Re: Bug#1128861: [PATCH] lockd: fix TEST handling when not all permissions are available.

Ben Hutchings
2026-03-25T20:29:35Z

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On Wed, 2026-03-25 at 18:08 +1100, NeilBrown wrote:
> On Wed, 25 Mar 2026, Chuck Lever wrote:
> >
> > On Tue, Mar 24, 2026, at 6:13 AM, NeilBrown wrote:
> > > From: NeilBrown <neil@brown.name>
> > >
> > > The F_GETLK fcntl can work with either read access or write access or
> > > both. It can query F_RDLCK and F_WRLCK locks in either case.
> > >
> > > However lockd currently treats F_GETLK similar to F_SETLK in that read
> > > access is required to query an F_RDLCK lock and write access is required
> > > to query a F_WRLCK lock.
> > >
> > > This is wrong and can cause problem - e.g. when qemu accesses a
> > > read-only (e.g. iso) filesystem image over NFS (though why it queries
> > > if it can get a write lock - I don't know. But it does, and this works
> > > with local filesystems).
> > >
> > > So we need TEST requests to be handled differently. To do this:
> > >
> > > - change nlm_do_fopen() to accept O_RDWR as a mode and in that case
> > > succeed if either a O_RDONLY or O_WRONLY file can be opened.
> > > - change nlm_lookup_file() to accept a mode argument from caller,
> > > instead of deducing base on lock time, and pass that on to nlm_do_fopen()
> > > - change nlm4svc_retrieve_args() and nlmsvc_retrieve_args() to detect
> > > TEST requests and pass O_RDWR as a mode to nlm_lookup_file, passing
> > > the same mode as before for other requests. Also set
> > > lock->fl.c.flc_file to whichever file is available for TEST requests.
> > > - change nlmsvc_testlock() to also not calculate the mode, but to use
> > > whenever was stored in lock->fl.c.flc_file.
> > >
> > > Reported-by: Tj <tj.iam.tj@proton.me>
> > > Link: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1128861
> > > Fixes: 7f024fcd5c97 ("Keep read and write fds with each nlm_file")
> > > Signed-off-by: NeilBrown <neil@brown.name>
> >
> > Hi Neil, which kernels should this fix apply to?
> >
>
> v6.13 and later. So linux-6.18.y and linux-6.19.y

6.12.y is also affected since commit 4cc9b9f2bf4d was backported there
(triggering this bug report).

Ben.

>
> The Fixes: tag is actually wrong. This bug has been present forever.
> However a different bug that
> Commit: 4cc9b9f2bf4d ("nfsd: refine and rename NFSD_MAY_LOCK")
> fixed was hiding the bug.
>
> So it should probably be marked
> Fixes: 4cc9b9f2bf4d ("nfsd: refine and rename NFSD_MAY_LOCK")
> with an explanation.
>
> NeilBrown

--
Ben Hutchings
Theory and practice are closer in theory than in practice - John Levine

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