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   uint32_t e_entry = p.segments.at(0).start;  // convention
101   // Override e_entry
102   emit(e_entry);
103   // e_phoff -- immediately after ELF header
104   uint32_t e_phoff = 0x34;
105   emit(e_phoff);
106   // e_shoff; unused
107   uint32_t dummy32 = 0;
108   emit(dummy32);
109   // e_flags; unused
110   emit(dummy32);
111   // e_ehsize
112   uint16_t e_ehsize = 0x34;
113   emit(e_ehsize);
114   // e_phentsize
115   uint16_t e_phentsize = 0x20;
116   emit(e_phentsize);
117   // e_phnum
118   uint16_t e_phnum = SIZE(p.segments);
119   emit(e_phnum);
120   // e_shentsize
121   uint16_t dummy16 = 0x0;
122   emit(dummy16);
123   // e_shnum
124   emit(dummy16);
125   // e_shstrndx
126   emit(dummy16);
127 
128   uint32_t p_offset = /*size of ehdr*/0x34 + SIZE(p.segments)*0x20/*size of each phdr*/;
129   for (int i = 0;  i < SIZE(p.segments);  ++i) {
130     //// phdr
131     // p_type
132     uint32_t p_type = 0x1;
133     emit(p_type);
134     // p_offset
135     emit(p_offset);
136     // p_vaddr
137     uint32_t p_start = p.segments.at(i).start;
138     emit(p_start);
139     // p_paddr
140     emit(p_start);
141     // p_filesz
142     uint32_t size = num_words(p.segments.at(i));
143     assert(p_offset + size < SEGMENT_ALIGNMENT);
144     emit(size);
145     // p_memsz
146     emit(size);
147     // p_flags
148     uint32_t p_flags = (i == 0) ? /*r-x*/0x5 : /*rw-*/0x6;  // convention: only first segment is code
149     emit(p_flags);
150 
151     // p_align
152     // "As the system creates or augments a process image, it logically copies
153     // a file's segment to a virtual memory segment.  When—and if— the system
154     // physically reads the file depends on the program's execution behavior,
155     // system load, and so on.  A process does not require a physical page
156     // unless it references the logical page during execution, and processes
157     // commonly leave many pages unreferenced. Therefore delaying physical
158     // reads frequently obviates them, improving system performance. To obtain
159     // this efficiency in practice, executable and shared object files must
160     // have segment images whose file offsets and virtual addresses are
161     // congruent, modulo the page size." -- http://refspecs.linuxbase.org/elf/elf.pdf (page 95)
162     uint32_t p_align = 0x1000;  // default page size on linux
163     emit(p_align);
164     if (p_offset % p_align != p_start % p_align) {
165       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();
166       return;
167     }
168 
169     // prepare for next segment
170     p_offset += size;
171   }
172 #undef O
173 #undef emit
174 }
175 
176 void write_segment(const segment& s, ostream& out) {
177   for (int i = 0;  i < SIZE(s.lines);  ++i) {
178     const vector<word>& w = s.lines.at(i).words;
179     for (int j = 0;  j < SIZE(w);  ++j) {
180       uint8_t x = hex_byte(w.at(j).data);  // we're done with metadata by this point
181       out.write(reinterpret_cast<const char*>(&x), /*sizeof(byte)*/1);
182     }
183   }
184 }
185 
186 uint32_t num_words(const segment& s) {
187   uint32_t sum = 0;
188   for (int i = 0;  i < SIZE(s.lines);  ++i)
189     sum += SIZE(s.lines.at(i).words);
190   return sum;
191 }
192 
193 :(before "End Includes")
194 using std::ios;