e2fsck: take flock(LOCK_EX) on whole-disk device during filesystem check

During journal replay, e2fsck writes the primary superblock back to disk in multiple I/O operations. The payload lands before the checksum, leaving a transient window where the on-disk superblock has a bad checksum.

If udevd processes a change uevent during this window, libblkid probes the primary superblock, finds a checksum mismatch, and concludes the partition has no recognisable filesystem. udev then fires a remove event, wiping all symlinks in /dev/disk/by-label/ and /dev/disk/by-uuid/. Any mount unit that depends on those symlinks will fail.

udevd already serialises its own partition probes against whole-disk device access using flock(LOCK_SH|LOCK_NB); if EAGAIN is returned it requeues the event. Take advantage of this protocol by acquiring flock(LOCK_EX) on the whole-disk device before opening the filesystem. This forces udevd to defer all probes on that disk until e2fsck exits and the lock is released, by which point the filesystem is fully consistent. A shared lock would not work, since it does not conflict with the LOCK_SH that udevd takes.

The whole-disk device is resolved using libsystemd's sd-device API (sd_device_new_from_devnum(), sd_device_get_parent_with_subsystem_devtype()) rather than parsing sysfs, so the lock covers the same device node that udevd itself resolves and locks.

This locking is entirely optional at build time where configure checks for sd_device_get_parent_with_subsystem_devtype() via AC_CHECK_LIB, and e2fsck.c only compiles in the locking code when it is found. On platforms without libsystemd there is no udevd to race against in the first place, so nothing is lost by skipping it there.

The lock is enabled by default, but can be disabled by setting the lock_whole_disk boolean to false in the [options] stanza of e2fsck.conf. This is documented in e2fsck.conf(5).

---

Notes:
Changes in v4:
- Add an "lock_whole_disk" boolean to the [options] stanza of e2fsck.conf, defaulting to true, so the whole-disk lock can be disabled without rebuilding e2fsck (e.g. if another tool legitimately holds the disk). It is read in lock_whole_disk() after the S_ISBLK check, so regular-file images never consult it. It is ignored when built without libsystemd. Documented in e2fsck.conf(5).
- The flock(LOCK_SH) cannot be used as udevd takes flock(LOCK_SH|LOCK_NB) on the whole-disk device before probing, and shared locks do not conflict with each other, so a LOCK_SH held by e2fsck would not make udevd's flock() fail with EAGAIN and udevd would still probe the half-written superblock. Only LOCK_EX causes udevd to requeue the event.
- Added a new lock_whole_disk option in e2fsck.conf
- No changes to the configure/libsystemd detection or to the lock_whole_disk() logic itself.

v3: https://lore.kernel.org/linux-ext4/[email protected]/

Changes in v3:
- Resolve the parent whole-disk device using libsystemd's sd-device API (sd_device_new_from_devnum(), sd_device_get_parent_with_subsystem_devtype(), sd_device_get_devname()) instead of hand-parsing sysfs paths
- Gate the feature on autoconf detecting sd_device_get_parent_with_subsystem_devtype() via AC_CHECK_LIB; compile it out entirely when libsystemd is absent, rather than falling back to another implementation
- Use sd_device_new_from_devnum() (available since libsystemd 240) rather than sd_device_new_from_devname() used by the systemd.io example (requires 251+), to keep the version floor as low as possible
- Keep lock_whole_disk()'s signature unconditional and provide a stub returning -1 on the #else branch, so main()'s primary code path has no #ifdefs in it

v2: https://lore.kernel.org/linux-ext4/[email protected]/

Changes in v2:
- Guard the whole feature (function, call site, lock_fd, cleanup) rather than just the function body, so it is fully absent from non-Linux builds
- Collapse the three HAVE_SYS_*_H header guards into a single conditional
- Reuse a single dev_path[] buffer instead of three single-use stack buffers
- Keep translatable warning strings on one line each, even past 80 columns
- Drop the blanket "could not lock" warning that fired even when the target was not a block device at all (e.g. a regular file), which broke most of "make check"; lock_whole_disk() now only warns at the point of a genuine failure
- No change needed for the Ctrl-C/EINTR concern: the SIGINT handler is installed before lock_whole_disk() runs and is not SA_RESTART, so a blocked flock() is interrupted and the existing E2F_FLAG_CANCEL check unwinds cleanly

v1: https://lore.kernel.org/linux-ext4/[email protected]/

 MCONFIG.in              |  1 +
 configure               | 71 +++++++++++++++++++++++++++++
 configure.ac            | 14 ++++++
 e2fsck/Makefile.in      |  6 +--
 e2fsck/e2fsck.conf.5.in | 14 ++++++
 e2fsck/unix.c           | 98 +++++++++++++++++++++++++++++++++++++++++
 lib/config.h.in         |  6 +++
 7 files changed, 207 insertions(+), 3 deletions(-)

index d66e2f3b..d3055955 100644
@@ -137,6 +137,7 @@ LIBE2P = $(LIB)/libe2p@LIB_EXT@
 LIBEXT2FS = $(LIB)/libext2fs@LIB_EXT@
 LIBUUID = @LIBUUID@ @SOCKET_LIB@
 LIBMAGIC = @MAGIC_LIB@
+LIBSYSTEMD = @SYSTEMD_LIB@
 LIBFUSE = @FUSE_LIB@
 LIBSUPPORT = $(LIBINTL) $(LIB)/libsupport@STATIC_LIB_EXT@
 LIBBLKID = @LIBBLKID@ @PRIVATE_LIBS_CMT@ $(LIBUUID)

