0) ? /*r-x*/0x5 : /*rw-*/0x6; // convention: only first segment is code
emit(p_flags);
// p_align
// "As the system creates or augments a process image, it logically copies
// a file's segment to a virtual memory segment. When—and if— the system
// physically reads the file depends on the program's execution behavior,
// system load, and so on. A process does not require a physical page
// unless it references the logical page during execution, and processes
// commonly leave many pages unreferenced. Therefore delaying physical
// reads frequently obviates them, improving system performance. To obtain
// this efficiency in practice, executable and shared object files must
// have segment images whose file offsets and virtual addresses are
// congruent, modulo the page size." -- http://refspecs.linuxbase.org/elf/elf.pdf (page 95)
uint32_t p_align = 0x1000; // default page size on linux
emit(p_align);
if (p_offset % p_align != p_start % p_align) {
raise << "segment starting at 0x" << HEXWORD << p_start << " is improperly aligned; alignment for p_offset " << p_offset << " should be " << (p_offset % p_align) << " but is " << (p_start % p_align) << '\n' << end();
return;
}
// prepare for next segment
p_offset += size;
}
#undef O
#undef emit
}
void write_segment(const segment& s, ostream& out) {
for (int i = 0; i < SIZE(s.lines); ++i) {
const vector<word>& w = s.lines.at(i).words;
for (int j = 0; j < SIZE(w); ++j) {
uint8_t x = hex_byte(w.at(j).data); // we're done with metadata by this point
out.write(reinterpret_cast<const char*>(&x), /*sizeof(byte)*/1);
}
}
}
uint32_t num_words(const segment& s) {
uint32_t sum = 0;
for (int i = 0; i < SIZE(s.lines); ++i)
sum += SIZE(s.lines.at(i).words);
return sum;
}
:(before "End Includes")
using std::ios;
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