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

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

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 in e2fsprogs is very old and out of date.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
> 
>