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