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file-systems: Add support for exFAT.

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* gnu/build/file-systems.scm (%exfat-endianness): New syntax.
(exfat-superblock?, exfat-bytes-per-sector-shift)
(exfat-sectors-per-cluster-shift, exfat-root-directory-offset)
(exfat-cluster-size, read-exfat-superblock+root-directory-cluster)
(read-exfat-superblock, exfat-superblock-volume-name)
(exfat-superblock-uuid, check-exfat-file-system): New procedures.
(%partition-label-readers, %partition-uuid-readers): Register them.

Change-Id: I08bd3147d2d67e5766c9381ae2159bc01530b814
This commit is contained in:
Tobias Geerinckx-Rice 2023-11-05 01:00:00 +01:00
parent c9a28a87e8
commit 191e0f9e52
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1 changed files with 116 additions and 1 deletions
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gnu/build/file-systems.scm
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@ -3,7 +3,7 @@
;;; Copyright © 2016, 2017 David Craven <david@craven.ch>
;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
;;; Copyright © 2019 Guillaume Le Vaillant <glv@posteo.net>
;;; Copyright © 2019–2021 Tobias Geerinckx-Rice <me@tobias.gr>
;;; Copyright © 2019–2021, 2024 Tobias Geerinckx-Rice <me@tobias.gr>
;;; Copyright © 2019 David C. Trudgian <dave@trudgian.net>
;;; Copyright © 2020 Maxim Cournoyer <maxim.cournoyer@gmail.com>
;;; Copyright © 2022 Oleg Pykhalov <go.wigust@gmail.com>
@ -448,6 +448,116 @@ fix only those considered safe to repair automatically."
(_ 'fatal-error))
'pass))
;;;
;;; exFAT file systems.
;;;
;; <https://learn.microsoft.com/en-us/windows/win32/fileio/exfat-specification>.
(define-syntax %exfat-endianness
(identifier-syntax (endianness little)))
(define (exfat-superblock? sblock)
"Return #t when SBLOCK, a bytevector of at least length 512, is an exFAT
superblock, called main boot sector in the exFAT specification."
(and (bytevector=? (string->utf8 "EXFAT ")
(sub-bytevector sblock 3 8)) ;FileSystemName
(bytevector=? (make-bytevector 53 0)
(sub-bytevector sblock 11 53)) ;MustBeZero
(bytevector=? #vu8(#x55 #xaa)
(sub-bytevector sblock 510 2)))) ;BootSignature
(define (exfat-bytes-per-sector-shift sblock)
(bytevector-u8-ref sblock 108))
(define (exfat-sectors-per-cluster-shift sblock)
(bytevector-u8-ref sblock 109))
(define (exfat-root-directory-offset sblock)
(let ((cluster-heap-offset (bytevector-u32-ref sblock 88
%exfat-endianness))
(root-directory-cluster (bytevector-u32-ref sblock 96
%exfat-endianness)))
(define (cluster->sector cluster)
(let ((first-data-cluster 2))
(+ cluster-heap-offset (ash (- cluster first-data-cluster)
(exfat-sectors-per-cluster-shift sblock)))))
(ash (cluster->sector root-directory-cluster)
(exfat-bytes-per-sector-shift sblock))))
(define (exfat-cluster-size sblock)
(ash 1 (+ (exfat-bytes-per-sector-shift sblock)
(exfat-sectors-per-cluster-shift sblock))))
;; exFAT stores the volume name in a directory entry with no fixed location. We
;; search an arbitrary number of entries before giving up: 128 for devices <256M
;; (4K clusters), 1024 for those <32G (32K clusters), or 4096 for others (128K).
;; It's silly but mostly matches what util-linux's libblkid does with its higher
;; arbitrary number of 10,000 tries. To match that, we'd not only have to look
;; up subsequent clusters in the FAT, but redesign this entire module not to
;; assume that all file systems have a `superblock' that both fits neatly into
;; RAM, and just happens to politely contain all the metadata we'll ever need.
