passwd source code

Question

I am trying to look at the source code of the passwd and gpasswd programs. These tools check the effective UID set to zero to complete execution successfully. But I am a noob to C and couldn't find the line of code that checks if the process is being run by UID == 0. I would appreciate any hints or solutions.

Answer 1

If you’re referring to the “shadow-utils” implementations of passwd, you’ll find the test here:

    /*
     * The program behaves differently when executed by root than when
     * executed by a normal user.
     */
    amroot = (getuid () == 0);

In gpasswd, the test is defined in two phases; first the UID is stored:

    /*
     * Make a note of whether or not this command was invoked by root.
     * This will be used to bypass certain checks later on. Also, set
     * the real user ID to match the effective user ID. This will
     * prevent the invoker from issuing signals which would interfere
     * with this command.
     */
    bywho = getuid ();

then that’s used in the amroot macro:

/* Indicate if gpasswd was called by root */
#define amroot  (0 == bywho)