[PATCH] mkfs: acquire flock before modifying the device superblock

Wu Guanghao
2022-10-14T08:41:41Z

We noticed that systemd has an issue about symlink unreliable caused by
formatting filesystem and systemd operating on same device.
Issue Link: https://github.com/systemd/systemd/issues/23746

According to systemd doc, a BSD flock needs to be acquired before
formatting the device.
Related Link: https://systemd.io/BLOCK_DEVICE_LOCKING/

So we acquire flock after opening the device but before
writing superblock.

Signed-off-by: wuguanghao <wuguanghao3@huawei.com>
---
mkfs/xfs_mkfs.c | 26 ++++++++++++++++++++++++++
1 file changed, 26 insertions(+)

diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c
index 9dd0e79c..b83cb043 100644
--- a/mkfs/xfs_mkfs.c
+++ b/mkfs/xfs_mkfs.c
@@ -13,6 +13,7 @@
#include "libfrog/crc32cselftest.h"
#include "proto.h"
#include <ini.h>
+#include <sys/file.h>

#define TERABYTES(count, blog) ((uint64_t)(count) << (40 - (blog)))
#define GIGABYTES(count, blog) ((uint64_t)(count) << (30 - (blog)))
@@ -2758,6 +2759,30 @@ _("log stripe unit (%d bytes) is too large (maximum is 256KiB)\n"

}

+static void
+lock_device(dev_t dev, int flag, char *name)
+{
+ int fd = libxfs_device_to_fd(dev);
+ int readonly = flag & LIBXFS_ISREADONLY;
+
+ if (!readonly && fd > 0)
+ if (flock(fd, LOCK_EX) != 0) {
+ fprintf(stderr, "%s: failed to get lock.\n", name);
+ exit(1);
+ }
+}
+
+static void
+lock_devices(struct libxfs_xinit *xi)
+{
+ if (!xi->disfile)
+ lock_device(xi->ddev, xi->dcreat, xi->dname);
+ if (xi->logdev && !xi->lisfile)
+ lock_device(xi->logdev, xi->lcreat, xi->logname);
+ if (xi->rtdev && !xi->risfile)
+ lock_device(xi->rtdev, xi->rcreat, xi->rtname);
+}
+
static void
open_devices(
struct mkfs_params *cfg,
@@ -4208,6 +4233,7 @@ main(
* Open and validate the device configurations
*/
open_devices(&cfg, &xi);
+ lock_devices(&xi);
validate_overwrite(dfile, force_overwrite);
validate_datadev(&cfg, &cli);
validate_logdev(&cfg, &cli, &logfile);
--
2.27.0