[PATCH] generic/492: bypass the blkid cache when probing the label

[PATCH] generic/492: bypass the blkid cache when probing the label

[Baokun Li]

blkid may serve a stale label from its cache instead of probing the
device, so the label just set via xfs_io can be missed and the output
mismatch is non-deterministic.

Pass "-c /dev/null" so that blkid always probes the on-disk superblock
directly.  Unlike the low-level probing mode (-p), which requires
util-linux 2.16 or later, this is also supported by the legacy blkid
shipped with e2fsprogs.

Fixes: c3c9630968a6 ("generic: test online label ioctl")
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
---
 tests/generic/492 | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/tests/generic/492 b/tests/generic/492
index 51a46b510453..0b4f69b80f17 100755
--- a/tests/generic/492
+++ b/tests/generic/492
@@ -28,14 +28,15 @@ $XFS_IO_PROG -c "label -c" $SCRATCH_MNT
 $XFS_IO_PROG -c "label" $SCRATCH_MNT
 
 # And that userspace can see it now, while mounted
-# NB: some blkid has trailing whitespace, filter it out here
+# NB: bypass the blkid cache which may hold a stale label, and filter
+# out the trailing whitespace some blkid versions emit
 $XFS_IO_PROG -c "label -s label.$seq" $SCRATCH_MNT
 $XFS_IO_PROG -c "label" $SCRATCH_MNT
-blkid -s LABEL $SCRATCH_DEV | _filter_scratch | sed -e "s/ $//g"
+blkid -c /dev/null -s LABEL $SCRATCH_DEV | _filter_scratch | sed -e "s/ $//g"
 
 # And that the it is still there when it's unmounted
 _scratch_unmount
-blkid -s LABEL $SCRATCH_DEV | _filter_scratch | sed -e "s/ $//g"
+blkid -c /dev/null -s LABEL $SCRATCH_DEV | _filter_scratch | sed -e "s/ $//g"
 
 # And that it persists after a remount
 _scratch_mount
-- 
2.43.7

Re: [PATCH] generic/492: bypass the blkid cache when probing the label

[Darrick J. Wong]

On Sun, Sep 20, 2026 at 05:41:37PM +0800, Baokun Li wrote:
> blkid may serve a stale label from its cache instead of probing the
> device, so the label just set via xfs_io can be missed and the output
> mismatch is non-deterministic.
> 
> Pass "-c /dev/null" so that blkid always probes the on-disk superblock
> directly.  Unlike the low-level probing mode (-p), which requires
> util-linux 2.16 or later, this is also supported by the legacy blkid
> shipped with e2fsprogs.
> 
> Fixes: c3c9630968a6 ("generic: test online label ioctl")
> Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>

Seems fine to me; I guess blkid could be racing with whatever updates
the cache...udev?

Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> ---
>  tests/generic/492 | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/tests/generic/492 b/tests/generic/492
> index 51a46b510453..0b4f69b80f17 100755
> --- a/tests/generic/492
> +++ b/tests/generic/492
> @@ -28,14 +28,15 @@ $XFS_IO_PROG -c "label -c" $SCRATCH_MNT
>  $XFS_IO_PROG -c "label" $SCRATCH_MNT
>  
>  # And that userspace can see it now, while mounted
> -# NB: some blkid has trailing whitespace, filter it out here
> +# NB: bypass the blkid cache which may hold a stale label, and filter
> +# out the trailing whitespace some blkid versions emit
>  $XFS_IO_PROG -c "label -s label.$seq" $SCRATCH_MNT
>  $XFS_IO_PROG -c "label" $SCRATCH_MNT
> -blkid -s LABEL $SCRATCH_DEV | _filter_scratch | sed -e "s/ $//g"
> +blkid -c /dev/null -s LABEL $SCRATCH_DEV | _filter_scratch | sed -e "s/ $//g"
>  
>  # And that the it is still there when it's unmounted
>  _scratch_unmount
> -blkid -s LABEL $SCRATCH_DEV | _filter_scratch | sed -e "s/ $//g"
> +blkid -c /dev/null -s LABEL $SCRATCH_DEV | _filter_scratch | sed -e "s/ $//g"
>  
>  # And that it persists after a remount
>  _scratch_mount
> -- 
> 2.43.7
> 
> 

Re: [PATCH] generic/492: bypass the blkid cache when probing the label

[Baokun Li]

Hi Darrick,

On 2026/9/21 13:27, Darrick J. Wong wrote:
> On Sun, Sep 20, 2026 at 05:41:37PM +0800, Baokun Li wrote:
>> blkid may serve a stale label from its cache instead of probing the
>> device, so the label just set via xfs_io can be missed and the output
>> mismatch is non-deterministic.
>>
>> Pass "-c /dev/null" so that blkid always probes the on-disk superblock
>> directly.  Unlike the low-level probing mode (-p), which requires
>> util-linux 2.16 or later, this is also supported by the legacy blkid
>> shipped with e2fsprogs.
>>
>> Fixes: c3c9630968a6 ("generic: test online label ioctl")
>> Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
> Seems fine to me; I guess blkid could be racing with whatever updates
> the cache...udev?


