On Wed, 25 Mar 2026, Chuck Lever wrote:
>
> On Tue, Mar 24, 2026, at 6:13 AM, NeilBrown wrote:
> > From: NeilBrown <[email protected]>
> >
> > 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 <[email protected]>
> > 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 <[email protected]>
>
> Hi Neil, which kernels should this fix apply to?
>
v6.13 and later. So linux-6.18.y and linux-6.19.y
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
