Re: [PATCH v4] kernel: Conditionally support non-root users, groups and capabilities

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 11 Feb 2015 17:03:36 +0200 Iulia Manda <iulia.manda21@gmail.com> wrote:

> There are a lot of embedded systems that run most or all of their functionality
> in init, running as root:root. For these systems, supporting multiple users is
> not necessary.
> 
> This patch adds a new symbol, CONFIG_MULTIUSER, that makes support for non-root
> users, non-root groups, and capabilities optional. It is enabled under
> CONFIG_EXPERT menu.
> 
> When this symbol is not defined, UID and GID are zero in any possible case
> and processes always have all capabilities.
> 
> The following syscalls are compiled out: setuid, setregid, setgid,
> setreuid, setresuid, getresuid, setresgid, getresgid, setgroups, getgroups,
> setfsuid, setfsgid, capget, capset.
> 
> Also, groups.c is compiled out completely.
> 
> In kernel/capability.c, capable function was moved in order to avoid adding two
> ifdef blocks.
> 
> This change saves about 25 KB on a defconfig build.
> 
> The kernel was booted in Qemu. All the common functionalities work. Adding
> users/groups is not possible, failing with -ENOSYS.
> 
> Bloat-o-meter output:
> add/remove: 7/87 grow/shrink: 19/397 up/down: 1675/-26325 (-24650)

The objective seems OK to me - in some kernel applications non-root
users are never needed and it's better to not force those systems to
carry a lump of code they will never use.

How significant is the 25k saving?  On a realistic kernel build for
such a target, what proportion of kernel memory usage are we talking
about here?

Did you look at moving sys_setregid into groups.c (or elsewhere) to
save an ifdef?

I assume that more savings can be squeezed out - there will be fields
in the task_struct and other places which will never be used.  ifdefing
those out will get messy.  Have you done any investigation here?