Homework 4

1. (100 points) In the Puzzle for the Day for February 23, the function mpopen provides a safe environment for the execution of the given command. Specifically, it first deletes the caller's environment, and sets only the following environment variables: PATH (to "/bin:/usr/bin:/usr/ucb:/usr/sbin:/sbin") IFS (to " "), SHELL (to the current value of the SHELL environment variable, or to the Bourne shell if SHELL is not set) and TZ (to "PST8PDT"). It closes all file descriptors that will not be used. It sets all signals to their default value, and umask to 077, the effective UID to the real UID, and the effective GID to the real GID. It then spawns the shell named in SHELL to execute the given command.

   The stream opened by mpopen is closed by mpclose.

   The interface to mpopen and mpclose is identical to that of popen(3) and pclose(3), as is their functionality.

   You are to write these two library functions.

   Hint: The process environment should be changed after the fork(2). if you change it before the fork, you will change the parent process' environment, which is not correct!

   Note: you may use any language you like, but the result must be a compiled library that can be called from a C program. We will supply our own driver for grading!

Extra Credit

2. Implement a variant of the system(3) command that uses the same protections as the mpopen/mpclose library functions, above.
3. Implement a set of library functions to enable the caller to control the environment set in mpopen:
   1. int le_set(char *env) takes an environment variable and associated value in the form "VAR=value". Subsequent calls to mpopen will add that environment variable and value to the subprocess. If env is "VAR=", the variable is defined to have the empty value. If env is "VAR", the value from the current environment is used. This function returns 1 if the argument was successfully added, and 0 if not.
   2. int le_unset(char *env) deletes an environment variable from the environment of subsequent calls to mpopen. The argument env names the variable, so to delet "VAR", say "le_unset("VAR")". This function returns 1 if the argument was successfully deleted, and 0 if not.
   3. void le_clobber(void) forces the next call to mpopen to use the standard mpopen environment.
   4. int le_umask(int umask) changes the value of umask as indicated in the environment of subsequent calls to mpopen. This function returns 1 if the change succeeds, and 0 if not.