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

[Wu Guanghao <wuguanghao3@huawei.com>]

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