[PATCH][BROKEN] common/rc: fix check for disabled kmemleak

[Amir Goldstein <amir73il@gmail.com>]

With kernel commit b353756b2b71 ("kmemleak: always register debugfs file")
that was merged to v4.19-rc3, the kmemleak debugfs knob exists even if
kmemleak is disabled, but returns EBUSY on write.

Suppress EBUSY errors from _check_kmemleak() by removing write permission
from knob on failure to initialize kmemleak.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---

Guys,

With kernel v4.19-rc3, tests started printing noisy messages about failure
to write to kmemleak knob.

Problem is that kmemleak was disabled on my system but the test for
kmemleak enabled we have in place was broken by the kernel change.

I tried this patch to hack around the new kernel behavior, but something
is broken in this patch as it doesn't suppress all errors.
For me, I prefer to have kmemleak enabled anyway, so I gave up on this
hack and enabled kmemleak.

If someone is interested in taking over, be my guest.

FYI, to enable kmemleak on kvm-xfstests I needed to configure
CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE=5000
# CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF is not set

Thanks,
Amir.

 common/rc | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/common/rc b/common/rc
index ec631ad9..cafc83b4 100644
--- a/common/rc
+++ b/common/rc
@@ -3514,8 +3514,14 @@ _init_kmemleak()
 
 	# Disable the automatic scan so that we can control it completely,
 	# then dump all the leaks recorded so far.
-	echo "scan=off" > "$kern_knob"
-	_capture_kmemleak /dev/null
+	if echo "scan=off" > "$kern_knob" 2>/dev/null; then
+		_capture_kmemleak /dev/null
+	else
+		# Since kernel v4.19-rc3, the knob exists even if kmemleak is
+		# disabled, but returns EBUSY on write. Suppress EBUSY errors
+		# from _check_kmemleak() by removing write permission from knob
+		chmod a-w "$kern_knob"
+	fi
 }
 
 # check kmemleak log
-- 
2.17.1