guix system: 'reconfigure' disallows downgrades by default.

This is similar to what 9744cc7b46 did for
'guix pull'.

* guix/scripts/system/reconfigure.scm (ensure-forward-reconfigure)
(warn-about-backward-reconfigure, channel-relations)
(check-forward-update): New procedures.
* guix/scripts/system.scm (perform-action): Add #:validate-reconfigure.
Call 'check-forward-update' when ACTION is 'reconfigure.
(%options, show-help): Add "--allow-downgrades".
(%default-options): Add 'validate-reconfigure' key.
(process-action): Pass #:validate-reconfigure to 'perform-action'.
* doc/guix.texi (Invoking guix system): Document 'guix system describe'
more prominently, and document '--allow-downgrades'.

doc/guix.texi

@@ -28388,11 +28388,16 @@ an older system generation at boot time should you need it.
 Upon completion, the new system is deployed under
 @file{/run/current-system}.  This directory contains @dfn{provenance
 meta-data}: the list of channels in use (@pxref{Channels}) and
-@var{file} itself, when available.  This information is useful should
+@var{file} itself, when available.  You can view it by running:
-you later want to inspect how this particular generation was built.
 
-In fact, assuming @var{file} is self-contained, you can later rebuild
+@example
-generation @var{n} of your operating system with:
+guix system describe
+@end example
+
+This information is useful should you later want to inspect how this
+particular generation was built.  In fact, assuming @var{file} is
+self-contained, you can later rebuild generation @var{n} of your
+operating system with:
 
 @example
 guix time-machine \
@@ -28406,6 +28411,12 @@ system is not just a binary artifact: @emph{it carries its own source}.
 @xref{Service Reference, @code{provenance-service-type}}, for more
 information on provenance tracking.
 
+By default, @command{reconfigure} @emph{prevents you from downgrading
+your system}, which could (re)introduce security vulnerabilities and
+also cause problems with ``stateful'' services such as database
+management systems.  You can override that behavior by passing
+@option{--allow-downgrades}.
+
 @item switch-generation
 @cindex generations
 Switch to an existing system generation.  This action atomically
@@ -28732,6 +28743,22 @@ appear in the @code{operating-system} declaration actually exist
 needed at boot time are listed in @code{initrd-modules} (@pxref{Initial
 RAM Disk}).  Passing this option skips these tests altogether.
 
+@item --allow-downgrades
+Instruct @command{guix system reconfigure} to allow system downgrades.
+
+By default, @command{reconfigure} prevents you from downgrading your
+system.  It achieves that by comparing the provenance info of your
+system (shown by @command{guix system describe}) with that of your
+@command{guix} command (shown by @command{guix describe}).  If the
+commits for @command{guix} are not descendants of those used for your
+system, @command{guix system reconfigure} errors out.  Passing
+@option{--allow-downgrades} allows you to bypass these checks.
+
+@quotation Note
+Make sure you understand its security implications before using
+@option{--allow-downgrades}.
+@end quotation
+
 @cindex on-error
 @cindex on-error strategy
 @cindex error strategy

guix/scripts/system.scm

@@ -736,6 +736,7 @@ and TARGET arguments."
 
 (define* (perform-action action os
                          #:key
+                         (validate-reconfigure ensure-forward-reconfigure)
                          save-provenance?
                          skip-safety-checks?
                          install-bootloader?
@@ -778,7 +779,8 @@ static checks."
          (operating-system-bootcfg os menu-entries)))
 
   (when (eq? action 'reconfigure)
-    (maybe-suggest-running-guix-pull))
+    (maybe-suggest-running-guix-pull)
+    (check-forward-update validate-reconfigure))
 
   ;; Check whether the declared file systems exist.  This is better than
   ;; instantiating a broken configuration.  Assume that we can only check if
@@ -926,6 +928,9 @@ Some ACTIONS support additional ARGS.\n"))
   (display (G_ "
   -e, --expression=EXPR  consider the operating-system EXPR evaluates to
                          instead of reading FILE, when applicable"))
+  (display (G_ "
+      --allow-downgrades for 'reconfigure', allow downgrades to earlier
+                         channel revisions"))
   (display (G_ "
       --on-error=STRATEGY
                          apply STRATEGY (one of nothing-special, backtrace,
@@ -981,6 +986,11 @@ Some ACTIONS support additional ARGS.\n"))
          (option '(#\d "derivation") #f #f
                  (lambda (opt name arg result)
                    (alist-cons 'derivations-only? #t result)))
+         (option '("allow-downgrades") #f #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'validate-reconfigure
+                               warn-about-backward-reconfigure
+                               result)))
          (option '("on-error") #t #f
                  (lambda (opt name arg result)
                    (alist-cons 'on-error (string->symbol arg)
@@ -1053,6 +1063,7 @@ Some ACTIONS support additional ARGS.\n"))
     (graft? . #t)
     (debug . 0)
     (verbosity . #f)                              ;default
+    (validate-reconfigure . ,ensure-forward-reconfigure)
     (file-system-type . "ext4")
     (image-size . guess)
     (install-bootloader? . #t)))
@@ -1138,6 +1149,8 @@ resulting from command-line parsing."
                                #:use-substitutes? (assoc-ref opts 'substitutes?)
                                #:skip-safety-checks?
                                (assoc-ref opts 'skip-safety-checks?)
+                               #:validate-reconfigure
+                               (assoc-ref opts 'validate-reconfigure)
                                #:file-system-type (assoc-ref opts 'file-system-type)
                                #:image-size (assoc-ref opts 'image-size)
                                #:full-boot? (assoc-ref opts 'full-boot?)

