xfs: fix online repair probing when CONFIG_XFS_ONLINE_REPAIR=n

commit 66314e9a57a050f95cb0ebac904f5ab047a8926e upstream.

I received a report from the release engineering side of the house that
xfs_scrub without the -n flag (aka fix it mode) would try to fix a
broken filesystem even on a kernel that doesn't have online repair built
into it:

 # xfs_scrub -dTvn /mnt/test
 EXPERIMENTAL xfs_scrub program in use! Use at your own risk!
 Phase 1: Find filesystem geometry.
 /mnt/test: using 1 threads to scrub.
 Phase 1: Memory used: 132k/0k (108k/25k), time:  0.00/ 0.00/ 0.00s
 <snip>
 Phase 4: Repair filesystem.
 <snip>
 Info: /mnt/test/some/victimdir directory entries: Attempting repair. (repair.c line 351)
 Corruption: /mnt/test/some/victimdir directory entries: Repair unsuccessful; offline repair required. (repair.c line 204)

Source: https://blogs.oracle.com/linux/post/xfs-online-filesystem-repair



It is strange that xfs_scrub doesn't refuse to run, because the kernel
is supposed to return EOPNOTSUPP if we actually needed to run a repair,
and xfs_io's repair subcommand will perror that.  And yet:

 # xfs_io -x -c 'repair probe' /mnt/test
 #

The first problem is commit dcb660f9 (4.15) which should have had
xchk_probe set the CORRUPT OFLAG so that any of the repair machinery
will get called at all.

It turns out that some refactoring that happened in the 6.6-6.8 era
broke the operation of this corner case.  What we *really* want to
happen is that all the predicates that would steer xfs_scrub_metadata()
towards calling xrep_attempt() should function the same way that they do
when repair is compiled in; and then xrep_attempt gets to return the
fatal EOPNOTSUPP error code that causes the probe to fail.

Instead, commit 8336a64e (6.6) started the failwhale swimming by
hoisting OFLAG checking logic into a helper whose non-repair stub always
returns false, causing scrub to return "repair not needed" when in fact
the repair is not supported.  Prior to that commit, the oflag checking
that was open-coded in scrub.c worked correctly.

Similarly, in commit 4bdfd7d1 (6.8) we hoisted the IFLAG_REPAIR
and ALREADY_FIXED logic into a helper whose non-repair stub always
returns false, so we never enter the if test body that would have called
xrep_attempt, let alone fail to decode the OFLAGs correctly.

The final insult (yes, we're doing The Naked Gun now) is commit
48a72f60 (6.8) in which we hoisted the "are we going to try a
repair?" predicate into yet another function with a non-repair stub
always returns false.

Fix xchk_probe to trigger xrep_probe if repair is enabled, or return
EOPNOTSUPP directly if it is not.  For all the other scrub types, we
need to fix the header predicates so that the ->repair functions (which
are all xrep_notsupported) get called to return EOPNOTSUPP.  Commit
48a72 is tagged here because the scrub code prior to LTS 6.12 are
incomplete and not worth patching.

Reported-by: David Flynn <david.flynn@oracle.com>
Cc: <stable@vger.kernel.org> # v6.8
Fixes: 8336a64e ("xfs: don't complain about unfixed metadata when repairs were injected")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>