Subject: [PATCH v2] mapped-devices: luks: Support passing --allow-discards during open

From: [email-masked]
Date: Sun, 9 Mar 2025 16:55:49 +0100

From: Sisiutl <[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.

Signed-off-by: Sören Tempel <[email-masked]>
---
Not the author of the original patchset, but I needed this for my
own setup as well so I might as well pick up the slack.  I made
the following changes since the v1:

* Mention allow-discards? in the docstring of open-luks-device.
* Reference the new option in luks-device-mapping-with-options.
* Expand the related documentation in doc/guix.texi.
* Revise the commit message slightly.
* Restore the linefeed.

 doc/guix.texi                 | 11 +++++++++-
 gnu/system/mapped-devices.scm | 39 ++++++++++++++++++++---------------
 2 files changed, 32 insertions(+), 18 deletions(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index 05c855c5ea..bc3ba1f2ed 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -18461,7 +18461,7 @@ 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
@@ -18483,6 +18483,15 @@ Mapped Devices
  (type (luks-device-mapping-with-options
         #:key-file "/crypto.key")))
 @end lisp
+
+If @code{allow-discards?} is provided, then the use of discard (TRIM)
+requests is allowed for the underlying device.  This is useful for
+Solid State Drives.  However, this option can have a negative security
+impact because it can make filesystem-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..c3eaf9ff6e 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 +235,19 @@ (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 (list "open" "--type" "luks" partition #$target))
+                    (cryptsetup-flags (if allow-discards?
+                                          (cons "--allow-discards" cryptsetup-flags)
+                                          cryptsetup-flags)))
+               ;; We want to fallback to the password unlock if the keyfile fails.
+               (or (and keyfile
+                        (zero? (apply system*/tty
+                                      #$(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 +289,15 @@ (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."
+(key-file and allow-discards?) 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: c4f297a664869a18126b66eb5209de1fcceb42d8