Homework 4
                               DEADLINE: Due Feb. 11

SUBMISSION:
  As always, you will submit a tarball (hw4.tar.gz) and a README file.
  You _must_ verify 'make check', or explain in README what isn't working.
  In addition, you must include open.txt, read.txt, and files.txt (see below).

1.  Rewrite xv6-sh.c to work on modern Linux.
    For full credit, you can change at most 30 lines.
    Your file must be called 'mysh.c'.
    You must make the minimum of changes to sh.c.  We will run:
      diff xv6-sh.c mysh.c
    where xv6-sh.c is the original in xv6 and mysh.c is code that works
    in Linux.
    In particular, some more recent syscalls that you will need are:
      fprintf(stderr, <FORMAT>, <ARGS>);
      fgets(<STRING>, <SIZE>, stdin);
      execvp(...);
    [ NOTE: 'man gets' exists in Linux, but the man page says to use fgets. ]
    [ WARNING:  This early sh.c can only handle 10 args on command line. ]

2.  Read about the syscall 'man 2 stat'.  Then write a short program,
     inode.c, that will print the inode number of a file.

3.  Rewrite xv6-ls.c to work on modern Linux.
    For full credit, you can change at most 35 lines.
    Your file must be called 'myls.c'.
    You must make the minimum of changes to sh.c.  We will run:
      diff xv6-ls.c myls.c | grep '^>' | wc -l
    where xv6-ls.c is the original in xv6 and myls.c is code that works
    in Linux.
    You will also need to add near the top (after the '#include'):
      #define DIRSIZ 40
    In particular, some more recent syscalls that you will need are:
      struct dirent readdir(DIR *dirp);  [ SEE 'man 3 readdir' ]
        [ HINT: CHANGE 'struct dirent' to the higher-level type, 'DIR' ]
      DIR *opendir(const char *name);
      int closedir(DIR *dirp);
      For an example program using these, see:
        https://pubs.opengroup.org/onlinepubs/007904875/functions/readdir_r.html
        [ Search on "EXAMPLES" ]
    NOTE:  'man 2 stat' will show you the entries of 'struct stat'.
    NOTE:  If you see errors in fprintf, such as 'long int' is not 'int',
           then try casting the argument to an int:  fprintf("%d\n", (int)XXX);
    [ HINT: Needs dirent.h ; Change your '#include' among other small changes ]
    [ HINT: In 'man 2 stat', note that 'file type' is st_mode.  Note also
            that 'man 2 stat' says to look at 'man 7 inode' for 'file type'. ]

4.  You must include a file, open.txt, which explains how 'open' is implemented.

    Read ulibc.c:stat() to see how xv6 computes the inode.
    As you will see, 'open()' is used to get a file descriptor for
    the path (filename), and then 'fstat()' is used to take that
    file descriptor and determine the inode.  Let's explore how
    the xv6 operating system converts from the filename.

    Start with sys_open() in sysfile.c.  You will see that eventually,
    namex() in fs.c is called.  Let's assume that the argument
    'path' for namex() is '/usr/bin/ls'.  Describe all steps inside
    the execution of namex(), including processing of directories
    in its pathname, until it returns with the 'inode *' (the inode pointer).

5.  You must include a file, read.txt, which explains how 'read' is implemented.

    Read 'man 2 read' for review.
    Now, start with 'sys_read()' in sysfile.c.  As you will see, this
    calls fileread() in file.c.  This uses a 'struct file *' (where
    a file descriptor indirectly points to a 'struct file').
    Eventually, readi() is called, based on the inode.  Describe
    how readi() reads from an ordinary file into the destination, dst.
    NOTE:  The function bread() means to read a block, based
           on its block number, 'blockno'.  You do not have to
           explain how 'bread()' works.

6.  You must include a file permissions.txt with the answer for this question.
    In Linux, 'man 2 open' has two kinds of permissions.  Quoting from
    the man page:
      "The  argument  flags  must  include  one of the following access
       modes: O_RDONLY, O_WRONLY, or O_RDWR."
    But quoting again from the man page:
      "The following symbolic constants are provided for mode:", with
      "S_IRWXU  00700 user (file owner) has read, write, and execute
         permission", etc.
    a.  Does xv6 have "access modes" or "read, write, and execute permissions"
        or both?
    b.  Indicate in what file and names that xv6 uses for
        "access modes", "read/write/execute permissions", or both.
        (If in part a, you decided only one permission exists in xv6,
         then describe only the one type of permission.)

7.  You must include a file files.txt with the answer for this question.
    In xv6, we see:
      (i) fd's (e.g., open() returns a fd); and
      (ii) "struct file"; and
      (iii) "struct inode".
    Describe how the three data structures, "fd", "struct file", and
    "struct inode", are related to each other.  In your explanation,
    you must cite the source of your reasoning.  This means referring
    either to a filename and a function within that file (or else
    a .h filename and a datatype such as a struct), for each "step"
    in the code that contributes to the relationship that you are describing.
    (NOTE:  Linux also has these same three data structures.)