syscalls: Add 'add-to-entropy-count'.

* guix/build/syscalls.scm (RNDADDTOENTCNT): New variable.
(add-to-entropy-count): New procedure.
* tests/syscalls.scm ("add-to-entropy-count"): New test.

guix/build/syscalls.scm

@@ -68,6 +68,7 @@ (define-module (guix build syscalls)
             statfs
             free-disk-space
             device-in-use?
+            add-to-entropy-count
 
             processes
             mkdtemp!
@@ -706,6 +707,33 @@ (define* (device-in-use? device)
              (list (strerror err))
              (list err))))))
 
+
+;;;
+;;; Random.
+;;;
+
+;; From <uapi/linux/random.h>.
+(define RNDADDTOENTCNT #x40045201)
+
+(define (add-to-entropy-count port-or-fd n)
+  "Add N to the kernel's entropy count (the value that can be read from
+/proc/sys/kernel/random/entropy_avail).  PORT-OR-FD must correspond to
+/dev/urandom or /dev/random.  Raise to 'system-error with EPERM when the
+caller lacks root privileges."
+  (let ((fd  (if (port? port-or-fd)
+                 (fileno port-or-fd)
+                 port-or-fd))
+        (box (make-bytevector (sizeof int))))
+    (bytevector-sint-set! box 0 n (native-endianness)
+                          (sizeof int))
+    (let-values (((ret err)
+                  (%ioctl fd RNDADDTOENTCNT
+                          (bytevector->pointer box))))
+      (unless (zero? err)
+        (throw 'system-error "add-to-entropy-count" "~A"
+               (list (strerror err))
+               (list err))))))
+
 
 ;;;
 ;;; Containers.

tests/syscalls.scm

@@ -567,6 +567,19 @@ (define perform-container-tests?
   (let ((result (call-with-input-file "/var/run/utmpx" read-utmpx)))
     (or (utmpx? result) (eof-object? result))))
 
+(when (zero? (getuid))
+  (test-skip 1))
+(test-equal "add-to-entropy-count"
+  EPERM
+  (call-with-output-file "/dev/urandom"
+    (lambda (port)
+      (catch 'system-error
+        (lambda ()
+          (add-to-entropy-count port 77)
+          #f)
+        (lambda args
+          (system-error-errno args))))))
+
 (test-end)
 
 (false-if-exception (delete-file temp-file))