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Verify that UID / GID system call verification is performed in the critical RCU section

Task

I have a small kernel module that I wrote for my RaspBerry Pi 2, which implements an additional system call to generate power metrics. I would like to change the system call so that it is called only when it is issued by a special user (for example, the "root" or "pi" user). Otherwise, the call simply skips the main body and returns success.

Background work

I examined this question in detail, and I found a similar question in SO , but from my point of view there are many problems from my point of view (below).

Question

  • A related question notes what struct task_structcontains an element of a pointer to struct cred, as defined in linux/sched.hand linux/cred.h. The last of the two headers does not exist on my system (s), and the first does not show an element pointer declaration struct cred. It makes sense?
    • Stupid mistake. This is present in the kernel headers (i.e. /usr/src/linux-headers-$(uname -r)/include/linux/cred.h) I searched in the gcc-build headers in /usr/include/linux.
  • Even if the above worked, it does not mention whether I will get a real, efficient or saved UID for the process . Is it even possible to get each of these three values ​​from a system call?
    • cred.h already contains all this data.
  • , , , /etc/group?
    • cred.h .

Update

, :

, , RCU.

+4
2

. :

, uid, , .

, , , . , . RCU . , .

, , , , ( , ). - , - . , , .

, . make_kuid, uid_eq .

- syscall, /proc.

. blogpost : http://codingtragedy.blogspot.com/2015/04/weird-stuff-thread-credentials-in-linux.html

+2

-, . , : , libc userland. Linux (, Unix-), , , proc sysfs.

, , (, "root" "pi" ).

. , . . , 0 namespace ( , ! , , , ). ( capability, root.)

, , - sysfs. - - (, sysfs, , ). sysfs /sys. - , chgrp, chmod setfacl . , .

sysfs show , . , .

+2

Source: https://habr.com/ru/post/1652386/

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