patch: Ics2017 两处框架相关的bug

[DGGua]

我校课程使用nemu 2017版本进行实验，2017版本中nemu框架两处位置会在最新编译器中提示warning，因Werror转为error。这里两处问题我认为应该不是教学目标，所以提一个patch修复

• src/cpu/exec/exec.c： nemu_backup/include/cpu/decode.h中DecodeInfo结构体有assembly和asm_buf两个char数组：

  typedef struct {
  uint32_t opcode;
  vaddr_t seq_eip;  // sequential eip
  bool is_operand_size_16;
  uint8_t ext_opcode;
  bool is_jmp;
  vaddr_t jmp_eip;
  Operand src, dest, src2;
  #ifdef DEBUG
  char assembly[80];
  char asm_buf[128];
  char *p;
  #endif
  } DecodeInfo;

  在src/cpu/exec/exec.c中执行了

  strcat(decoding.asm_buf, decoding.assembly);

  这里编译器会报警：

  /usr/include/x86_64-linux-gnu/bits/string_fortified.h:130:10: error: ‘__builtin___strcat_chk’ accessing 81 or more bytes at offsets 264 and 184 may overlap 1 byte at offset 264 [-Werror=restrict]
  130 |   return __builtin___strcat_chk (__dest, __src, __glibc_objsize (__dest));
    |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors
  make: *** [Makefile:25: build/obj/cpu/exec/exec.o] Error 1

  安全起见这里改为strncat

• nemu/src/monitor/diff-test/protocol.c： calloc 使用错误

  static struct gdb_conn* gdb_begin(int fd) {
  struct gdb_conn *conn = calloc(1, sizeof(struct gdb_conn *));
  if (conn == NULL)
  err(1, "calloc");