Not a race, and not related to udev.  It's a long story; details below.

In the xfstests-bld appliance blkid is e2fsprogs' legacy blkid and its
cache is /etc/blkid.tab; the only writers are blkid itself and
e2fsprogs' fsck (both probe with the cache enabled).  systemd's udev
uses libblkid's low-level probe API and never touches this file.

What we hit is a strictly sequential sequence that depends on two
"same second" coincidences.  The cache validity check (blkid_verify()
in lib/blkid/probe.c) is:

    now >= TIME && mtime(seconds) <= TIME && age < 2s

Both sides are in seconds, and only the 2s window applies to a fresh
blkid process (BLKID_BID_FL_VERIFIED is not persistent).

mkfs only advances the device mtime and never updates the cache; the fs
label ioctl updates neither, so right after setting a new label a
cached lookup can still return the old one.

The trap is set by _check_scratch_fs: it runs "fsck -t ext4 ...", which
probes the device with the cache enabled and writes the entry back with
TIME=now and the current label state.

Here is a sequential 490/491/492 run that trips it (timestamps from a
CI log, ext4/bigalloc_1k):

    M-2  09:09:44  end of 490: fsck finds the previous entry stale
                   (age >= 2s) -> probes -> tab = {no LABEL, TIME=44}

    M-1  09:09:45  start of 491: mkfs -> device mtime = 45
                   (mkfs doesn't update the cache; tab stays TIME=44)
    M    09:09:46  end of 491: unmount + _check_scratch_fs
                   -> fsck sees mtime(45) > TIME(44) -> probes
                   -> tab = {no LABEL, TIME=46}          <- trap

    M    09:09:46  start of 492: mkfs -> device mtime = 46
    M    09:09:46  mount; ioctl label="label.492" (mtime unchanged)
    M    09:09:46  blkid(35): mtime(46) <= TIME(46), age < 2s
                   -> cache deemed valid, no probe -> prints no LABEL
    M+1  09:09:47  umount; blkid(39): still < 2s -> again
    M+1  09:09:47  check reports output mismatch (two lines missing)

The difference between the e2fsprogs and util-linux libblkid is the
time granularity: the former stores seconds only (TIME="%ld",
bid_time = time(0), compares st_mtime <= bid_time), while the latter
stores usec (TIME="sec.usec") and has an nsec tie-break in the compare,
so the same-second case is much easier to hit with the e2fsprogs blkid.

The patch avoids this by probing with -c /dev/null (the legacy blkid
has no -p).  Reproduced on ext4/bigalloc_1k: unpatched 10 of 65 runs
fail, patched 120/120 pass.


>
> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
>
> --D


Thanks for the review!

Cheers,
Baokun

Re: [PATCH] generic/492: bypass the blkid cache when probing the label

[Darrick J. Wong]

On Mon, Sep 21, 2026 at 05:07:48PM +0800, Baokun Li wrote:
> Hi Darrick,
> 
> On 2026/9/21 13:27, Darrick J. Wong wrote:
> > On Sun, Sep 20, 2026 at 05:41:37PM +0800, Baokun Li wrote:
> >> blkid may serve a stale label from its cache instead of probing the
> >> device, so the label just set via xfs_io can be missed and the output
> >> mismatch is non-deterministic.
> >>
> >> Pass "-c /dev/null" so that blkid always probes the on-disk superblock
> >> directly.  Unlike the low-level probing mode (-p), which requires
> >> util-linux 2.16 or later, this is also supported by the legacy blkid
> >> shipped with e2fsprogs.
> >>
> >> Fixes: c3c9630968a6 ("generic: test online label ioctl")
> >> Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
> > Seems fine to me; I guess blkid could be racing with whatever updates
> > the cache...udev?
> 
> 
> Not a race, and not related to udev.  It's a long story; details below.
> 
> In the xfstests-bld appliance blkid is e2fsprogs' legacy blkid and its
> cache is /etc/blkid.tab; the only writers are blkid itself and
> e2fsprogs' fsck (both probe with the cache enabled).  systemd's udev
> uses libblkid's low-level probe API and never touches this file.
> 
> What we hit is a strictly sequential sequence that depends on two
> "same second" coincidences.  The cache validity check (blkid_verify()
> in lib/blkid/probe.c) is:
> 
>     now >= TIME && mtime(seconds) <= TIME && age < 2s
> 
> Both sides are in seconds, and only the 2s window applies to a fresh
> blkid process (BLKID_BID_FL_VERIFIED is not persistent).
> 
> mkfs only advances the device mtime and never updates the cache; the fs
> label ioctl updates neither, so right after setting a new label a
> cached lookup can still return the old one.
> 
> The trap is set by _check_scratch_fs: it runs "fsck -t ext4 ...", which
> probes the device with the cache enabled and writes the entry back with
> TIME=now and the current label state.
> 
> Here is a sequential 490/491/492 run that trips it (timestamps from a
> CI log, ext4/bigalloc_1k):
> 
>     M-2  09:09:44  end of 490: fsck finds the previous entry stale
>                    (age >= 2s) -> probes -> tab = {no LABEL, TIME=44}
> 
>     M-1  09:09:45  start of 491: mkfs -> device mtime = 45
>                    (mkfs doesn't update the cache; tab stays TIME=44)
>     M    09:09:46  end of 491: unmount + _check_scratch_fs
>                    -> fsck sees mtime(45) > TIME(44) -> probes
>                    -> tab = {no LABEL, TIME=46}          <- trap
> 
>     M    09:09:46  start of 492: mkfs -> device mtime = 46
>     M    09:09:46  mount; ioctl label="label.492" (mtime unchanged)
>     M    09:09:46  blkid(35): mtime(46) <= TIME(46), age < 2s
>                    -> cache deemed valid, no probe -> prints no LABEL
>     M+1  09:09:47  umount; blkid(39): still < 2s -> again
>     M+1  09:09:47  check reports output mismatch (two lines missing)
> 
> The difference between the e2fsprogs and util-linux libblkid is the
> time granularity: the former stores seconds only (TIME="%ld",
> bid_time = time(0), compares st_mtime <= bid_time), while the latter
> stores usec (TIME="sec.usec") and has an nsec tie-break in the compare,
> so the same-second case is much easier to hit with the e2fsprogs blkid.

