[cyphar]

Huh, I assumed (just as you did) that this would obviously not work -- but you're right that GCC ignores this and allows the assignment anyway.

However it turns out that you still get a build error, and even the more explicit versions also give you a error:

  kernel/cred.c:763:17: error: assignment of member ‘euid’ in read-only object
    763 |  current_euid() = GLOBAL_ROOT_UID;
        |                 ^
  kernel/cred.c:764:23: error: assignment of member ‘euid’ in read-only object
    764 |  current_cred()->euid = GLOBAL_ROOT_UID;
        |                       ^
  kernel/cred.c:765:22: error: assignment of member ‘euid’ in read-only object
    765 |  current->cred->euid = GLOBAL_ROOT_UID;
        |                      ^

So it is blocked but not for the reason I thought. current_cred() returns a const pointer and all of the cred pointers in task_struct are also const. So you'd need to do something more like:

  ((struct cred *)current_cred())->euid = GLOBAL_ROOT_UID;

Which is well beyond "eyebrow-raising" territory.