https://github.com/akkartik/mu/blob/master/subx/012elf.cc

      1 //: Loading SubX programs from ELF binaries.
      2 //: This will allow us to run them natively on a Linux kernel.
      3 //: Based on https://github.com/kragen/stoneknifeforth/blob/702d2ebe1b/386.c
      4
      5 :(before "End Main")
      6 assert(argc > 1);
      7 if (is_equal(argv[1], "run")) {
      8   START_TRACING_UNTIL_END_OF_SCOPE;
      9   trace(2, "run") << "=== Starting to run" << end();
     10   assert(argc > 2);
     11   reset();
     12   cerr << std::hex;
     13   load_elf(argv[2], argc, argv);
     14   while (EIP < End_of_program)  // weak final-gasp termination check
     15     run_one_instruction();
     16   raise << "executed past end of the world: " << EIP << " vs " << End_of_program << '\n' << end();
     17   return 1;
     18 }
     19
     20 :(code)
     21 void load_elf(const string& filename, int argc, char* argv[]) {
     22   int fd = open(filename.c_str(), O_RDONLY);
     23   if (fd < 0) raise << filename.c_str() << ": open" << perr() << '\n' << die();
     24   off_t size = lseek(fd, 0, SEEK_END);
     25   lseek(fd, 0, SEEK_SET);
     26   uint8_t* elf_contents = static_cast<uint8_t*>(malloc(size));
     27   if (elf_contents == NULL) raise << "malloc(" << size << ')' << perr() << '\n' << die();
     28   ssize_t read_size = read(fd, elf_contents, size);
     29   if (size != read_size) raise << "read → " << size << " (!= " << read_size << ')' << perr() << '\n' << die();
     30   load_elf_contents(elf_contents, size, argc, argv);
     31   free(elf_contents);
     32 }
     33
     34 void load_elf_contents(uint8_t* elf_contents, size_t size, int argc, char* argv[]) {
     35   uint8_t magic[5] = {0};
     36   memcpy(magic, elf_contents, 4);
     37   if (memcmp(magic, "\177ELF", 4) != 0)
     38     raise << "Invalid ELF file; starts with \"" << magic << '"' << die();
     39   if (elf_contents[4] != 1)
     40     raise << "Only 32-bit ELF files (4-byte words; virtual addresses up to 4GB) supported.\n" << die();
     41   if (elf_contents[5] != 1)
     42     raise << "Only little-endian ELF files supported.\n" << die();
     43   // unused: remaining 10 bytes of e_ident
     44   uint32_t e_machine_type = u32_in(&elf_contents[16]);
     45   if (e_machine_type != 0x00030002)
     46     raise << "ELF type/machine 0x" << HEXWORD << e_machine_type << " isn't i386 executable\n" << die();
     47   // unused: e_version. We only support version 1, and later versions will be backwards compatible.
     48   uint32_t e_entry = u32_in(&elf_contents[24]);
     49   uint32_t e_phoff = u32_in(&elf_contents[28]);
     50   // unused: e_shoff
     51   // unused: e_flags
     52   uint32_t e_ehsize = u16_in(&elf_contents[40]);
     53   if (e_ehsize < 52) raise << "Invalid binary; ELF header too small\n" << die();
     54   uint32_t e_phentsize = u16_in(&elf_contents[42]);
     55   uint32_t e_phnum = u16_in(&elf_contents[44]);
     56   trace(90, "load") << e_phnum << " entries in the program header, each " << e_phentsize << " bytes long" << end();
     57   // unused: e_shentsize
     58   // unused: e_shnum
     59   // unused: e_shstrndx
     60
     61   set<uint32_t> overlap;  // to detect overlapping segments
     62   for (size_t i = 0;  i < e_phnum;  ++i)
     63     load_segment_from_program_header(elf_contents, i, size, e_phoff + i*e_phentsize, e_ehsize, overlap);
     64
     65   // initialize code and stack
     66   assert(overlap.find(STACK_SEGMENT) == overlap.end());
     67   Mem.push_back(vma(STACK_SEGMENT));
     68   assert(overlap.find(AFTER_STACK) == overlap.end());
     69   Reg[ESP].u = AFTER_STACK;
     70   Reg[EBP].u = 0;
     71   EIP = e_entry;
     72
     73   // initialize args on stack
     74   // no envp for now
     75   // we wastefully use a separate page of memory for argv
     76   Mem.push_back(vma(ARGV_DATA_SEGMENT));
     77   uint32_t argv_data = ARGV_DATA_SEGMENT;
     78   for (int i = argc-1;  i >= /*skip 'subx_bin' and 'run'*/2;  --i) {
     79     push(argv_data);
     80     for (size_t j = 0;  j <= strlen(argv[i]);  ++j) {
     81       assert(overlap.find(argv_data) == overlap.end());  // don't bother comparing ARGV and STACK
     82       write_mem_u8(argv_data, argv[i][j]);
     83       argv_data += sizeof(char);
     84       assert(argv_data < ARGV_DATA_SEGMENT + SEGMENT_ALIGNMENT);
     85     }
     86   }
     87   push(argc-/*skip 'subx_bin' and 'run'*/2);
     88 }
     89
     90 void push(uint32_t val) {
     91   Reg[ESP].u -= 4;