[add ext4 list to cc]

Perhaps xfstests-bld should use the system blkid then?  IIRC the one
in e2fsprogs is very old and out of date.

--D

> 
> The patch avoids this by probing with -c /dev/null (the legacy blkid
> has no -p).  Reproduced on ext4/bigalloc_1k: unpatched 10 of 65 runs
> fail, patched 120/120 pass.
> 
> 
> >
> > Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
> >
> > --D
> 
> 
> Thanks for the review!
> 
> Cheers,
> Baokun
> 
> 

Re: [PATCH] generic/492: bypass the blkid cache when probing the label

[Theodore Tso]

On Mon, Sep 21, 2026 at 04:49:10PM -0500, Darrick J. Wong wrote:
> > The difference between the e2fsprogs and util-linux libblkid is the
> > time granularity: the former stores seconds only (TIME="%ld",
> > bid_time = time(0), compares st_mtime <= bid_time), while the latter
> > stores usec (TIME="sec.usec") and has an nsec tie-break in the compare,
> > so the same-second case is much easier to hit with the e2fsprogs blkid.
> 
> Perhaps xfstests-bld should use the system blkid then?  IIRC the one
> in e2fsprogs is very old and out of date.

Historically, xfstests-bld was built in environments where libblkid
wasn't available.  This included Android's runtime (which uses the
bionic libc), and grte (Google Runtime Environment)[1].

[1] https://github.com/bazelment/lrte/blob/master/grte/grte-build

However, as more and more tests require utilities from util-linux,
xfstests-bld was taught to build util-linux.  And if util-linux is
being built, then we don't use e2fsprogs-libs any more, which is where
libblkid is built.  From build-all:

if test -z "$USE_LOCAL_E2FSLIBS" -o -d util-linux; then
    SKIP_E2FSLIBS=yes
fi

I've always built xfstesets-bld with util-linux, even though it is
listed as an optional repository.  This is because (a) util-linux is
needed for Android and other non-standard Linux environments, and (b)
the version of util-linux used by earlier Debian Stable was too old
for some of the newer tests in fstests and blktests.

If you aren't building xfstests-bld with util-linux, it falls back to
building libblkid from e2fsprogs-libs.  HOWEVER, we don't actually
blkid, and libblkid is always built statically.  So what seems to be
going on is that one of the programs built by xfstests-bld is using
the older libblkid, and that results in the blkid cache being writen
with a second granularity.

I've never noticed this issue because I've always built with
util-linux.  The way to solve this is to require building with the
system libblkid if util-linux is not being built, and to drop
e2fsprogs-libs from xfstests-bld.  Or just build xfstests-bld with
util-linux.  :-)

Cheers,

					- Ted

Re: [PATCH] generic/492: bypass the blkid cache when probing the label

[Baokun Li]

