Is there a way to find out what user owns a process from the process's task_struct?
linux-kernel, multithreading, permissions, pid, process
Solution
Unfortunately, the user/group ids are no longer stored in the task struct, but instead in a separate privilege structure that's dynamically allocated and shared between all tasks that have the same ids. This in turn creates a situation where `setuid` can fail due to resource exhaustion, and failure of `setuid` to prop privileges is an infamous source of vulnerabilities...
Anyway, it's in these members of the `task_struct`:
const struct cred __rcu *real_cred; /* objective and real subjective task
* credentials (COW) */
const struct cred __rcu *cred; /* effective (overridable) subjective task
* credentials (COW) */
Problem
I have a `task_struct *` that I got by calling `find_task_by_vpid(get_pid())`. I'd like to figure out what user owns that process so that I can do some permission checking in the system call I'm writing, but looking through the `task_struct` source code hasn't helped much. The only thing that looked helpful is the `loginuid`, but for some reason the kernel won't compile if I try to access it like this: `my_task_struct->loginuid`. Is there another way to get the user who called the process from the `task_struct`?