Getting "Syscall param execve(argv) points to unaddressable byte(s)" in valgrind

Question

Running the following C program with valgrind --leak-check=yes results in valgrind giving an output indicating that

Syscall param execve(argv) points to unaddressable byte(s)

The program is as follows:

int main() {
  const int NUM_ARGS = 3;
  char** run_arguments = malloc(sizeof(char*)*NUM_ARGS);
  run_arguments[0] = "ls";
  run_arguments[1] = "-l";
  run_arguments[2] = "--color";
  char* full_path = "/bin/ls";
  int pid = fork();
  if (pid == 0)
    execv(full_path,run_arguments);
  else {
    int status;
    waitpid(pid,&status,WUNTRACED);
    free(run_arguments);
  }
  return 0;
}

According to valgrind, the problem occurs on the line execv(full_path,run_arguments);, and the problem originates from the malloc done on the line char** run_arguments = malloc(sizeof(char*)*NUM_ARGS);.

What mistake have I made that causes valgrind to give this output?

Answer 1

The argument list must be terminated by a NULL pointer. Add one element to the run_arguments array, and have it be a NULL pointer.

Without the null pointer argument, the exec functions will go out of bounds in their search for the terminator, and will treat every non-null element as an argument that should be passed to the program. That leads to undefined behavior.

This is clearly noted in the exec manual page.