Re: [PATCH e2fsprogs] tune2fs: touch the device node after setting the label via ioctl

["Darrick J. Wong" <djwong@kernel.org>]

[cc linux-fsdevel]

On Wed, Sep 23, 2026 at 09:22:23PM +0800, Baokun Li wrote:
> When the file system is mounted, tune2fs sets the label through
> FS_IOC_SETFSLABEL, which updates the label without touching the
> mtime of the device node.  Consumers such as blkid use that mtime
> to validate their cached superblock information, so the old label
> can still be returned right after the change:
> 
>   blkid -s LABEL /dev/sdb    # gets "old" and caches it
>   tune2fs -L new /dev/sdb    # online path, node mtime unchanged
>   blkid -s LABEL /dev/sdb    # still gets "old" within 2s
> 
> The offline path is unaffected because writing the superblock
> through the device node updates its mtime as a side effect.  The
> ioctl path has no such side effect, so add an explicit utime() on
> the device after a successful FS_IOC_SETFSLABEL.
> 
> The return value is ignored; if the touch fails, the behavior is
> no worse than before.
> 
> Suggested-by: Theodore Ts'o <tytso@mit.edu>
> Link: https://patch.msgid.link/arKZu30pMe0ZavVA@mit.edu
> Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
> ---
>  misc/tune2fs.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/misc/tune2fs.c b/misc/tune2fs.c
> index 2d85fb704e15..87b8e6830bb9 100644
> --- a/misc/tune2fs.c
> +++ b/misc/tune2fs.c
> @@ -48,6 +48,7 @@ extern int optind;
>  #endif
>  #include <string.h>
>  #include <time.h>
> +#include <utime.h>
>  #include <unistd.h>
>  #include <sys/types.h>
>  #include <libgen.h>
> @@ -3167,6 +3168,14 @@ static int handle_fslabel(int setlabel)
>  		return 1;
>  	}
>  	close(fd);
> +
> +	/*
> +	 * FS_IOC_SETFSLABEL does not touch the device mtime that
> +	 * blkid uses to validate its cache; touch the device so
> +	 * mtime-based consumers see the change.
> +	 */
> +	utime(device_name, NULL);

For everyone on fsdevel who might be seeing this for the first time,
there was a problem report in which it was discovered that the libblkid
cache invalidates its knowledge if something updates the block device
mtime.  This is done whenever userspace tools update a block device
(e.g. mkfs) but not done by the kernel when it rewrites the primary
superblock (e.g. FS_IOC_SETFSLABEL).

This patch fixes tune2fs to add the missing mtime update, but I think
a better way to solve this is to fix the ~8 or so implementations inside
the kernel.  And maybe the FS_IOC_SETFSUUID implementations too.

A bigger question, then, is whether filesystems should bump mtime /any/
time they update their own superblock?  There aren't any published specs
mandating this behavior by the kernel, but I suppose it falls under
"someone observed a behavior and started relying on it" :P

Thoughts?

--D

[1] https://lore.kernel.org/linux-ext4/20260920094137.2749428-1-libaokun@linux.alibaba.com/

> +
>  	return 0;
>  #else
>  	return -1;
> -- 
> 2.43.7
> 
>