4482

1 files changed, 159 insertions, 0 deletions

diff --git a/subx/028translate.cc b/subx/028translate.cc
new file mode 100644
index 00000000..f3e30126
--- /dev/null
+++ b/subx/028translate.cc
@@ -0,0 +1,159 @@
+//: 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;
+  assert(argc > 3);
+  program p;
+  ifstream fin(argv[2]);
+  if (!fin) {
+    cerr << "could not open " << argv[2] << '\n';
+    return 1;
+  }
+  parse(fin, p);
+  if (trace_contains_errors()) return 1;
+  transform(p);
+  if (trace_contains_errors()) return 1;
+  save_elf(p, argv[3]);
+  if (trace_contains_errors()) unlink(argv[3]);
+  return 0;
+}
+
+:(code)
+// write out a program to a bare-bones ELF file
+void save_elf(const program& p, const char* filename) {
+  ofstream out(filename, 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
+    emit(p.segments.at(i).start);
+    // p_paddr
+    emit(p.segments.at(i).start);
+    // p_filesz
+    uint32_t size = size_of(p.segments.at(i));
+    assert(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.segments.at(i).start % p_align) {
+      raise << "segment starting at 0x" << HEXWORD << p.segments.at(i).start << " is improperly aligned; alignment for p_offset " << p_offset << " should be " << (p_offset % p_align) << " but is " << (p.segments.at(i).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 size_of(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;