https://github.com/akkartik/mu/blob/master/subx/020syscalls.cc

      1 :(before "End Initialize Op Names")
      2 put_new(Name, "cd", "software interrupt (int)");
      3
      4 :(before "End Single-Byte Opcodes")
      5 case 0xcd: {  // int imm8 (software interrupt)
      6   trace(Callstack_depth+1, "run") << "syscall" << end();
      7   uint8_t code = next();
      8   if (code != 0x80) {
      9     raise << "Unimplemented interrupt code " << HEXBYTE << code << '\n' << end();
     10     raise << "  Only `int 80h` supported for now.\n" << end();
     11     break;
     12   }
     13   process_int80();
     14   break;
     15 }
     16
     17 :(code)
     18 void process_int80() {
     19   switch (Reg[EAX].u) {
     20   case 1:
     21     exit(/*exit code*/Reg[EBX].u);
     22     break;
     23   case 3:
     24     trace(Callstack_depth+1, "run") << "read: " << Reg[EBX].u << ' ' << Reg[ECX].u << ' ' << Reg[EDX].u << end();
     25     Reg[EAX].i = read(/*file descriptor*/Reg[EBX].u, /*memory buffer*/mem_addr_u8(Reg[ECX].u), /*size*/Reg[EDX].u);
     26     trace(Callstack_depth+1, "run") << "result: " << Reg[EAX].i << end();
     27     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
     28     break;
     29   case 4:
     30     trace(Callstack_depth+1, "run") << "write: " << Reg[EBX].u << ' ' << Reg[ECX].u << ' ' << Reg[EDX].u << end();
     31     trace(Callstack_depth+1, "run") << Reg[ECX].u << " => " << mem_addr_string(Reg[ECX].u, Reg[EDX].u) << end();
     32     Reg[EAX].i = write(/*file descriptor*/Reg[EBX].u, /*memory buffer*/mem_addr_u8(Reg[ECX].u), /*size*/Reg[EDX].u);
     33     trace(Callstack_depth+1, "run") << "result: " << Reg[EAX].i << end();
     34     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
     35     break;
     36   case 5: {
     37     check_flags(ECX);
     38     check_mode(EDX);
     39     trace(Callstack_depth+1, "run") << "open: " << Reg[EBX].u << ' ' << Reg[ECX].u << end();
     40     trace(Callstack_depth+1, "run") << Reg[EBX].u << " => " << mem_addr_kernel_string(Reg[EBX].u) << end();
     41     Reg[EAX].i = open(/*filename*/mem_addr_kernel_string(Reg[EBX].u), /*flags*/Reg[ECX].u, /*mode*/0640);
     42     trace(Callstack_depth+1, "run") << "result: " << Reg[EAX].i << end();
     43     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
     44     break;
     45   }
     46   case 6:
     47     trace(Callstack_depth+1, "run") << "close: " << Reg[EBX].u << end();
     48     Reg[EAX].i = close(/*file descriptor*/Reg[EBX].u);
     49     trace(Callstack_depth+1, "run") << "result: " << Reg[EAX].i << end();
     50     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
     51     break;
     52   case 8:
     53     check_mode(ECX);
     54     trace(Callstack_depth+1, "run") << "creat: " << Reg[EBX].u << end();
     55     trace(Callstack_depth+1, "run") << Reg[EBX].u << " => " << mem_addr_kernel_string(Reg[EBX].u) << end();
     56     Reg[EAX].i = creat(/*filename*/mem_addr_kernel_string(Reg[EBX].u), /*mode*/0640);
     57     trace(Callstack_depth+1, "run") << "result: " << Reg[EAX].i << end();
     58     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
     59     break;
     60   case 10:
     61     trace(Callstack_depth+1, "run") << "unlink: " << Reg[EBX].u << end();
     62     trace(Callstack_depth+1, "run") << Reg[EBX].u << " => " << mem_addr_kernel_string(Reg[EBX].u) << end();
     63     Reg[EAX].i = unlink(/*filename*/mem_addr_kernel_string(Reg[EBX].u));
     64     trace(Callstack_depth+1, "run") << "result: " << Reg[EAX].i << end();
     65     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
     66     break;
     67   case 38:
     68     trace(Callstack_depth+1, "run") << "rename: " << Reg[EBX].u << " -> " << Reg[ECX].u << end();
     69     trace(Callstack_depth+1, "run") << Reg[EBX].u << " => " << mem_addr_kernel_string(Reg[EBX].u) << end();
     70     trace(Callstack_depth+1, "run") << Reg[ECX].u << " => " << mem_addr_kernel_string(Reg[ECX].u) << end();
     71     Reg[EAX].i = rename(/*old filename*/mem_addr_kernel_string(Reg[EBX].u), /*new filename*/mem_addr_kernel_string(Reg[ECX].u));
     72     trace(Callstack_depth+1, "run") << "result: " << Reg[EAX].i << end();
     73     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
     74     break;
     75   case 45:  // brk: modify size of data segment
     76     trace(Callstack_depth+1, "run") << "grow data segment to " << Reg[EBX].u << end();
     77     grow_data_segment(/*new end address*/Reg[EBX].u);
     78     break;
     79   case 90:  // mmap: allocate memory outside existing segment allocations
     80     trace(Callstack_depth+1, "run") << "mmap: allocate new segment" << end();
     81     // Ignore most arguments for now: address hint, protection flags, sharing flags, fd, offset.
     82     // We only support anonymous maps.
     83     Reg[EAX].u = new_segment(/*length*/read_mem_u32(Reg[EBX].u+0x4));
     84     trace(Callstack_depth+1, "run") << "result: " << Reg[EAX].u << end();
     85     break;
     86   default:
     87     raise << HEXWORD << EIP << ": unimplemented syscall " << Reg[EAX].u << '\n' << end();
     88   }
     89 }
     90
     91 // SubX is oblivious to file permissions, directories, symbolic links, terminals, and much else besides.
     92 // Also ignoring any concurrency considerations for now.
     93 void check_flags(int reg) {
     94   uint32_t flags = Reg[reg].u;
     95   if (flags != ((flags & O_RDONLY) | (flags & O_WRONLY))) {
     96     cerr << HEXWORD << EIP << ": most POSIX flags to the open() syscall are not supported. Just O_RDONLY and O_WRONLY for now. Zero concurrent access support.\n";
     97     exit(1);
     98   }
     99   if ((flags & O_RDONLY) && (flags & O_WRONLY)) {
    100     cerr << HEXWORD << EIP << ": can't open a file for both reading and writing at once. See http://man7.org/linux/man-pages/man2/open.2.html.\n";
    101     exit(1);
    102   }
    103 }
    104
    105 void check_mode(int reg) {
    106   if (Reg[reg].u != 0600) {
    107     cerr << HEXWORD << EIP << ": SubX is oblivious to file permissions; register " << reg << " must be 0.\n";
    108     exit(1);
    109   }
    110 }
    111
    112 :(before "End Globals")
    113 uint32_t Next_segment = 0xb0000000;  // 0xc0000000 and up is reserved for Linux kernel
    114 const uint32_t SPACE_FOR_SEGMENT = 0x01000000;
    115 :(code)
    116 uint32_t new_segment(uint32_t length) {
    117   uint32_t result = Next_segment;
    118   Mem.push_back(vma(Next_segment, Next_segment+length));
    119   Next_segment -= SPACE_FOR_SEGMENT;
    120   return result;
    121 }