On 2026/9/22 11:43, Theodore Tso wrote:
> On Mon, Sep 21, 2026 at 04:49:10PM -0500, Darrick J. Wong wrote:
>>> The difference between the e2fsprogs and util-linux libblkid is the
>>> time granularity: the former stores seconds only (TIME="%ld",
>>> bid_time = time(0), compares st_mtime <= bid_time), while the latter
>>> stores usec (TIME="sec.usec") and has an nsec tie-break in the compare,
>>> so the same-second case is much easier to hit with the e2fsprogs blkid.
>> Perhaps xfstests-bld should use the system blkid then?  IIRC the one
>> in e2fsprogs is very old and out of date.
> Historically, xfstests-bld was built in environments where libblkid
> wasn't available.  This included Android's runtime (which uses the
> bionic libc), and grte (Google Runtime Environment)[1].
>
> [1] https://github.com/bazelment/lrte/blob/master/grte/grte-build
>
> However, as more and more tests require utilities from util-linux,
> xfstests-bld was taught to build util-linux.  And if util-linux is
> being built, then we don't use e2fsprogs-libs any more, which is where
> libblkid is built.  From build-all:
>
> if test -z "$USE_LOCAL_E2FSLIBS" -o -d util-linux; then
>     SKIP_E2FSLIBS=yes
> fi
>
> I've always built xfstesets-bld with util-linux, even though it is
> listed as an optional repository.  This is because (a) util-linux is
> needed for Android and other non-standard Linux environments, and (b)
> the version of util-linux used by earlier Debian Stable was too old
> for some of the newer tests in fstests and blktests.
>
> If you aren't building xfstests-bld with util-linux, it falls back to
> building libblkid from e2fsprogs-libs.  HOWEVER, we don't actually
> blkid, and libblkid is always built statically.  So what seems to be
> going on is that one of the programs built by xfstests-bld is using
> the older libblkid, and that results in the blkid cache being writen
> with a second granularity.
>
> I've never noticed this issue because I've always built with
> util-linux.  The way to solve this is to require building with the
> system libblkid if util-linux is not being built, and to drop
> e2fsprogs-libs from xfstests-bld.  Or just build xfstests-bld with
> util-linux.  :-)


Thanks for the detailed explanation.

I checked my environment again though -- it turned out not to be
xfstests-bld at all.  xfstests-bld never touches e2fsprogs.

What happened is that I had manually run "make install" of a locally
patched e2fsprogs in the test VM, which overwrote util-linux's blkid
and fsck.

Rebuilding e2fsprogs with these options disabled made the test pass:

    ./configure --disable-libblkid --disable-fsck \
            --disable-libuuid --disable-uuidd

I ran unpatched generic/492 60 times on each side in a VM: 12/60
failures with the hand-installed strays, 0/60 with util-linux's
blkid/fsck.

That said, I think the patch is still needed, because
FS_IOC_SETFSLABEL does not update the block device's mtime:

    blkid -s LABEL ...    # get A
    FS_IOC_SETFSLABEL     # set B (device mtime unchanged)
    blkid -s LABEL ...    # still get A (within 2s)

The first read writes a cache entry with TIME=now, and the ioctl
leaves the device mtime untouched, so a second read within the 2s
window still trusts the cache.

I reproduced this with util-linux 2.41.5 too, so
"blkid -c /dev/null" is what keeps the test deterministic
regardless of which blkid the image carries.


Cheers,
Baokun

Re: [PATCH] generic/492: bypass the blkid cache when probing the label

[Darrick J. Wong]

On Tue, Sep 22, 2026 at 03:17:27PM +0800, Baokun Li wrote:
> On 2026/9/22 11:43, Theodore Tso wrote:
> > On Mon, Sep 21, 2026 at 04:49:10PM -0500, Darrick J. Wong wrote:
> >>> The difference between the e2fsprogs and util-linux libblkid is the
> >>> time granularity: the former stores seconds only (TIME="%ld",
> >>> bid_time = time(0), compares st_mtime <= bid_time), while the latter
> >>> stores usec (TIME="sec.usec") and has an nsec tie-break in the compare,
> >>> so the same-second case is much easier to hit with the e2fsprogs blkid.
> >> Perhaps xfstests-bld should use the system blkid then?  IIRC the one
> >> in e2fsprogs is very old and out of date.
> > Historically, xfstests-bld was built in environments where libblkid
> > wasn't available.  This included Android's runtime (which uses the
> > bionic libc), and grte (Google Runtime Environment)[1].
> >
> > [1] https://github.com/bazelment/lrte/blob/master/grte/grte-build
> >
> > However, as more and more tests require utilities from util-linux,
> > xfstests-bld was taught to build util-linux.  And if util-linux is
> > being built, then we don't use e2fsprogs-libs any more, which is where
> > libblkid is built.  From build-all:
> >
> > if test -z "$USE_LOCAL_E2FSLIBS" -o -d util-linux; then
> >     SKIP_E2FSLIBS=yes
> > fi
> >
> > I've always built xfstesets-bld with util-linux, even though it is
> > listed as an optional repository.  This is because (a) util-linux is
> > needed for Android and other non-standard Linux environments, and (b)
> > the version of util-linux used by earlier Debian Stable was too old
> > for some of the newer tests in fstests and blktests.
> >
> > If you aren't building xfstests-bld with util-linux, it falls back to
> > building libblkid from e2fsprogs-libs.  HOWEVER, we don't actually
> > blkid, and libblkid is always built statically.  So what seems to be
> > going on is that one of the programs built by xfstests-bld is using
> > the older libblkid, and that results in the blkid cache being writen
> > with a second granularity.
> >
> > I've never noticed this issue because I've always built with
> > util-linux.  The way to solve this is to require building with the
> > system libblkid if util-linux is not being built, and to drop
> > e2fsprogs-libs from xfstests-bld.  Or just build xfstests-bld with
> > util-linux.  :-)
> 
> 
> Thanks for the detailed explanation.
> 
> I checked my environment again though -- it turned out not to be
> xfstests-bld at all.  xfstests-bld never touches e2fsprogs.
> 
> What happened is that I had manually run "make install" of a locally
> patched e2fsprogs in the test VM, which overwrote util-linux's blkid
> and fsck.
> 
> Rebuilding e2fsprogs with these options disabled made the test pass:
> 
>     ./configure --disable-libblkid --disable-fsck \
>             --disable-libuuid --disable-uuidd
> 
> I ran unpatched generic/492 60 times on each side in a VM: 12/60
> failures with the hand-installed strays, 0/60 with util-linux's
> blkid/fsck.
> 
> That said, I think the patch is still needed, because
> FS_IOC_SETFSLABEL does not update the block device's mtime:

Many filesystems /never/ update the bdev mtime, why would they?

>     blkid -s LABEL ...    # get A
>     FS_IOC_SETFSLABEL     # set B (device mtime unchanged)
>     blkid -s LABEL ...    # still get A (within 2s)
> 
> The first read writes a cache entry with TIME=now, and the ioctl
> leaves the device mtime untouched, so a second read within the 2s
> window still trusts the cache.
> 
> I reproduced this with util-linux 2.41.5 too, so
> "blkid -c /dev/null" is what keeps the test deterministic
> regardless of which blkid the image carries.

<nod> That was the problem I thought you were solving here ;)

--D

> 
> Cheers,
> Baokun
> 
> 

Re: [PATCH] generic/492: bypass the blkid cache when probing the label

[Baokun Li]

On 2026/9/22 22:53, Darrick J. Wong wrote:
> On Tue, Sep 22, 2026 at 03:17:27PM +0800, Baokun Li wrote:
>> On 2026/9/22 11:43, Theodore Tso wrote:
>>> On Mon, Sep 21, 2026 at 04:49:10PM -0500, Darrick J. Wong wrote:
>>>>> The difference between the e2fsprogs and util-linux libblkid is the
>>>>> time granularity: the former stores seconds only (TIME="%ld",
>>>>> bid_time = time(0), compares st_mtime <= bid_time), while the latter
>>>>> stores usec (TIME="sec.usec") and has an nsec tie-break in the compare,
>>>>> so the same-second case is much easier to hit with the e2fsprogs blkid.
>>>> Perhaps xfstests-bld should use the system blkid then?  IIRC the one
>>>> in e2fsprogs is very old and out of date.
>>> Historically, xfstests-bld was built in environments where libblkid
>>> wasn't available.  This included Android's runtime (which uses the
>>> bionic libc), and grte (Google Runtime Environment)[1].
>>>
>>> [1] https://github.com/bazelment/lrte/blob/master/grte/grte-build
>>>
>>> However, as more and more tests require utilities from util-linux,
>>> xfstests-bld was taught to build util-linux.  And if util-linux is
>>> being built, then we don't use e2fsprogs-libs any more, which is where
>>> libblkid is built.  From build-all:
>>>
>>> if test -z "$USE_LOCAL_E2FSLIBS" -o -d util-linux; then
>>>     SKIP_E2FSLIBS=yes
>>> fi
>>>
>>> I've always built xfstesets-bld with util-linux, even though it is
>>> listed as an optional repository.  This is because (a) util-linux is
>>> needed for Android and other non-standard Linux environments, and (b)
>>> the version of util-linux used by earlier Debian Stable was too old
>>> for some of the newer tests in fstests and blktests.
>>>
>>> If you aren't building xfstests-bld with util-linux, it falls back to
>>> building libblkid from e2fsprogs-libs.  HOWEVER, we don't actually
>>> blkid, and libblkid is always built statically.  So what seems to be
>>> going on is that one of the programs built by xfstests-bld is using
>>> the older libblkid, and that results in the blkid cache being writen
>>> with a second granularity.
>>>
>>> I've never noticed this issue because I've always built with
>>> util-linux.  The way to solve this is to require building with the
>>> system libblkid if util-linux is not being built, and to drop
>>> e2fsprogs-libs from xfstests-bld.  Or just build xfstests-bld with
>>> util-linux.  :-)
>>
>> Thanks for the detailed explanation.
>>
>> I checked my environment again though -- it turned out not to be
>> xfstests-bld at all.  xfstests-bld never touches e2fsprogs.
>>
>> What happened is that I had manually run "make install" of a locally
>> patched e2fsprogs in the test VM, which overwrote util-linux's blkid
>> and fsck.
>>
>> Rebuilding e2fsprogs with these options disabled made the test pass:
>>
>>     ./configure --disable-libblkid --disable-fsck \
>>             --disable-libuuid --disable-uuidd
>>
>> I ran unpatched generic/492 60 times on each side in a VM: 12/60
>> failures with the hand-installed strays, 0/60 with util-linux's
>> blkid/fsck.
>>
>> That said, I think the patch is still needed, because
>> FS_IOC_SETFSLABEL does not update the block device's mtime:
> Many filesystems /never/ update the bdev mtime, why would they?