(define (read-exfat-superblock+root-directory-cluster device sblock)
"Return as a bytevector the raw contents of DEVICE's exFAT `superblock' from
main boot sector up to the RootDirectoryCluster expected to contain the volume
label."
(let* ((span (+ (exfat-root-directory-offset sblock)
(exfat-cluster-size sblock))))
(read-superblock device 0 span (const #t))))
(define (read-exfat-superblock device)
"Return the raw contents of DEVICE's exFAT superblock as a bytevector, or #f
if DEVICE does not contain an exFAT file system."
(and=> (read-superblock device 0 512 exfat-superblock?)
(cut read-exfat-superblock+root-directory-cluster device <>)))
(define (exfat-superblock-volume-name sblock)
"Return as a string the volume name belonging to an exFAT file system
beginning with bytevector SBLOCK, which includes directory entries.
Return #f if no volume name was found within our arbitrary limit."
;; Defined in section 7.3 of the exFAT specification.
(let ((root-directory (exfat-root-directory-offset sblock))
(cluster-size (exfat-cluster-size sblock))
(entry-size 32)) ;bytes
(let loop ((cluster-offset 0))
(if (< cluster-offset cluster-size)
(let ((offset (+ root-directory cluster-offset)))
(match (bytevector-u8-ref sblock offset)
((or #x00 ;end of directory
#x03) ;type 3 & not in use
#f)
(#x83 ;type 3 & in use
(let* ((length (min 11 (bytevector-u8-ref sblock (+ 1 offset))))
(label (sub-bytevector sblock (+ 2 offset) (* 2 length))))
(utf16->string label %exfat-endianness)))
(_ (loop (+ entry-size offset)))))
#f))))
(define (exfat-superblock-uuid sblock)
"Return the Volume Serial Number of exFAT superblock SBLOCK as a bytevector.
This 4-byte identifier is guaranteed to exist, unlike the optional 16-byte
Volume GUID from section 7.5 of the exFAT specification."
(sub-bytevector sblock 100 4))
(define (check-exfat-file-system device force? repair)
"Return the health of an unmounted exFAT file system on DEVICE. If FORCE?
is true, check the file system even if it's marked as clean. If REPAIR is
false, do not write to the file system to fix errors. If it's #t, fix all
errors. Otherwise, fix only those considered safe to repair automatically."
(match (status:exit-val
(apply system*/tty "fsck.exfat" "-sv"
`(,@(if force? '("-b") '())
,@(match repair
(#f '("-n"))
(#t '("-y"))
(_ '("-p")))
,device)))
(0 'pass)
(1 'errors-corrected)
(2 'reboot-required)
(_ 'fatal-error)))
;;;
;;; FAT32 file systems.
@ -937,6 +1047,8 @@ partition field reader that returned a value."
bcachefs-superblock-volume-name)
(partition-field-reader read-btrfs-superblock
btrfs-superblock-volume-name)
(partition-field-reader read-exfat-superblock
exfat-superblock-volume-name)
(partition-field-reader read-fat32-superblock
fat32-superblock-volume-name)
(partition-field-reader read-fat16-superblock
@ -959,6 +1071,8 @@ partition field reader that returned a value."
bcachefs-superblock-external-uuid)
(partition-field-reader read-btrfs-superblock
btrfs-superblock-uuid)
(partition-field-reader read-exfat-superblock
exfat-superblock-uuid)
(partition-field-reader read-fat32-superblock
fat32-superblock-uuid)
(partition-field-reader read-fat16-superblock
@ -1079,6 +1193,7 @@ an exception in such cases but perform the nearest sane action."
((string-prefix? "ext" type) check-ext2-file-system)
((string-prefix? "bcachefs" type) check-bcachefs-file-system)
((string-prefix? "btrfs" type) check-btrfs-file-system)
((string-suffix? "exfat" type) check-exfat-file-system)
((string-suffix? "fat" type) check-fat-file-system)
((string-prefix? "jfs" type) check-jfs-file-system)
((string-prefix? "f2fs" type) check-f2fs-file-system)