[Buildroot] The best way to add a new group to /etc/group

[Buildroot] The best way to add a new group to /etc/group

[Óscar Gómez Fuente]

Hi,

I'm building a new package in buildroot and I would like to know the best
way to add a new group.

I have to add a new group called: test_group. In my system (with
INIT_SYSV). I have a user called root and I have to add it to this group
too.

I've read the section "17.5.2. generic-package reference" in the buidroot
manual and find this variable: NAMEPACKAGE_USERS that it's very interesting
because automatically it adds the user you want and the uid and guid can be
computed automatically, so is very useful.

Does anyone know if there is something similar to add a group?

Righ now, I'm using this command in my package.mk but it's a lilttle bit
owful and very dangerous because I don't know if the guid 1001 is free or
not.

# Added usergroup test_group
@echo "test_group:x:1001:root" 1>>$(TARGET_DIR)/etc/group

Oscar Gomez Fuente
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[Buildroot] The best way to add a new group to /etc/group

[Yann E. MORIN]

?scar, All,

On 2016-03-18 10:38 +0100, ?scar G?mez Fuente spake thusly:
> I'm building a new package in buildroot and I would like to know the best
> way to add a new group.
> 
> I have to add a new group called: test_group. In my system (with
> INIT_SYSV). I have a user called root and I have to add it to this group
> too.
> 
> I've read the section "17.5.2. generic-package reference" in the buidroot
> manual and find this variable: NAMEPACKAGE_USERS that it's very interesting
> because automatically it adds the user you want and the uid and guid can be
> computed automatically, so is very useful.
> 
> Does anyone know if there is something similar to add a group?

Yes, it's simple: specify the username as just a plain dash '-', as
explained in the manual:
    https://buildroot.org/downloads/manual/manual.html#makeuser-syntax

    ---8<---
    username is the desired user name (aka login name) for the user. It
    can not be root, and must be unique. If set to -, then just a group
    will be created.
    ---8<---

Regards,
Yann E. MORIN.

-- 
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| +33 662 376 056 | Software  Designer | \ / CAMPAIGN     |  ___               |
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[Buildroot] The best way to add a new group to /etc/group

[Arnout Vandecappelle]

On 03/18/16 18:43, Yann E. MORIN wrote:
> ?scar, All,
>
> On 2016-03-18 10:38 +0100, ?scar G?mez Fuente spake thusly:
>> I'm building a new package in buildroot and I would like to know the best
>> way to add a new group.
>>
>> I have to add a new group called: test_group. In my system (with
>> INIT_SYSV). I have a user called root and I have to add it to this group
>> too.
>>
>> I've read the section "17.5.2. generic-package reference" in the buidroot
>> manual and find this variable: NAMEPACKAGE_USERS that it's very interesting
>> because automatically it adds the user you want and the uid and guid can be
>> computed automatically, so is very useful.
>>
>> Does anyone know if there is something similar to add a group?
>
> Yes, it's simple: specify the username as just a plain dash '-', as
> explained in the manual:
>      https://buildroot.org/downloads/manual/manual.html#makeuser-syntax
>
>      ---8<---
>      username is the desired user name (aka login name) for the user. It
>      can not be root, and must be unique. If set to -, then just a group
>      will be created.

  But as far as I know, there is no way to add users to a group, no? So to add 
the root user to this new group, the only option is to explicitly update the 
groups file.

  Regards,
  Arnout

>      ---8<---
>
> Regards,
> Yann E. MORIN.
>


-- 
Arnout Vandecappelle                          arnout at mind be
Senior Embedded Software Architect            +32-16-286500
Essensium/Mind                                http://www.mind.be
G.Geenslaan 9, 3001 Leuven, Belgium           BE 872 984 063 RPR Leuven
LinkedIn profile: http://www.linkedin.com/in/arnoutvandecappelle
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[Buildroot] The best way to add a new group to /etc/group

[Yann E. MORIN]

Arnout, All,

On 2016-03-18 22:19 +0100, Arnout Vandecappelle spake thusly:
> On 03/18/16 18:43, Yann E. MORIN wrote:
> >On 2016-03-18 10:38 +0100, ?scar G?mez Fuente spake thusly:
> >>I'm building a new package in buildroot and I would like to know the best
> >>way to add a new group.
> >>
> >>I have to add a new group called: test_group. In my system (with
> >>INIT_SYSV). I have a user called root and I have to add it to this group
> >>too.
> >>
> >>I've read the section "17.5.2. generic-package reference" in the buidroot
> >>manual and find this variable: NAMEPACKAGE_USERS that it's very interesting
> >>because automatically it adds the user you want and the uid and guid can be
> >>computed automatically, so is very useful.
> >>
> >>Does anyone know if there is something similar to add a group?
> >
> >Yes, it's simple: specify the username as just a plain dash '-', as
> >explained in the manual:
> >     https://buildroot.org/downloads/manual/manual.html#makeuser-syntax
> >
> >     ---8<---
> >     username is the desired user name (aka login name) for the user. It
> >     can not be root, and must be unique. If set to -, then just a group
> >     will be created.
> 
>  But as far as I know, there is no way to add users to a group, no?

I'm not sure to understand.

When adding a new user, you have to specify its primary group. There is
no way in UNIX to create a user that is not part of any group, so the
primary group is mandatory. So at least the user is part of that group.
There is nothing that prevents you from adding a user to an existing
group, or to create a brand new group for that user. In any case, you
have de facto added the user to its primary group.

Furthermore, when adding a user, you can also specify the additional
groups it will belong to (also explained in the manual).

Finally, defining the same user twice is in fact allowed. The manual is
not much clear about it, as it states that the user "must be unique."
What that means is that, when adding a user with a specific UID, it has
to be a unique combination. Some examples will better explain that:

Correct:
    FOO_USERS = foo - [...]
    BAR_USERS = foo - [...]

Correct:
    FOO_USERS = foo 1 [...]
    BAR_USERS = foo 1 [...]

Correct:
    FOO_USERS = foo - [...]
    BAR_USERS = foo 1 [...]

Not correct:
    FOO_USERS = foo 1 [...]
    BAR_USERS = foo 2 [...]

Similarly, the primary group must match for that user. Similarly, when
adding a group with a specific GID, it has to also be a unique GID.

What that means is that all definitions of a user must be "compatible",
and that all definitions of a group must be "compatible".

> So to
> add the root user to this new group, the only option is to explicitly update
> the groups file.

Indeed, it is not allowed to define the root user. We could allow that,
and ensure it gets UID zero, has primary group set to root and has GID
zero, as well as a few other similar sanity checks, so that it would be
possible to add the user root to any arbitrary groups.

And I doubt it is even sane to begin with... ;-/

However, root being root, it can become member of any group it pleases,
either by way of newgrp(1) in the shell, or programtaically with
setgid(2). So I fail to see what the issue here really is.

Regards,
Yann E. MORIN.

-- 
.-----------------.--------------------.------------------.--------------------.
|  Yann E. MORIN  | Real-Time Embedded | /"\ ASCII RIBBON | Erics' conspiracy: |
| +33 662 376 056 | Software  Designer | \ / CAMPAIGN     |  ___               |
| +33 223 225 172 `------------.-------:  X  AGAINST      |  \e/  There is no  |
| http://ymorin.is-a-geek.org/ | _/*\_ | / \ HTML MAIL    |   v   conspiracy.  |
'------------------------------^-------^------------------^--------------------'