Not from FS_IOC_SETFSLABEL, at least -- as far as I can see, none of
the filesystems implementing it updates the mtime, and even btrfs does
not do it from its label ioctl.  btrfs does have update_dev_time() for
the add/remove case (5a1972bd9fd4, so that libblkid does not keep
reading a stale cache), but that is not wired into its label ioctl.

I can see why.  What blkid keys on is the mtime of the /dev/xxx node,
and once the mount is set up we hold no reference to that path anymore
-- the node can be deleted or re-created, and s_id only keeps an
informational name.

Worse, one block device can have any number of /dev nodes pointing at
it; an ioctl on a mountpoint fd cannot tell which one the caller means,
nor find them all.  So the kernel has no well-defined node to update
here.

And it gets easy in userspace: for tools that take the device as an
argument (tune2fs, e2label, xfs_admin), one utime(device_name, NULL)
is enough to guarantee that after set the label of a device with
the tool, blkid -s <device> won't see the old label.

The tricky counterpart is the standard way to invoke this ioctl: a
mountpoint as the argument, xfs_io being the typical case.  It seems
the only option there is what the kernel would have to do anyway --
resolve the device behind the mount and update that node's mtime.

Do you have any thoughts?


>>     blkid -s LABEL ...    # get A
>>     FS_IOC_SETFSLABEL     # set B (device mtime unchanged)
>>     blkid -s LABEL ...    # still get A (within 2s)
>>
>> The first read writes a cache entry with TIME=now, and the ioctl
>> leaves the device mtime untouched, so a second read within the 2s
>> window still trusts the cache.
>>
>> I reproduced this with util-linux 2.41.5 too, so
>> "blkid -c /dev/null" is what keeps the test deterministic
>> regardless of which blkid the image carries.
> <nod> That was the problem I thought you were solving here ;)

Yep, that's the idea.  The point was to stay away from blkid's
cache complexity -- testing how a userspace tool caches things is
not really what a kernel test should be doing.


Cheers,
Baokun

Re: [PATCH] generic/492: bypass the blkid cache when probing the label

[Theodore Tso]

On Tue, Sep 22, 2026 at 03:17:27PM -0500, Baokun Li wrote:
> I checked my environment again though -- it turned out not to be
> xfstests-bld at all.  xfstests-bld never touches e2fsprogs.
> 
> What happened is that I had manually run "make install" of a locally
> patched e2fsprogs in the test VM, which overwrote util-linux's blkid
> and fsck.

What e2fsprogs's configure script will do is that it checks to see if
the blkid libraries are available for building against.  That is, if
you are using Debian or Ubuntu, you need to have the libblkid-dev
package install.  Among other things, this makes the following files
available:

/usr/include/blkid/blkid.h
/usr/lib/x86_64-linux-gnu/libblkid.a
/usr/lib/x86_64-linux-gnu/libblkid.so

... which are the files needed to build against libblkid and then link
against it statically or dynamically.  If these files aren't
available, then e2fsprogs will assume that it needs to build the local
libblkid, which is what is needed when building on Android, MacOS,
NetBSD, Open Solaris, etc.

> Rebuilding e2fsprogs with these options disabled made the test pass:
> 
>     ./configure --disable-libblkid --disable-fsck \
>             --disable-libuuid --disable-uuidd

This works, but this wll also disable those e2fsprogs features that
require those libraries.  So mke2fs will not generate UUID's in the
superblock, e2fsck won't handle LABEL=xyzzy specifiers, etc.
Fortunately xfstests don't depend on these featuers, so this will work.

The way I build with a locally patched e2fsprogs is that I'll build it
using the Debian build tools, which will enforce building with the
necessary prereqsuite packages, and then I'll put the built packages
in the directory test-appliance/debs and then build a fresh test
appliance image.  Or I'll just upload the packages to the VM and then
install it before launching the tests.