guix/scripts/system/reconfigure.scm

@@ -34,9 +34,18 @@
   #:use-module (guix monads)
   #:use-module (guix store)
   #:use-module ((guix self) #:select (make-config.scm))
+  #:autoload   (guix describe) (current-profile)
+  #:use-module (guix channels)
+  #:autoload   (guix git) (update-cached-checkout)
+  #:use-module (guix i18n)
+  #:use-module (guix diagnostics)
+  #:use-module ((guix utils) #:select (&fix-hint))
   #:use-module (ice-9 match)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-11)
+  #:use-module (srfi srfi-34)
+  #:use-module (srfi srfi-35)
+  #:use-module ((guix config) #:select (%guix-package-name))
   #:export (switch-system-program
             switch-to-system
 
@@ -44,7 +53,11 @@
             upgrade-shepherd-services
 
             install-bootloader-program
-            install-bootloader))
+            install-bootloader
+
+            check-forward-update
+            ensure-forward-reconfigure
+            warn-about-backward-reconfigure))
 
 ;;; Commentary:
 ;;;
@@ -266,3 +279,85 @@ additional configurations specified by MENU-ENTRIES can be selected."
                                                             bootcfg-file
                                                             device
                                                             target))))))
+
+
+;;;
+;;; Downgrade detection.
+;;;
+
+(define (ensure-forward-reconfigure channel start commit relation)
+  "Raise an error if RELATION is not 'ancestor, meaning that START is not an
+ancestor of COMMIT, unless CHANNEL specifies a commit."
+  (match relation
+    ('ancestor #t)
+    ('self #t)
+    (_
+     (raise (make-compound-condition
+             (condition
+              (&message (message
+                         (format #f (G_ "\
+aborting reconfiguration because commit ~a of channel '~a' is not a descendant of ~a")
+                                 commit (channel-name channel)
+                                 start)))
+              (&fix-hint
+               (hint (G_ "Use @option{--allow-downgrades} to force
+this downgrade.")))))))))
+
+(define (warn-about-backward-reconfigure channel start commit relation)
+  "Warn about non-forward updates of CHANNEL from START to COMMIT, without
+aborting."
+  (match relation
+    ((or 'ancestor 'self)
+     #t)
+    ('descendant
+     (warning (G_ "rolling back channel '~a' from ~a to ~a~%")
+              (channel-name channel) start commit))
+    ('unrelated
+     (warning (G_ "moving channel '~a' from ~a to unrelated commit ~a~%")
+              (channel-name channel) start commit))))
+
+(define (channel-relations old new)
+  "Return a list of channel/relation pairs, where each relation is a symbol as
+returned by 'commit-relation' denoting how commits of channels in OLD relate
+to commits of channels in NEW."
+  (filter-map (lambda (old)
+                (let ((new (find (lambda (channel)
+                                   (eq? (channel-name channel)
+                                        (channel-name old)))
+                                 new)))
+                  (and new
+                       (let-values (((checkout commit relation)
+                                     (update-cached-checkout
+                                      (channel-url new)
+                                      #:ref
+                                      `(commit . ,(channel-commit new))
+                                      #:starting-commit
+                                      (channel-commit old)
+                                      #:check-out? #f)))
+                         (list new
+                               (channel-commit old) (channel-commit new)
+                               relation)))))
+              old))
+
+(define* (check-forward-update #:optional
+                               (validate-reconfigure ensure-forward-reconfigure))
+  "Call VALIDATE-RECONFIGURE passing it, for each channel, the channel, the
+currently-deployed commit (as returned by 'guix system describe') and the
+target commit (as returned by 'guix describe')."
+  ;; TODO: Make that functionality available to 'guix deploy'.
+  (define new
+    (or (and=> (current-profile) profile-channels)
+        '()))
+
+  (define old
+    (system-provenance "/run/current-system"))
+
+  (when (null? old)
+    (warning (G_ "cannot determine provenance for /run/current-system~%")))
+  (when (and (null? new) (not (getenv "GUIX_UNINSTALLED")))
+    (warning (G_ "cannot determine provenance of ~a~%") %guix-package-name))
+
+  (for-each (match-lambda
+              ((channel old new relation)
+               (validate-reconfigure channel old new relation)))
+            (channel-relations old new)))