How to get current process's UID and EUID in Linux Kernel 4.2?

Question

As LDD3 chapter 6 p175 show, we can get current process UID and EUID by current->uid and current->euid. But the definition of struct task_struct of Linux Kernel 4.2 don't contain fields named by uid or euid any more. So, I wonder if there are any other methods to get UID and EUID ? Thanks!

Answer 1

.uid and .euid fields were moved to struct cred, which is now exposed as .cred field in struct task_struct. It was done in this commit: CRED: Separate task security context from task_struct. If you look at diff for include/linux/sched.h file, you can notice this change:

-   uid_t uid,euid,suid,fsuid;
-   gid_t gid,egid,sgid,fsgid;
+   struct cred *cred;  /* actual/objective task credentials */

So now instead of:

current->uid;
current->euid;

you should use:

const struct cred *cred = current_cred();

cred->uid;
cred->euid;

Notice that current_cred() function should be used to access .cred field, as it's RCU pointer.

Check out also check_same_owner() implementation for example.