bug#22050: [PATCH v3 1/2] linux-boot: Add make-static-device-nodes.

[ludo@gnu.org (Ludovic Courtès)]

Hello!

AIUI this will solve lack of /dev/fuse at startup (among other things),
right?  I always wondered why it wasn’t showing up automatically.

Danny Milosavljevic <dannym@scratchpost.org> skribis:

> * gnu/build/linux-boot.scm (make-static-device-nodes): New variable.
> ---
>  gnu/build/linux-boot.scm | 63 ++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 63 insertions(+)
>
> diff --git a/gnu/build/linux-boot.scm b/gnu/build/linux-boot.scm
> index 2547f1e0a..f6eea96e3 100644
> --- a/gnu/build/linux-boot.scm
> +++ b/gnu/build/linux-boot.scm
> @@ -24,6 +24,8 @@
>    #:use-module (srfi srfi-1)
>    #:use-module (srfi srfi-26)
>    #:use-module (ice-9 match)
> +  #:use-module (ice-9 rdelim)
> +  #:use-module (ice-9 popen)
>    #:use-module (ice-9 ftw)
>    #:use-module (guix build utils)
>    #:use-module ((guix build syscalls)
> @@ -35,6 +37,7 @@
>              linux-command-line
>              find-long-option
>              make-essential-device-nodes
> +            make-static-device-nodes
>              configure-qemu-networking
>  
>              bind-mount
> @@ -105,6 +108,66 @@ with the given MAJOR number, starting with MINOR."
>               'block-special #o644 (device-number major (+ minor i)))
>        (loop (+ i 1)))))
>  
> +(define (tmpfiles-mknod name type mode-string device-number-string)
> +  "Given a NAME, TYPE, MODE-STRING, DEVICE-NUMBER-STRING,
> +   call mknod with the respective numbers."
> +  (let* ((mode (string->number mode-string 8))
> +         (device-number-parts (string-split device-number-string #\:)))
> +    (match device-number-parts
> +     ((major-device-number-string minor-device-number-string)
> +      (let ((major-device-number (string->number major-device-number-string))
> +            (minor-device-number (string->number minor-device-number-string)))
> +        (mknod name type #o660 (device-number major-device-number
> +                                              minor-device-number))))
> +     (_ #f))))

That’s a surprising name ;-), and I would suggest separating parsing
from node creation (see below).

> +(define (log-static-device-system-error name callback)
> +  "Call CALLBACK.  If it fails, print an error message."
> +  (catch 'system-error
> +    (lambda ()
> +      (callback))
> +    (lambda args
> +      (format #t "could not create node '~a'~%" name))))

Rather:

  (define (report-system-error . args)
    (let ((errno (system-error-errno args)))
      (format (current-error-port) "could not create…: ~a~%" (strerror errno))))

  (define-syntax-rule (catch-system-error exp)
    (catch 'system-error
      (lambda ()
        exp)
      report-system-error))

(The term ‘callback’ is never used in Scheme; we just write ‘proc’ or
‘thunk’.)

> +(define* (make-static-device-nodes kmod-executable-name)
> +  "Invoke and handle 'kmod static-nodes' output."
> +  ;; "kmod static-nodes --format=tmpfiles" output format:
> +  ;;   c! /dev/fuse 0600 - - - 10:229
> +  ;;   d /dev/vfio 0755 - - -

I checked what kmod does and in fact it just reads
$LINUX_MODULE_DIRECTORY/*/modules.devname, which has a format similar to
what you’re parsing here:

--8<---------------cut here---------------start------------->8---
$ cat $LINUX_MODULE_DIRECTORY/*/modules.devname
# Device nodes to trigger on-demand module loading.
autofs4 autofs c10:235
fuse fuse c10:229
cuse cuse c10:203
btrfs btrfs-control c10:234
userio userio c10:240
vfio vfio/vfio c10:196
hci_vhci vhci c10:137
uhid uhid c10:239
vhost_net vhost-net c10:238
vhost_vsock vhost-vsock c10:241
snd_timer snd/timer c116:33
snd_seq snd/seq c116:1
--8<---------------cut here---------------end--------------->8---

Could we read that directly instead of invoking ‘kmod’?

What about having a ‘static-device-nodes’ procedure that would parse
that and return a list of <device-node>, where:

  ;; TYPE is 'char or 'block, MAJOR and MINOR are integers.
  (define-record-type <device-node>
    (device-node name type major minor module)
    device-node?
    …)

and then:

  (define create-device-node
    (match-lambda
      (($ <device-node> name type major minor)
       (mknod …))))

finally:

  (for-each create-device-node (static-device-nodes))

?

Thanks for fixing this!

Ludo’.