Subject: [bug#73654] [PATCH v4] mapped-devices: luks: Support passing --allow-discards during open

From: [email-masked]
Date: Sun, 16 Mar 2025 12:49:50 +0100

From: Sören Tempel <[email-masked]>

* gnu/system/mapped-devices.scm (open-luks-device): Support opening
LUKS devices with the --allow-discards option.
* gnu/system/mapped-devices.scm (luks-device-mapping-with-options):
Pass through the allow-discards? keyword argument.
* doc/guix.texi (Mapped Devices): Update documentation for the
luks-device-mapping-with-options procedure.

Co-authored-by: Sisiutl <[email-masked]>
---
Changes since v3: Fix replacement of “Solid State Disks” with “solid
state disks” in doc/guix.texi.  That is, only perform this replacement
locally on the added text and not the whole document.

 doc/guix.texi                 | 11 +++++++++-
 gnu/system/mapped-devices.scm | 39 +++++++++++++++++++++--------------
 2 files changed, 33 insertions(+), 17 deletions(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index b1b6d98e74..6eb9fcb8ee 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -18402,7 +18402,7 @@ Mapped Devices
 @code{dm-crypt} Linux kernel module.
 @end defvar
 
-@deffn {Procedure} luks-device-mapping-with-options [#:key-file]
+@deffn {Procedure} luks-device-mapping-with-options [#:key-file #:allow-discards?]
 Return a @code{luks-device-mapping} object, which defines LUKS block
 device encryption using the @command{cryptsetup} command from the
 package with the same name.  It relies on the @code{dm-crypt} Linux
@@ -18424,6 +18424,15 @@ Mapped Devices
  (type (luks-device-mapping-with-options
         #:key-file "/crypto.key")))
 @end lisp
+
+
+@code{allow-discards?} allows the use of discard (TRIM) requests for the
+underlying device.  This is useful for solid state drives.  However,
+this option can have a negative security impact because it can make
+file system level operations visible on the physical device.  For more
+information, refer to the description of the @code{--allow-discards}
+option in the @code{cryptsetup-open(8)} man page.
+
 @end deffn
 
 @defvar raid-device-mapping
diff --git a/gnu/system/mapped-devices.scm b/gnu/system/mapped-devices.scm
index 931c371425..3a8f0d66fe 100644
--- a/gnu/system/mapped-devices.scm
+++ b/gnu/system/mapped-devices.scm
@@ -194,9 +194,10 @@ (define (check-device-initrd-modules device linux-modules location)
 ;;; Common device mappings.
 ;;;
 
-(define* (open-luks-device source targets #:key key-file)
+(define* (open-luks-device source targets #:key key-file allow-discards?)
   "Return a gexp that maps SOURCE to TARGET as a LUKS device, using
-'cryptsetup'."
+'cryptsetup'.  When ALLOW-DISCARDS? is true, the use of discard (TRIM) requests is
+allowed for the underlying device."
   (with-imported-modules (source-module-closure
                           '((gnu build file-systems)
                             (guix build utils))) ;; For mkdir-p
@@ -234,17 +235,21 @@ (define* (open-luks-device source targets #:key key-file)
                                             (loop (- tries-left 1))))))
                           (error "LUKS partition not found" source))
                       source)))
-             ;; We want to fallback to the password unlock if the keyfile fails.
-             (or (and keyfile
-                      (zero? (system*/tty
-                              #$(file-append cryptsetup-static "/sbin/cryptsetup")
-                              "open" "--type" "luks"
-                              "--key-file" keyfile
-                              partition #$target)))
-                 (zero? (system*/tty
-                         #$(file-append cryptsetup-static "/sbin/cryptsetup")
-                         "open" "--type" "luks"
-                         partition #$target)))))))))
+             (let ((cryptsetup-flags (cons*
+                                       "open" "--type" "luks" partition #$target
+                                       (if allow-discards?
+                                           '("--allow-discards")
+                                           '()))))
+               ;; We want to fallback to the password unlock if the keyfile fails.
+               (or (and keyfile
+                        (zero?
+                          (apply system*/tty
+                                 #$(file-append cryptsetup-static "/sbin/cryptsetup")
+                                 "--key-file" keyfile
+                                 cryptsetup-flags)))
+                   (zero? (apply system*/tty
+                                 #$(file-append cryptsetup-static "/sbin/cryptsetup")
+                                 cryptsetup-flags))))))))))
 
 (define (close-luks-device source targets)
   "Return a gexp that closes TARGET, a LUKS device."
@@ -286,13 +291,15 @@ (define luks-device-mapping
               ((gnu build file-systems)
                #:select (find-partition-by-luks-uuid system*/tty))))))
 
-(define* (luks-device-mapping-with-options #:key key-file)
+(define* (luks-device-mapping-with-options #:key key-file allow-discards?)
   "Return a luks-device-mapping object with open modified to pass the arguments
 into the open-luks-device procedure."
   (mapped-device-kind
    (inherit luks-device-mapping)
-   (open (λ (source targets) (open-luks-device source targets
-                                               #:key-file key-file)))))
+   (open (λ (source targets)
+           (open-luks-device source targets
+                             #:key-file key-file
+                             #:allow-discards? allow-discards?)))))
 
 (define (open-raid-device sources targets)
   "Return a gexp that assembles SOURCES (a list of devices) to the RAID device

base-commit: f2b3c36bee8c232b026a66de93db38e13fbd7076