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       trace(2, "parse") << argv[i] << end();
 38       ifstream fin(argv[i]);
 39       if (!fin) {
 40         cerr << "could not open " << argv[i] << '\n';
 41         return 1;
 42       }
 43       parse(fin, p);
 44       if (trace_contains_errors()) return 1;
 45     }
 46   }
 47   if (p.segments.empty()) {
 48     print_translate_usage();
 49     cerr << "nothing to do; must provide at least one file to read\n";
 50     exit(1);
 51   }
 52   if (output_filename.empty()) {
 53     print_translate_usage();
 54     cerr << "must provide a filename to write to using '-o'\n";
 55     exit(1);
 56   }
 57   trace(2, "transform") << "begin" << end();
 58   transform(p);
 59   if (trace_contains_errors()) return 1;
 60   trace(2, "translate") << "begin" << end();
 61   save_elf(p, output_filename);
 62   if (trace_contains_errors()) {
 63     unlink(output_filename.c_str());
 64     return 1;
 65   }
 66   // End subx translate
 67   return 0;
 68 }
 69 
 70 :(code)
 71 void print_translate_usage() {
 72   cerr << "Usage: subx translate file1 file2 ... -o output\n";
 73 }
 74 
 75 // write out a program to a bare-bones ELF file
 76 void save_elf(const program& p, const string& filename) {
 77   ofstream out(filename.c_str(), ios::binary);
 78   save_elf(p, out);
 79   out.close();
 80 }
 81 
 82 void save_elf(const program& p, ostream& out) {
 83   // validation: stay consistent with the self-hosted translator
 84   if (p.entry == 0) {
 85     raise << "no 'Entry' label found\n" << end();
 86     return;
 87   }
 88   if (find(p, "data") == NULL) {
 89     raise << "must include a 'data' segment\n" << end();
 90     return;
 91   }
 92   // processing
 93   write_elf_header(out, p);
 94   for (size_t i = 0;  i < p.segments.size();  ++i)
 95     write_segment(p.segments.at(i), out);
 96 }
 97 
 98 void write_elf_header(ostream& out, const program& p) {
 99   char c = '\0';
100 #define O(X)  c = (X); out.write(&c, sizeof(c))
101 // host is required to be little-endian
102 #define emit(X)  out.write(reinterpret_cast<const char*>(&X), sizeof(X))
103   //// ehdr
104   // e_ident
105   O(0x7f); O(/*E*/0x45); O(/*L*/0x4c); O(/*F*/0x46);
106     O(0x1);  // 32-bit format
107     O(0x1);  // little-endian
108     O(0x1); O(0x0);
109   for (size_t i = 0;  i < 8;  ++i) { O(0x0); }
110   // e_type
111   O(0x02); O(0x00);
112   // e_machine
113   O(0x03); O(0x00);
114   // e_version
115   O(0x01); O(0x00); O(0x00); O(0x00);
116   // e_entry
117   uint32_t e_entry = p.entry;
118   // Override e_entry
119   emit(e_entry);
120   // e_phoff -- immediately after ELF header
121   uint32_t e_phoff = 0x34;
122   emit(e_phoff);
123   // e_shoff; unused
124   uint32_t dummy32 = 0;
125   emit(dummy32);
126   // e_flags; unused
127   emit(dummy32);
128   // e_ehsize
129   uint16_t e_ehsize = 0x34;
130   emit(e_ehsize);
131   // e_phentsize
132   uint16_t e_phentsize = 0x20;
133   emit(e_phentsize);
134   // e_phnum
135   uint16_t e_phnum = SIZE(p.segments);
136   emit(e_phnum);
137   // e_shentsize
138   uint16_t dummy16 = 0x0;
139   emit(dummy16);
140   // e_shnum
141   emit(dummy16);
142   // e_shstrndx
143   emit(dummy16);
144 
145   uint32_t p_offset = /*size of ehdr*/0x34 + SIZE(p.segments)*0x20/*size of each phdr*/;
146   for (int i = 0;  i < SIZE(p.segments);  ++i) {
147     const segment& curr = p.segments.at(i);
148     //// phdr
149     // p_type
150     uint32_t p_type = 0x1;
151     emit(p_type);
152     // p_offset
153     emit(p_offset);
154     // p_vaddr
155     uint32_t p_start = curr.start;
156     emit(p_start);
157     // p_paddr
158     emit(p_start);
159     // p_filesz
160     uint32_t size = num_words(curr);
161     assert(p_offset + size < SEGMENT_ALIGNMENT);
162     emit(size);
163     // p_memsz
164     emit(size);
165     // p_flags
166     uint32_t p_flags = (curr.name == "code") ? /*r-x*/0x5 : /*rw-*/0x6;
167     emit(p_flags);
168 
169     // p_align
170     // "As the system creates or augments a process image, it logically copies
171     // a file's segment to a virtual memory segment.  When—and if— the system
172     // physically reads the file depends on the program's execution behavior,
173     // system load, and so on.  A process does not require a physical page
174     // unless it references the logical page during execution, and processes
175     // commonly leave many pages unreferenced. Therefore delaying physical
176     // reads frequently obviates them, improving system performance. To obtain
177     // this efficiency in practice, executable and shared object files must
178     // have segment images whose file offsets and virtual addresses are
179     // congruent, modulo the page size." -- http://refspecs.linuxbase.org/elf/elf.pdf (page 95)
180     uint32_t p_align = 0x1000;  // default page size on linux
181     emit(p_align);
182     if (p_offset % p_align != p_start % p_align) {
183       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();
184       return;
185     }
186 
187     // prepare for next segment
188     p_offset += size;
189   }
190 #undef O
191 #undef emit
192 }
193 
194 void write_segment(const segment& s, ostream& out) {
195   for (int i = 0;  i < SIZE(s.lines);  ++i) {
196     const vector<word>& w = s.lines.at(i).words;
197     for (int j = 0;  j < SIZE(w);  ++j) {
198       uint8_t x = hex_byte(w.at(j).data);  // we're done with metadata by this point
199       out.write(reinterpret_cast<const char*>(&x), /*sizeof(byte)*/1);
200     }
201   }
202 }
203 
204 uint32_t num_words(const segment& s) {
205   uint32_t sum = 0;
206   for (int i = 0;  i < SIZE(s.lines);  ++i)
207     sum += SIZE(s.lines.at(i).words);
208   return sum;
209 }
210 
211 :(before "End Includes")
212 using std::ios;