Process Credentials

• This is chapter 9. You should read it.
• I used a feature not in this chapter for the demo
  • #include <stdlib.h>
  • int mkstemp(char *template);
  • This command takes a template, which must end in XXXXXX
    • And includes a path reference if desired.
    • and will create a unique file name (if possible)
    • The file will be opened (r,w)
    • And with O_EXCL
    • with permissions 0600 (user read, write)
    • And a file descriptor will be returned
    • The template is modified after the call.
  • There are other versions, but we don't need them here.
    • different options for opening
    • Different options for naming.
• Each process carries a number of user ids
  • The real user and group id identify the user/group to which the process belongs.
  • The shell gets these from /etc/passwd
  • New processes inherit these from their parents.
  • #include <unistd.h>
  • #include <sys/types.h>
  • uid_t getuid(void);
  • gid_t getgid(void);
  • look at myID
  • run newgrp and look again.
• Effective User/Group ID
  • Used when a process tries to make a privileged system call
  • Used for sending signals
  • Also used for file access ...
  • If the effective ID is 0, the process is privileged.
  • Changed via set*id
  • Or setuid files.
• Set User ID files.
  • A file can be marked to change the user or group id.
  • This is dangerous.
  • This is easy/hard
    • setuid-bit, setgid bit.
    • chmod u+s filename
    • chmod g+s filename
    • But you can't do this for a file you don't own.
    • And you can't do this on some NFS mounted file systems.
    • And I bet there are new security things that watch this as well.
  • If these are set, the program is run with effective ids as listed.