Откуда UID / GID процесса, если не от указателя кредита процесса?

Задачи не имеют struct cred. У них есть два структурных кредита:

 * A task has two security pointers.  task->real_cred points to the objective
 * context that defines that task's actual details.  The objective part of this
 * context is used whenever that task is acted upon.
 *
 * task->cred points to the subjective context that defines the details of how
 * that task is going to act upon another object.  This may be overridden
 * temporarily to point to another security context, but normally points to the
 * same context as task->real_cred.

Я проверил, какой /procвам показывает, но вы, наверное, догадываетесь :-П.

(См. fs/proc/, используя https://elixir.bootlin.com. Файл «status» procfs определен в fs/proc/base.c.)