system: Relax subordinate ID validation.

https://codeberg.org/guix/guix/issues/3925 raised the inabilityy of the
subordinate IDs service of handling externally managed sub{u,g}id file
entries. This patch relaxes the checks in place for existing ranges,
by allowing subid-range records lower than %subordinate-id-min, leaving
all the space from subid 0 to %subordinate-id-min - 1 to external
users. Generic ranges are still allocated within %subordinate-id-min and
%subordinate-id-max.

* gnu/build/accounts.scm (<unused-id-range>)[min]: Change default value
to 0, allowing subid-ranges with a start lesser than
%subordinate-id-min.
(allocate-generic-range): Allocate generic ranges starting from
%subordinate-id-min, leaving ranges starting before %subordinate-id-min.
(allocate-specific-range): Move bounds check to...
(allocate-subids): ...here. Now bound validation is applied only to user
provided ranges.
* tests/accounts.scm: Test new behavior.

Fixes: guix/guix#3925
Change-Id: Id923b122c97a20f148684f5fb144fd9422810612
Signed-off-by: Ludovic Courtès <ludo@gnu.org>
Merges: #4235

gnu/build/accounts.scm

@@ -359,7 +359,7 @@ to it atomically and set the appropriate permissions."
   (left    unused-subid-range-left   ;previous unused subuid range or #f
            (default #f))
   (min     unused-subid-range-min    ;lower bound of this unused subuid range
-           (default %subordinate-id-min))
+           (default 0))
   (max     unused-subid-range-max    ;upper bound
            (default %subordinate-id-max))
   (right   unused-subid-range-right  ;next unused subuid range or #f
@@ -555,7 +555,10 @@ will be marked as used in it."
   (define actual-range
     (subid-range
      (inherit range)
-     (start allocation-start)))
+     ;; New IDs are only allocated between %subordinate-id-min and
+     ;; %subordinate-id-max.
+     (start
+      (max allocation-start %subordinate-id-min))))
 
   (if (within-interval? allocation actual-range)
       (values
@@ -603,13 +606,6 @@ is visited to find the best unused range that can hold RANGE."
   (define range-end
     (subid-range-end range))
 
-  (unless (and (subordinate-id? range-start)
-               (subordinate-id? range-end))
-    (raise
-     (condition
-      (&invalid-subid-range-error
-       (range range)))))
-
   (define less?
     (< range-end allocation-start))
   (define more?
@@ -802,12 +798,23 @@ new UIDs."
 (define* (allocate-subids ranges #:optional (current-ranges '()))
   "Return a list of subids entries for RANGES, a list of <subid-range>.  IDs
 found in CURRENT-RANGES, a list of subid entries, are reused."
+  ;; Ranges from disk must always have a start.
   (let ((generic (any (compose not subid-range-has-start?) current-ranges)))
     (when generic
       (raise
        (condition
         (&specific-subid-range-expected-error
          (range generic))))))
+  (for-each
+   (lambda (range)
+     ;; New ranges must always be included in the current supported set.
+     (unless (or (not (subid-range-has-start? range))
+                 (and (subordinate-id? (subid-range-start range))
+                      (subordinate-id? (subid-range-end range))))
+       (raise
+        (condition (&invalid-subid-range-error (range range))))))
+   ranges)
+
   (define sorted-ranges
     (stable-sort ranges
                  subid-range-less))

tests/accounts.scm

@@ -271,6 +271,23 @@ ada:100600:300\n")
                                       (start %subordinate-id-min)
                                       (count 100)))))
 
+(test-equal "allocate-subids with externally managed state"
+  (list (subid-entry (name "guix-daemon") (start 904) (count 1))
+        (subid-entry (name "alice") (start 1085) (count 100))
+        (subid-entry (name "t") (start %subordinate-id-min) (count 899))
+        (subid-entry (name "x") (start 100899) (count 200)))
+  (allocate-subids (list
+                    (subid-range (name "x") (count 200))
+                    (subid-range (name "t") (count 899)))
+                   ;; Test use case from
+                   ;; https://codeberg.org/guix/guix/issues/3925
+                   (list (subid-range (name "guix-daemon")
+                                      (start 904)
+                                      (count 1))
+                         (subid-range (name "alice")
+                                      (start 1085)
+                                      (count 100)))))
+
 (test-equal "allocate-subids with requested IDs ranges"
   ;; Make sure the requested sub ID for "k" and "root" are honored.
   (list (subid-entry (name "x") (start %subordinate-id-min) (count 200))