> That said, I think the patch is still needed, because
> FS_IOC_SETFSLABEL does not update the block device's mtime:
> 
>     blkid -s LABEL ...    # get A
>     FS_IOC_SETFSLABEL     # set B (device mtime unchanged)
>     blkid -s LABEL ...    # still get A (within 2s)
> 
> The first read writes a cache entry with TIME=now, and the ioctl
> leaves the device mtime untouched, so a second read within the 2s
> window still trusts the cache.

The patch to xfstests might still be appropriate, but it seems to me
that tune2fs should get fixed to update the device mtime (if it can),
and perhaps the kernel should try to update block device inode as well
as part of the FS_IOC_SETFSLABEL ioctl to address potential corner
cases with systemd/udev.

						- Ted

Re: [PATCH] generic/492: bypass the blkid cache when probing the label

[Baokun Li]

On 2026/9/22 23:56, Theodore Tso wrote:
> On Tue, Sep 22, 2026 at 03:17:27PM -0500, Baokun Li wrote:
>> I checked my environment again though -- it turned out not to be
>> xfstests-bld at all.  xfstests-bld never touches e2fsprogs.
>>
>> What happened is that I had manually run "make install" of a locally
>> patched e2fsprogs in the test VM, which overwrote util-linux's blkid
>> and fsck.
> What e2fsprogs's configure script will do is that it checks to see if
> the blkid libraries are available for building against.  That is, if
> you are using Debian or Ubuntu, you need to have the libblkid-dev
> package install.  Among other things, this makes the following files
> available:
>
> /usr/include/blkid/blkid.h
> /usr/lib/x86_64-linux-gnu/libblkid.a
> /usr/lib/x86_64-linux-gnu/libblkid.so
>
> ... which are the files needed to build against libblkid and then link
> against it statically or dynamically.  If these files aren't
> available, then e2fsprogs will assume that it needs to build the local
> libblkid, which is what is needed when building on Android, MacOS,
> NetBSD, Open Solaris, etc.

Indeed, I originally built and installed it manually in an
environment without libblkid-dev.

>> Rebuilding e2fsprogs with these options disabled made the test pass:
>>
>>     ./configure --disable-libblkid --disable-fsck \
>>             --disable-libuuid --disable-uuidd
> This works, but this wll also disable those e2fsprogs features that
> require those libraries.  So mke2fs will not generate UUID's in the
> superblock, e2fsck won't handle LABEL=xyzzy specifiers, etc.
> Fortunately xfstests don't depend on these featuers, so this will work.


Hmm, what I disabled is exactly the set duplicated by util-linux:
the blkid/uuid libs and UUID/LABEL= handling all live there, so
the tools just link the system ones -- there is no functional loss.

> The way I build with a locally patched e2fsprogs is that I'll build it
> using the Debian build tools, which will enforce building with the
> necessary prereqsuite packages, and then I'll put the built packages
> in the directory test-appliance/debs and then build a fresh test
> appliance image.  Or I'll just upload the packages to the VM and then
> install it before launching the tests.

That sounds much better than a hand install -- I'll give it a try.

>> That said, I think the patch is still needed, because
>> FS_IOC_SETFSLABEL does not update the block device's mtime:
>>
>>     blkid -s LABEL ...    # get A
>>     FS_IOC_SETFSLABEL     # set B (device mtime unchanged)
>>     blkid -s LABEL ...    # still get A (within 2s)
>>
>> The first read writes a cache entry with TIME=now, and the ioctl
>> leaves the device mtime untouched, so a second read within the 2s
>> window still trusts the cache.
> The patch to xfstests might still be appropriate, but it seems to me
> that tune2fs should get fixed to update the device mtime (if it can),
> and perhaps the kernel should try to update block device inode as well
> as part of the FS_IOC_SETFSLABEL ioctl to address potential corner
> cases with systemd/udev.


Yes -- we need to add a utime() in tune2fs's ioctl path to update
the device mtime; I'll send a patch shortly.

For the kernel side, we don't have the dev path that the caller
is using, so it is not doable there: the inode we can reach
inside the ioctl is not the one userspace sees.  Details are in my
reply to Darrick:

https://lore.kernel.org/all/eeeab8d8-dfd3-45ff-9f30-d83c527210f2@linux.alibaba.com

Also, a thought that just came up: on the blkid side, would it be
feasible to not trust the cache for mounted devices and always
re-probe?  The mount / set label / umount / get label (within 2s)
window would remain, but it would be much smaller.


Thanks,
Baokun

Re: [PATCH] generic/492: bypass the blkid cache when probing the label

[Theodore Tso]

On Wed, Sep 23, 2026 at 06:02:38PM -0500, Baokun Li wrote:
> >>     ./configure --disable-libblkid --disable-fsck \
> >>             --disable-libuuid --disable-uuidd
> > This works, but this wll also disable those e2fsprogs features that
> > require those libraries.  So mke2fs will not generate UUID's in the
> > superblock, e2fsck won't handle LABEL=xyzzy specifiers, etc.
> > Fortunately xfstests don't depend on these featuers, so this will work.
> 
> Hmm, what I disabled is exactly the set duplicated by util-linux:
> the blkid/uuid libs and UUID/LABEL= handling all live there, so
> the tools just link the system ones -- there is no functional loss.

