syscalls: Deduplicate device number conversion.

* guix/cpio.scm (device-number, device->major+minor):
Move to, and subsequently import from, …
* guix/build/syscalls.scm (device-number, device-number->major+minor):
…here.  Note the slight name change.
(mounts): Replace 16-bit open code with a DEVICE-NUMBER call.
* gnu/build/linux-boot.scm (device-number):
Remove duplicate 16-bit implementation in favour of the one above.
(resume-if-hibernated): Reuse DEVICE-NUMBER->MAJOR+MINOR.

gnu/build/linux-boot.scm

@@ -25,6 +25,7 @@ (define-module (gnu build linux-boot)
   #:autoload   (system repl repl) (start-repl)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-9)
+  #:use-module (srfi srfi-11)
   #:use-module (srfi srfi-26)
   #:use-module (ice-9 match)
   #:use-module (ice-9 rdelim)
@@ -44,7 +45,6 @@ (define-module (gnu build linux-boot)
             make-static-device-nodes
             configure-qemu-networking
 
-            device-number
             boot-system))
 
 ;;; Commentary:
@@ -134,14 +134,9 @@ (define (string->major:minor string)
            ;; is found on the command line; our canonicalize-device-spec gives
            ;; up after 20 seconds.  We could emulate the former by looping…
            (device (canonicalize-device-spec spec))
-           (rdev (stat:rdev (stat device)))
-           ;; For backwards compatibility, device numbering is a baroque affair.
-           ;; This is the full 64-bit scheme used by glibc's <sys/sysmacros.h>.
-           (major (logior (ash (logand #x00000000000fff00 rdev) -8)
-                          (ash (logand #xfffff00000000000 rdev) -32)))
-           (minor (logior      (logand #x00000000000000ff rdev)
-                          (ash (logand #x00000ffffff00000 rdev) -12))))
-      (format #f "~a:~a" major minor)))
+           (rdev (stat:rdev (stat device))))
+      (let-values (((major minor) (device-number->major+minor rdev)))
+        (format #f "~a:~a" major minor))))
 
   ;; Write the resume DEVICE to this magic file, using the MAJOR:MINOR device
   ;; numbers if possible.  The kernel will immediately try to resume from it.
@@ -392,11 +387,6 @@ (define* (configure-qemu-networking #:optional (interface "eth0"))
 
     (logand (network-interface-flags sock interface) IFF_UP)))
 
-(define (device-number major minor)
-  "Return the device number for the device with MAJOR and MINOR, for use as
-the last argument of `mknod'."
-  (+ (* major 256) minor))
-
 (define (pidof program)
   "Return the PID of the first presumed instance of PROGRAM."
   (let ((program (basename program)))

guix/build/syscalls.scm

@@ -7,6 +7,7 @@
 ;;; Copyright © 2020 Julien Lepiller <julien@lepiller.eu>
 ;;; Copyright © 2020 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
 ;;; Copyright © 2021 Chris Marusich <cmmarusich@gmail.com>
+;;; Copyright © 2021 Tobias Geerinckx-Rice <me@tobias.gr>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -56,6 +57,9 @@ (define-module (guix build syscalls)
 
             restart-on-EINTR
 
+            device-number
+            device-number->major+minor
+
             mount?
             mount-device-number
             mount-source
@@ -448,6 +452,29 @@ (define-as-needed proc (lambda* (args ...) body ...)))
            (module-define! (current-module) 'variable value)
            (module-export! (current-module) '(variable)))))))
 
+
+;;;
+;;; Block devices.
+;;;
+
+;; Convert between major:minor pairs and packed ‘device number’ representation.
+;; XXX These aren't syscalls, but if you squint very hard they are part of the
+;; FFI or however you want to justify me not finding a better fit… :-)
+(define (device-number major minor)     ; see glibc's <sys/sysmacros.h>
+  "Return the device number for the device with MAJOR and MINOR, for use as
+the last argument of `mknod'."
+  (logior (ash (logand #x00000fff major) 8)
+          (ash (logand #xfffff000 major) 32)
+               (logand #x000000ff minor)
+          (ash (logand #xffffff00 minor) 12)))
+
+(define (device-number->major+minor device)     ; see glibc's <sys/sysmacros.h>
+  "Return two values: the major and minor device numbers that make up DEVICE."
+  (values (logior (ash (logand #x00000000000fff00 device) -8)
+                  (ash (logand #xfffff00000000000 device) -32))
+          (logior      (logand #x00000000000000ff device)
+                  (ash (logand #x00000ffffff00000 device) -12))))
+
 
 ;;;
 ;;; File systems.
@@ -628,7 +655,7 @@ (define (mounts)
   (define (string->device-number str)
     (match (string-split str #\:)
       (((= string->number major) (= string->number minor))
-       (+ (* major 256) minor))))
+       (device-number major minor))))
 
   (call-with-input-file "/proc/self/mountinfo"
     (lambda (port)

guix/cpio.scm

@@ -18,6 +18,8 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (guix cpio)
+  #:use-module ((guix build syscalls) #:select (device-number
+                                                device-number->major+minor))
   #:use-module ((guix build utils) #:select (dump-port))
   #:use-module (srfi srfi-9)
   #:use-module (srfi srfi-11)
@@ -129,8 +131,8 @@ (define* (make-cpio-header #:key
                            (nlink 1) (mtime 0) (size 0)
                            (dev 0) (rdev 0) (name-size 0))
   "Return a new cpio file header."
-  (let-values (((major minor)   (device->major+minor dev))
-               ((rmajor rminor) (device->major+minor rdev)))
+  (let-values (((major minor)   (device-number->major+minor dev))
+               ((rmajor rminor) (device-number->major+minor rdev)))
     (%make-cpio-header MAGIC
                        inode mode uid gid
                        nlink mtime
@@ -154,21 +156,6 @@ (define (mode->type mode)
           (else
            (error "unsupported file type" mode)))))
 
-(define (device-number major minor)     ; see glibc's <sys/sysmacros.h>
-  "Return the device number for the device with MAJOR and MINOR, for use as
-the last argument of `mknod'."
-  (logior (ash (logand #x00000fff major) 8)
-          (ash (logand #xfffff000 major) 32)
-               (logand #x000000ff minor)
-          (ash (logand #xffffff00 minor) 12)))
-
-(define (device->major+minor device)     ; see glibc's <sys/sysmacros.h>
-  "Return two values: the major and minor device numbers that make up DEVICE."
-  (values (logior (ash (logand #x00000000000fff00 device) -8)
-                  (ash (logand #xfffff00000000000 device) -32))
-          (logior      (logand #x00000000000000ff device)
-                  (ash (logand #x00000ffffff00000 device) -12))))
-
 (define* (file->cpio-header file #:optional (file-name file)
                             #:key (stat lstat))
   "Return a cpio header corresponding to the info returned by STAT for FILE,