https://github.com/akkartik/mu/blob/main/linux/bootstrap/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   // Outside of tests, traces must be explicitly requested.
      9   if (Trace_file.is_open()) Trace_stream = new trace_stream;
     10   trace(2, "run") << "=== Starting to run" << end();
     11   if (argc <= 2) {
     12     raise << "Not enough arguments provided.\n" << die();
     13   }
     14   reset();
     15   cerr << std::hex;
     16   load_elf(argv[2], argc, argv);
     17   while (EIP < End_of_program)  // weak final-gasp termination check
     18     run_one_instruction();
     19   raise << "executed past end of the world: " << EIP << " vs " << End_of_program << '\n' << end();
     20   return 1;
     21 }
     22
     23 :(code)
     24 void load_elf(const string& filename, int argc, char* argv[]) {
     25   int fd = open(filename.c_str(), O_RDONLY);
     26   if (fd < 0) raise << filename.c_str() << ": open" << perr() << '\n' << die();
     27   off_t size = lseek(fd, 0, SEEK_END);
     28   lseek(fd, 0, SEEK_SET);
     29   uint8_t* elf_contents = static_cast<uint8_t*>(malloc(size));
     30   if (elf_contents == NULL) raise << "malloc(" << size << ')' << perr() << '\n' << die();
     31   ssize_t read_size = read(fd, elf_contents, size);
     32   if (size != read_size) raise << "read → " << size << " (!= " << read_size << ')' << perr() << '\n' << die();
     33   load_elf_contents(elf_contents, size, argc, argv);
     34   free(elf_contents);
     35 }
     36
     37 void load_elf_contents(uint8_t* elf_contents, size_t size, int argc, char* argv[]) {
     38   uint8_t magic[5] = {0};
     39   memcpy(magic, elf_contents, 4);
     40   if (memcmp(magic, "\177ELF", 4) != 0)
     41     raise << "Invalid ELF file; starts with \"" << magic << '"' << die();
     42   if (elf_contents[4] != 1)
     43     raise << "Only 32-bit ELF files (4-byte words; virtual addresses up to 4GB) supported.\n" << die();
     44   if (elf_contents[5] != 1)
     45     raise << "Only little-endian ELF files supported.\n" << die();
     46   // unused: remaining 10 bytes of e_ident
     47   uint32_t e_machine_type = u32_in(&elf_contents[16]);
     48   if (e_machine_type != 0x00030002)
     49     raise << "ELF type/machine 0x" << HEXWORD << e_machine_type << " isn't i386 executable\n" << die();
     50   // unused: e_version. We only support version 1, and later versions will be backwards compatible.
     51   uint32_t e_entry = u32_in(&elf_contents[24]);
     52   uint32_t e_phoff = u32_in(&elf_contents[28]);
     53   // unused: e_shoff
     54   // unused: e_flags
     55   uint32_t e_ehsize = u16_in(&elf_contents[40]);
     56   if (e_ehsize < 52) raise << "Invalid binary; ELF header too small\n" << die();
     57   uint32_t e_phentsize = u16_in(&elf_contents[42]);
     58   uint32_t e_phnum = u16_in(&elf_contents[44]);
     59   trace(90, "load") << e_phnum << " entries in the program header, each " << e_phentsize << " bytes long" << end();
     60   // unused: e_shentsize
     61   // unused: e_shnum
     62   // unused: e_shstrndx
     63
     64   set<uint32_t> overlap;  // to detect overlapping segments
     65   for (size_t i = 0;  i < e_phnum;  ++i)
     66     load_segment_from_program_header(elf_contents, i, size, e_phoff + i*e_phentsize, e_ehsize, overlap);
     67
     68   // initialize code and stack
     69   assert(overlap.find(STACK_SEGMENT) == overlap.end());
     70   Mem.push_back(vma(STACK_SEGMENT));
     71   assert(overlap.find(AFTER_STACK) == overlap.end());
     72   // The stack grows downward.