The fact that you needed the --disable options implies that e2fsprogs
wasn't able to use the util-linux supplied libraries.  If the
configure script determines libblkid and libuuid are available for
linking against, (e.g., libblkid-dev and uuid-dev are installed), the
built-in libblkid and libuuid file systems are disabled.

From the configure script output:

checking for blkid_get_cache in -lblkid... yes
Using system blkid library by default

checking for uuid_generate in -luuid... yes
Using system uuid by default
Disabling uuidd by default

In contrast, if you run the configure script on MacOS, NetBSD, or a
Linux system missing the libuuid / libblkid development files, you
will see:

checking for uuid_generate in -luuid... no
Enabling private uuid library by default
Building uuidd by default

> Also, a thought that just came up: on the blkid side, would it be
> feasible to not trust the cache for mounted devices and always
> re-probe?  The mount / set label / umount / get label (within 2s)
> window would remain, but it would be much smaller.

Maybe.  It's not clear how often people might be trying to query the
blkid data.  Also, figuring out whether or not a device is mounted is
not free.  That would mean that for every blkid query, it would have
to read /proc/mounts to make that determination.  And note that on
some data center servers, there could potentially be hundreds if not
thousands of entries in /proc/mounts, either because you have a huge
number of bind mounts or overlayfs, and/or because you might have a
huge number of read-only iscsi mounts to deliver software packages to
your container based Borg or Kubernetes jobs.

I suppose you could put in the blkid cache a hint as to whether the
file system is mounted, but that in itself could get stale and out of
date.

Cheers,

					- Ted

Re: [PATCH] generic/492: bypass the blkid cache when probing the label

[Baokun Li]

On 2026/9/24 10:25, Theodore Tso wrote:
> On Wed, Sep 23, 2026 at 06:02:38PM -0500, Baokun Li wrote:
>>>>     ./configure --disable-libblkid --disable-fsck \
>>>>             --disable-libuuid --disable-uuidd
>>> This works, but this wll also disable those e2fsprogs features that
>>> require those libraries.  So mke2fs will not generate UUID's in the
>>> superblock, e2fsck won't handle LABEL=xyzzy specifiers, etc.
>>> Fortunately xfstests don't depend on these featuers, so this will work.
>> Hmm, what I disabled is exactly the set duplicated by util-linux:
>> the blkid/uuid libs and UUID/LABEL= handling all live there, so
>> the tools just link the system ones -- there is no functional loss.
> The fact that you needed the --disable options implies that e2fsprogs
> wasn't able to use the util-linux supplied libraries.  If the
> configure script determines libblkid and libuuid are available for
> linking against, (e.g., libblkid-dev and uuid-dev are installed), the
> built-in libblkid and libuuid file systems are disabled.
>
> From the configure script output:
>
> checking for blkid_get_cache in -lblkid... yes
> Using system blkid library by default
>
> checking for uuid_generate in -luuid... yes
> Using system uuid by default
> Disabling uuidd by default
>
> In contrast, if you run the configure script on MacOS, NetBSD, or a
> Linux system missing the libuuid / libblkid development files, you
> will see:
>
> checking for uuid_generate in -luuid... no
> Enabling private uuid library by default
> Building uuidd by default

Right.  The flags went in only after the development packages were
installed.  The original hand build had no libblkid-dev, so it built
and installed the private copies, which is how the distro's
blkid/fsck/findfs got shadowed.

With libblkid-dev, uuid-dev and pkg-config installed, a plain
./configure now prints exactly your first output.  Compared with my
--disable build, the only extra program it produces is the fsck
wrapper.

>> Also, a thought that just came up: on the blkid side, would it be
>> feasible to not trust the cache for mounted devices and always
>> re-probe?  The mount / set label / umount / get label (within 2s)
>> window would remain, but it would be much smaller.
> Maybe.  It's not clear how often people might be trying to query the
> blkid data.  Also, figuring out whether or not a device is mounted is
> not free.  That would mean that for every blkid query, it would have
> to read /proc/mounts to make that determination.  And note that on
> some data center servers, there could potentially be hundreds if not
> thousands of entries in /proc/mounts, either because you have a huge
> number of bind mounts or overlayfs, and/or because you might have a
> huge number of read-only iscsi mounts to deliver software packages to
> your container based Borg or Kubernetes jobs.
>
> I suppose you could put in the blkid cache a hint as to whether the
> file system is mounted, but that in itself could get stale and out of
> date.

Agreed -- checking whether a device is mounted on every probe might
cost more than simply re-probing the device.  Updating the mtime on
the write side (e.g., in tune2fs, xfs_io, or the kernel) looks like
the better approach.


Cheers,
Baokun