# Mu - subx/028translate.cc

      1 //: The bedrock level 1 of abstraction is now done, and we're going to start
      2 //: building levels above it that make programming in x86 machine code a
      3 //: little more ergonomic.
      4 //:
      5 //: All levels will be "pass through by default". Whatever they don't
      6 //: understand they will silently pass through to lower levels.
      7 //:
      8 //: Since raw hex bytes of machine code are always possible to inject, SubX is
      9 //: not a language, and we aren't building a compiler. This is something
     10 //: deliberately leakier. Levels are more for improving auditing, checks and
     11 //: error messages rather than for hiding low-level details.
     12
     13 //: Translator workflow: read 'source' file. Run a series of transforms on it,
     14 //: each passing through what it doesn't understand. The final program should
     15 //: be just machine code, suitable to write to an ELF binary.
     16 //:
     17 //: Higher levels usually transform code on the basis of metadata.
     18
     19 :(before "End Main")
     20 if (is_equal(argv[1], "translate")) {
     21   START_TRACING_UNTIL_END_OF_SCOPE;
     22   reset();
     23   // Begin subx translate
     24   program p;
     25   string output_filename;
     26   for (int i = /*skip 'subx translate'*/2;  i < argc;  ++i) {
     27     if (is_equal(argv[i], "-o")) {
     28       ++i;
     29       if (i >= argc) {
     30         print_translate_usage();
     31         cerr << "'-o' must be followed by a filename to write results to\n";
     32         exit(1);
     33       }
     34       output_filename = argv[i];
     35     }
     36     else {
     37       ifstream fin(argv[i]);
     38       if (!fin) {
     39         cerr << "could not open " << argv[i] << '\n';
     40         return 1;
     41       }
     42       parse(fin, p);
     43       if (trace_contains_errors()) return 1;
     44     }
     45   }
     46   if (p.segments.empty()) {
     47     print_translate_usage();
     48     cerr << "nothing to do; must provide at least one file to read\n";
     49     exit(1);
     50   }
     51   if (output_filename.empty()) {
     52     print_translate_usage();
     53     cerr << "must provide a filename to write to using '-o'\n";
     54     exit(1);
     55   }
     56   transform(p);
     57   if (trace_contains_errors()) return 1;
     58   save_elf(p, output_filename);
     59   if (trace_contains_errors()) {
     60     unlink(output_filename.c_str());
     61     return 1;
     62   }
     63   // End subx translate
     64   return 0;
     65 }
     66
     67 :(code)
     68 void print_translate_usage() {
     69   cerr << "Usage: subx translate file1 file2 ... -o output\n";
     70 }
     71
     72 // write out a program to a bare-bones ELF file
     73 void save_elf(const program& p, const string& filename) {
     74   ofstream out(filename.c_str(), ios::binary);
     75   write_elf_header(out, p);
     76   for (size_t i = 0;  i < p.segments.size();  ++i)
     77     write_segment(p.segments.at(i), out);
     78   out.close();
     79 }
     80
     81 void write_elf_header(ostream& out, const program& p) {
     82   char c = '\0';
     83 #define O(X)  c = (X); out.write(&c, sizeof(c))
     84 // host is required to be little-endian
     85 #define emit(X)  out.write(reinterpret_cast<const char*>(&X), sizeof(X))
     86   //// ehdr
     87   // e_ident
     88   O(0x7f); O(/*E*/0x45); O(/*L*/0x4c); O(/*F*/0x46);
     89     O(0x1);  // 32-bit format
     90     O(0x1);  // little-endian
     91     O(0x1); O(0x0);
     92   for (size_t i = 0;  i < 8;  ++i) { O(0x0); }
     93   // e_type
     94   O(0x02); O(0x00);
     95   // e_machine
     96   O(0x03); O(0x00);
     97   // e_version
     98   O(0x01); O(0x00); O(0x00); O(0x00);
     99   // e_entry
    100   int e_entry = p.segments.at(0).start;  // convention
    101   emit(e_entry);
    102   // e_phoff -- immediately after ELF header
    103   int e_phoff = 0x34;
    104   emit(e_phoff);
    105   // e_shoff; unused
    106   int dummy32 = 0;
    107   emit(dummy32);
    108   // e_flags; unused
    109   emit(dummy32);
    110   // e_ehsize
    111   uint16_t e_ehsize = 0x34;
    112   emit(e_ehsize);
    113   // e_phentsize
    114   uint16_t e_phentsize = 0x20;
    115   emit(e_phentsize);
    116   // e_phnum
    117   uint16_t e_phnum = SIZE(p.segments);
    118   emit(e_phnum);
    119   // e_shentsize
    120   uint16_t dummy16 = 0x0;
    121   emit(dummy16);
    122   // e_shnum
    123   emit(dummy16);
    124   // e_shstrndx
    125   emit(dummy16);
    126
    127   uint32_t p_offset = /*size of ehdr*/0x34 + SIZE(p.segments)*0x20/*size of each phdr*/;
    128   for (int i = 0;  i < SIZE(p.segments);  ++i) {
    129     //// phdr
    130     // p_type
    131     uint32_t p_type = 0x1;
    132     emit(p_type);
    133     // p_offset
    134     emit(p_offset);
    135     // p_vaddr
    136     uint32_t p_start = p.segments.at(i).start;
    137     emit(p_start);
    138     // p_paddr
    139     emit(p_start);
    140     // p_filesz
    141     uint32_t size = num_words(p.segments.at(i));
    142     assert(p_offset + size < INITIAL_SEGMENT_SIZE);
    143     emit(size);
    144     // p_memsz
    145     emit(size);
    146     // p_flags
    147     uint32_t p_flags = (i == 0) ? /*r-x*/0x5 : /*rw-*/0x6;  // convention: only first segment is code
    148     emit(p_flags);
    149
    150     // p_align
    151     // "As the system creates or augments a process image, it logically copies
    152     // a file's segment to a virtual memory segment.  When—and if— the system
    153     // physically reads the file depends on the program's execution behavior,
    154     // system load, and so on.  A process does not require a physical page
    155     // unless it references the logical page during execution, and processes
    156     // commonly leave many pages unreferenced. Therefore delaying physical
    157     // reads frequently obviates them, improving system performance. To obtain
    158     // this efficiency in practice, executable and shared object files must
    159     // have segment images whose file offsets and virtual addresses are
    160     // congruent, modulo the page size." -- http://refspecs.linuxbase.org/elf/elf.pdf (page 95)
    161     uint32_t p_align = 0x1000;  // default page size on linux
    162     emit(p_align);
    163     if (p_offset % p_align != p_start % p_align) {
    164       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();
    165       return;
    166     }
    167
    168     // prepare for next segment
    169     p_offset += size;
    170   }
    171 #undef O
    172 #undef emit
    173 }
    174
    175 void write_segment(const segment& s, ostream& out) {
    176   for (int i = 0;  i < SIZE(s.lines);  ++i) {
    177     const vector<word>& w = s.lines.at(i).words;
    178     for (int j = 0;  j < SIZE(w);  ++j) {
    179       uint8_t x = hex_byte(w.at(j).data);  // we're done with metadata by this point
    180       out.write(reinterpret_cast<const char*>(&x), /*sizeof(byte)*/1);
    181     }
    182   }
    183 }
    184
    185 uint32_t num_words(const segment& s) {
    186   uint32_t sum = 0;
    187   for (int i = 0;  i < SIZE(s.lines);  ++i)
    188     sum += SIZE(s.lines.at(i).words);
    189   return sum;
    190 }
    191
    192 :(before "End Includes")
    193 using std::ios;