     73   Reg[ESP].u = AFTER_STACK;
     74   Reg[EBP].u = 0;
     75   EIP = e_entry;
     76
     77   // initialize args on stack
     78   // no envp for now
     79   // we wastefully use a separate page of memory for argv
     80   Mem.push_back(vma(ARGV_DATA_SEGMENT));
     81   uint32_t argv_data = ARGV_DATA_SEGMENT;
     82   for (int i = argc-1;  i >= /*skip 'subx_bin' and 'run'*/2;  --i) {
     83     push(argv_data);
     84     for (size_t j = 0;  j <= strlen(argv[i]);  ++j) {
     85       assert(overlap.find(argv_data) == overlap.end());  // don't bother comparing ARGV and STACK
     86       write_mem_u8(argv_data, argv[i][j]);
     87       argv_data += sizeof(char);
     88       assert(argv_data < ARGV_DATA_SEGMENT + SEGMENT_ALIGNMENT);
     89     }
     90   }
     91   push(argc-/*skip 'subx_bin' and 'run'*/2);
     92 }
     93
     94 void push(uint32_t val) {
     95   Reg[ESP].u -= 4;
     96   if (Reg[ESP].u < STACK_SEGMENT) {
     97     raise << "The stack overflowed its segment. "
     98           << "Maybe SPACE_FOR_SEGMENT should be larger? "
     99           << "Or you need to carve out an exception for the stack segment "
    100           << "to be larger.\n" << die();
    101   }
    102   trace(Callstack_depth+1, "run") << "decrementing ESP to 0x" << HEXWORD << Reg[ESP].u << end();
    103   trace(Callstack_depth+1, "run") << "pushing value 0x" << HEXWORD << val << end();
    104   write_mem_u32(Reg[ESP].u, val);
    105 }
    106
    107 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) {
    108   uint32_t p_type = u32_in(&elf_contents[offset]);
    109   trace(90, "load") << "program header at offset " << offset << ": type " << p_type << end();
    110   if (p_type != 1) {
    111     trace(90, "load") << "ignoring segment at offset " << offset << " of non PT_LOAD type " << p_type << " (see http://refspecs.linuxbase.org/elf/elf.pdf)" << end();
    112     return;
    113   }
    114   uint32_t p_offset = u32_in(&elf_contents[offset + 4]);
    115   uint32_t p_vaddr = u32_in(&elf_contents[offset + 8]);
    116   if (e_ehsize > p_vaddr) raise << "Invalid binary; program header overlaps ELF header\n" << die();
    117   // unused: p_paddr
    118   uint32_t p_filesz = u32_in(&elf_contents[offset + 16]);
    119   uint32_t p_memsz = u32_in(&elf_contents[offset + 20]);
    120   if (p_filesz != p_memsz)
    121     raise << "Can't yet handle segments where p_filesz != p_memsz (see http://refspecs.linuxbase.org/elf/elf.pdf)\n" << die();
    122
    123   if (p_offset + p_filesz > size)
    124     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();
    125   if (p_memsz >= SEGMENT_ALIGNMENT) {
    126     raise << "Code segment too small for SubX; for now please manually increase SEGMENT_ALIGNMENT.\n" << end();
    127     return;
    128   }
    129   trace(90, "load") << "blitting file offsets (" << p_offset << ", " << (p_offset+p_filesz) << ") to addresses (" << p_vaddr << ", " << (p_vaddr+p_memsz) << ')' << end();
    130   if (size > p_memsz) size = p_memsz;
    131   Mem.push_back(vma(p_vaddr));
    132   for (size_t i = 0;  i < p_filesz;  ++i) {
    133     assert(overlap.find(p_vaddr+i) == overlap.end());
    134     write_mem_u8(p_vaddr+i, elf_contents[p_offset+i]);
    135     overlap.insert(p_vaddr+i);
    136   }
    137   if (segment_index == 0 && End_of_program < p_vaddr+p_memsz)
    138     End_of_program = p_vaddr+p_memsz;
    139 }
    140
    141 :(before "End Includes")
    142 // Very primitive/fixed/insecure ELF segments for now.
    143 //   --- inaccessible:        0x00000000 -> 0x08047fff
    144 //   code:                    0x09000000 -> 0x09ffffff (specified in ELF binary)
    145 //   data:                    0x0a000000 -> 0x0affffff (specified in ELF binary)
    146 //                      --- heap gets mmap'd somewhere here ---
    147 //   stack:                   0xbdffffff -> 0xbd000000 (downward; not in ELF binary)
    148 //   argv hack:               0xbf000000 -> 0xbfffffff (not in ELF binary)
    149 //   --- reserved for kernel: 0xc0000000 -> ...
    150 const uint32_t START_HEAP        = 0x0b000000;
    151 const uint32_t END_HEAP          = 0xbd000000;
    152 const uint32_t STACK_SEGMENT     = 0xbd000000;
    153 const uint32_t AFTER_STACK       = 0xbe000000;
    154 const uint32_t ARGV_DATA_SEGMENT = 0xbf000000;
    155 // When updating the above memory map, don't forget to update `mmap`'s
    156 // implementation in the 'syscalls' layer.
    157 :(before "End Dump Info for Instruction")
    158 //? dump_stack();  // slow
    159 :(code)
    160 void dump_stack() {
    161   ostringstream out;
    162   trace(Callstack_depth+1, "run") << "stack:" << end();
    163   for (uint32_t a = AFTER_STACK-4;  a > Reg[ESP].u;  a -= 4)
    164     trace(Callstack_depth+2, "run") << "  0x" << HEXWORD << a << " => 0x" << HEXWORD << read_mem_u32(a) << end();
    165   trace(Callstack_depth+2, "run") << "  0x" << HEXWORD << Reg[ESP].u << " => 0x" << HEXWORD << read_mem_u32(Reg[ESP].u) << "  <=== ESP" << end();
    166   for (uint32_t a = Reg[ESP].u-4;  a > Reg[ESP].u-40;  a -= 4)
    167     trace(Callstack_depth+2, "run") << "  0x" << HEXWORD << a << " => 0x" << HEXWORD << read_mem_u32(a) << end();
    168 }
    169
    170 inline uint32_t u32_in(uint8_t* p) {
    171   return p[0] | p[1] << 8 | p[2] << 16 | p[3] << 24;
    172 }
    173
    174 inline uint16_t u16_in(uint8_t* p) {
    175   return p[0] | p[1] << 8;
    176 }
    177
    178 :(before "End Types")
    179 struct perr {};
    180 :(code)
    181 ostream& operator<<(ostream& os, perr /*unused*/) {
    182   if (errno)
    183     os << ": " << strerror(errno);
    184   return os;
    185 }
    186
    187 :(before "End Includes")
    188 #include <sys/types.h>
    189 #include <sys/stat.h>
    190 #include <fcntl.h>
    191 #include <stdarg.h>
    192 #include <errno.h>
    193 #include <unistd.h>