index 4921f81f..9f585aae 100644
@@ -1309,6 +1309,20 @@ if test "$ac_cv_func_dlopen" = yes ; then
 fi
 AC_SUBST(MAGIC_LIB)
 dnl
+dnl See if libsystemd has sd-device APIs for whole-disk BSD locking
+dnl
+SYSTEMD_LIB=
+AC_CHECK_LIB(systemd, sd_device_get_parent_with_subsystem_devtype,
+	[SYSTEMD_LIB=-lsystemd])
+if test -n "$SYSTEMD_LIB"; then
+	AC_CHECK_HEADERS([systemd/sd-device.h], , [SYSTEMD_LIB=])
+fi
+if test -n "$SYSTEMD_LIB"; then
+	AC_DEFINE(HAVE_SD_DEVICE_GET_PARENT_WITH_SUBSYSTEM_DEVTYPE, 1,
+		[Define to 1 if libsystemd has sd-device APIs for whole-disk locking])
+fi
+AC_SUBST(SYSTEMD_LIB)
+dnl
 dnl libarchive
 dnl
 AC_ARG_WITH([libarchive],

index fbb7b156..a050eafe 100644
@@ -17,20 +17,20 @@ MANPAGES=	e2fsck.8
 FMANPAGES=	e2fsck.conf.5
 
 LIBS= $(LIBSUPPORT) $(LIBEXT2FS) $(LIBCOM_ERR) $(LIBBLKID) $(LIBUUID) \
-	$(LIBINTL) $(LIBE2P) $(LIBMAGIC) $(SYSLIBS)
+	$(LIBINTL) $(LIBE2P) $(LIBMAGIC) $(LIBSYSTEMD) $(SYSLIBS)
 DEPLIBS= $(DEPLIBSUPPORT) $(LIBEXT2FS) $(DEPLIBCOM_ERR) $(DEPLIBBLKID) \
 	 $(DEPLIBUUID) $(DEPLIBE2P)
 
 STATIC_LIBS= $(STATIC_LIBSUPPORT) $(STATIC_LIBEXT2FS) $(STATIC_LIBCOM_ERR) \
 	     $(STATIC_LIBBLKID) $(STATIC_LIBUUID) $(LIBINTL) $(STATIC_LIBE2P) \
-	     $(LIBMAGIC) $(SYSLIBS)
+	     $(LIBMAGIC) $(LIBSYSTEMD) $(SYSLIBS)
 STATIC_DEPLIBS= $(DEPSTATIC_LIBSUPPORT) $(STATIC_LIBEXT2FS) \
 		$(DEPSTATIC_LIBCOM_ERR) $(DEPSTATIC_LIBBLKID) \
 		$(DEPSTATIC_LIBUUID) $(DEPSTATIC_LIBE2P)
 
 PROFILED_LIBS= $(PROFILED_LIBSUPPORT) $(PROFILED_LIBEXT2FS) \
 	       $(PROFILED_LIBCOM_ERR) $(PROFILED_LIBBLKID) $(PROFILED_LIBUUID) \
-	       $(PROFILED_LIBE2P) $(LIBINTL) $(LIBMAGIC) $(SYSLIBS)
+	       $(PROFILED_LIBE2P) $(LIBINTL) $(LIBMAGIC) $(LIBSYSTEMD) $(SYSLIBS)
 PROFILED_DEPLIBS= $(DEPPROFILED_LIBSUPPORT) $(PROFILED_LIBEXT2FS) \
 		  $(DEPPROFILED_LIBCOM_ERR) $(DEPPROFILED_LIBBLKID) \
 		  $(DEPPROFILED_LIBUUID) $(DEPPROFILED_LIBE2P)

index e82610d3..3b9a118c 100644
@@ -172,6 +172,20 @@ file system.  For large file systems, this can be gigabytes of memory.
 additional 5.7 GB memory if this optimization is enabled.)  This setting
 defaults to false.
 .TP
+.I lock_whole_disk
+If this boolean relation is true, e2fsck will take an exclusive
+.BR flock (2)
+on the whole-disk device containing the file system while it is being
+checked, following the protocol described at
+.IR https://systemd.io/BLOCK_DEVICE_LOCKING .
+This causes
+.BR systemd-udevd (8)
+to defer probing the device until e2fsck exits, so that it does not see
+a transiently inconsistent superblock while the journal is being replayed.
+This only has an effect when e2fsck was built with libsystemd support and
+is checking a block device; otherwise this relation is ignored.
+This setting defaults to true.
+.TP
 .I log_dir
 If the
 .I log_filename

index f129abfe..bc3ae4aa 100644
@@ -352,6 +352,9 @@
 /* Define to 1 if if struct sockaddr contains sa_len */
 #undef HAVE_SA_LEN
 
+/* Define to 1 if libsystemd has sd-device APIs for whole-disk locking */
+#undef HAVE_SD_DEVICE_GET_PARENT_WITH_SUBSYSTEM_DEVTYPE
+
 /* Define to 1 if you have the 'secure_getenv' function. */
 #undef HAVE_SECURE_GETENV
 

@@ -430,6 +433,9 @@
 /* Define to 1 if you have the 'sysconf' function. */
 #undef HAVE_SYSCONF
 
+/* Define to 1 if you have the <systemd/sd-device.h> header file. */
+#undef HAVE_SYSTEMD_SD_DEVICE_H
+
 /* Define to 1 if you have the <sys/acl.h> header file. */
 #undef HAVE_SYS_ACL_H
 

2.55.0