     92   trace(Callstack_depth+1, "run") << "decrementing ESP to 0x" << HEXWORD << Reg[ESP].u << end();
     93   trace(Callstack_depth+1, "run") << "pushing value 0x" << HEXWORD << val << end();
     94   write_mem_u32(Reg[ESP].u, val);
     95 }
     96
     97 void load_segment_from_program_header(uint8_t* elf_contents, int segment_index, size_t size, uint32_t offset, uint32_t e_ehsize, set<uint32_t>& overlap) {
     98   uint32_t p_type = u32_in(&elf_contents[offset]);
     99   trace(90, "load") << "program header at offset " << offset << ": type " << p_type << end();
    100   if (p_type != 1) {
    101     trace(90, "load") << "ignoring segment at offset " << offset << " of non PT_LOAD type " << p_type << " (see http://refspecs.linuxbase.org/elf/elf.pdf)" << end();
    102     return;
    103   }
    104   uint32_t p_offset = u32_in(&elf_contents[offset + 4]);
    105   uint32_t p_vaddr = u32_in(&elf_contents[offset + 8]);
    106   if (e_ehsize > p_vaddr) raise << "Invalid binary; program header overlaps ELF header\n" << die();
    107   // unused: p_paddr
    108   uint32_t p_filesz = u32_in(&elf_contents[offset + 16]);
    109   uint32_t p_memsz = u32_in(&elf_contents[offset + 20]);
    110   if (p_filesz != p_memsz)
    111     raise << "Can't yet handle segments where p_filesz != p_memsz (see http://refspecs.linuxbase.org/elf/elf.pdf)\n" << die();
    112
    113   if (p_offset + p_filesz > size)
    114     raise << "Invalid binary; segment at offset " << offset << " is too large: wants to end at " << p_offset+p_filesz << " but the file ends at " << size << '\n' << die();
    115   if (p_memsz >= SEGMENT_ALIGNMENT) {
    116     raise << "Code segment too small for SubX; for now please manually increase SEGMENT_ALIGNMENT.\n" << end();
    117     return;
    118   }
    119   trace(90, "load") << "blitting file offsets (" << p_offset << ", " << (p_offset+p_filesz) << ") to addresses (" << p_vaddr << ", " << (p_vaddr+p_memsz) << ')' << end();
    120   if (size > p_memsz) size = p_memsz;
    121   Mem.push_back(vma(p_vaddr));
    122   for (size_t i = 0;  i < p_filesz;  ++i) {
    123     assert(overlap.find(p_vaddr+i) == overlap.end());
    124     write_mem_u8(p_vaddr+i, elf_contents[p_offset+i]);
    125     overlap.insert(p_vaddr+i);
    126   }
    127   if (segment_index == 0 && End_of_program < p_vaddr+p_memsz)
    128     End_of_program = p_vaddr+p_memsz;
    129 }
    130
    131 :(before "End Includes")
    132 // Very primitive/fixed/insecure ELF segments for now: just consecutive VMAs.
    133 //   code: 0x09000000 -> 0x09ffffff
    134 //   data/heap: 0x0a000000 -> 0x0affffff
    135 //   stack: 0x0b000ffc -> 0x0b000000 (downward)
    136 const int CODE_SEGMENT = 0x09000000;
    137 const int DATA_SEGMENT = 0x0a000000;  // keep sync'd with `Heap.limit` in allocate.subx
    138 const int STACK_SEGMENT = 0x0b000000;
    139 const int AFTER_STACK = 0x0c000000;
    140 const int ARGV_DATA_SEGMENT = 0x0c000000;
    141 :(before "End Dump Info for Instruction")
    142 //? dump_stack();  // slow
    143 :(code)
    144 void dump_stack() {
    145   ostringstream out;
    146   trace(Callstack_depth+1, "run") << "stack:" << end();
    147   for (uint32_t a = AFTER_STACK-4;  a > Reg[ESP].u;  a -= 4)
    148     trace(Callstack_depth+2, "run") << "  0x" << HEXWORD << a << " => 0x" << HEXWORD << read_mem_u32(a) << end();
    149   trace(Callstack_depth+2, "run") << "  0x" << HEXWORD << Reg[ESP].u << " => 0x" << HEXWORD << read_mem_u32(Reg[ESP].u) << "  <=== ESP" << end();
    150   for (uint32_t a = Reg[ESP].u-4;  a > Reg[ESP].u-40;  a -= 4)
    151     trace(Callstack_depth+2, "run") << "  0x" << HEXWORD << a << " => 0x" << HEXWORD << read_mem_u32(a) << end();
    152 }
    153
    154 inline uint32_t u32_in(uint8_t* p) {
    155   return p[0] | p[1] << 8 | p[2] << 16 | p[3] << 24;
    156 }
    157
    158 inline uint16_t u16_in(uint8_t* p) {
    159   return p[0] | p[1] << 8;
    160 }
    161
    162 :(before "End Types")
    163 struct perr {};
    164 :(code)
    165 ostream& operator<<(ostream& os, perr /*unused*/) {
    166   if (errno)
    167     os << ": " << strerror(errno);
    168   return os;
    169 }
    170
    171 :(before "End Types")
    172 struct die {};
    173 :(code)
    174 ostream& operator<<(ostream& /*unused*/, die /*unused*/) {
    175   if (Trace_stream) Trace_stream->newline();
    176   exit(1);
    177 }
    178
    179 :(before "End Includes")
    180 #include <sys/types.h>
    181 #include <sys/stat.h>
    182 #include <fcntl.h>
    183 #include <stdarg.h>
    184 #include <errno.h>
    185 #include <unistd.h>