shepherd service, unexplained permission errors

shepherd service, unexplained permission errors

[Attila Lendvai]

dear Guix,

i have a daemon that i (try to) run/start through Shepherd's fork+exec-command, with a specific user and group. i can start it up using su or sudo, both work:

$ sudo --user zigbee2mqtt bash -c "guix shell node gcc-toolchain make python libuv -- bash -c 'cd /srv/zigbee2mqtt && npm start'"

# su zigbee2mqtt --command "guix shell node gcc-toolchain make python libuv -- bash -c 'cd /srv/zigbee2mqtt && npm start'"

but through Shepherd the daemon fails while starting up due to a permission denied error:

"Error: Permission denied, cannot open /dev/ttyUSB0"

$ ls -l /dev/ttyUSB0 
crw-rw---- 1 root dialout 188, 0 Jun 28 21:04 /dev/ttyUSB0

i created a user for that daemon, and added dialout as a supplementary group:

(user-account
 (name "zigbee2mqtt")
 (comment "zigbee2mqtt service")
 (group "homeaut")
 (supplementary-groups '("dialout"))
 (system? #true))

(user-group
 (name "homeaut")
 (system? #true))

i start the service like this:

(simple-service
  'zigbee2mqtt
  shepherd-root-service-type
  (list
   (shepherd-service
    (requirement '(...))
    (provision '(...))
    (documentation "")
    (start
     #~(lambda _
         (let* ((cmd (list
                      "guix" "shell"
                      "node" "gcc-toolchain" "make" "python" "libuv"
                      "--"
                      "bash" "-c" "cd /srv/zigbee2mqtt && npm start")))
           (fork+exec-command
            cmd
            #:log-file "/var/log/zigbee2mqtt.log"
            #:user "zigbee2mqtt"
            #:group "homeaut")))))))

so, my question boils down to this:

what is different in the two environments? what am i missing?

PS: i have another daemon where i have a very similar issue, so it's probably not a daemon-specific issue.

PS2: i know that i should package zigbee2mqtt, but i'm taking this shortcut because life is short.

-- 
• attila lendvai
• PGP: 963F 5D5F 45C7 DFCD 0A39
--
“The trouble is, you think you have time.”
	— Siddhartha Gutama, aka Buddha (c. 5th century BC)

Re: shepherd service, unexplained permission errors

[Felix Lechner via Development of GNU Guix and the GNU System distribution.]

Hi Attila,

On Wed, Jun 28, 2023 at 1:27 PM Attila Lendvai <attila@lendvai.name> wrote:
>
> but through Shepherd the daemon fails while starting up due to a permission denied error:
>
> i created a user for that daemon, and added dialout as a supplementary group:
>
>  (supplementary-groups '("dialout"))
>
>            (fork+exec-command
>             cmd
>             #:log-file "/var/log/zigbee2mqtt.log"
>             #:user "zigbee2mqtt"
>             #:group "homeaut")))))))

The declaration of Shepherd's fork+exec-command has another optional
parameter for "supplementary-groups" [1] which is passed straight to
exec-command. [2]

With a double negation, the language in the manual is a bit hard to
parse, but the explicit argument may be required in order for the
supplementary group to work the way you expect:

"Likewise, command will be run under the current group, unless the
group keyword argument is present and not false, and
supplementary-groups is not '(). " [3]

Kind regards
Felix

[1] https://git.savannah.gnu.org/cgit/shepherd.git/tree/modules/shepherd/service.scm#n1569
[2] https://git.savannah.gnu.org/cgit/shepherd.git/tree/modules/shepherd/service.scm#n1610
[3] https://www.gnu.org/software/shepherd/manual/shepherd.html#index-exec_002dcommand

Re: shepherd service, unexplained permission errors

[Attila Lendvai]

thank you Felix!

the broken expectation from me was that users "have" a group and supplementar-groups (as suggested by the (user-account #:group ...) constructor), and if i specify the user to fork+exec-command, then it sets "its" group, too (i tried with and without specifying a #:group).

in retrospect it should have been obvious, because the fork+exec-command API is on the abstraction level of linux, not that of guix.

the solution that works now is essentially:

(fork+exec-command
  cmd
  #:user "zigbee2mqtt"
  #:group '#$(user-account-group *zigbee2mqtt-user*)
  #:supplementary-groups
  '#$(user-account-supplementary-groups *zigbee2mqtt-user*))

thanks again,

-- 
• attila lendvai
• PGP: 963F 5D5F 45C7 DFCD 0A39
--
“Enlightenment is man's leaving his self-caused immaturity. Immaturity is the incapacity to use one's intelligence without the guidance of another.”
	— Immanuel Kant (1724–1804)