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

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

[Baokun Li]

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

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

[Darrick J. Wong]

[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
> 
>