[PATCH v8 05/12] test_sysfs: add support to use kernfs failure injection

Luis Chamberlain
2021-09-27T16:38:52Z

This extends test_sysfs with support for using the failure injection
wait completion and knobs to force a few race conditions which
demonstrates that kernfs active reference protection is sufficient
for kobject / device protection at higher layers.

This adds 4 new tests which tries to remove the device attribute
store operation in 4 different situations:

1) at the start of kernfs_kernfs_fop_write_iter()
2) before the of->mutex is held in kernfs_kernfs_fop_write_iter()
3) after the of->mutex is held in kernfs_kernfs_fop_write_iter()
4) after the kernfs node active reference is taken

A write fails in call cases except the last one, test number #32. There
is a good explanation for this: *once* kernfs_get_active() gets called
we have a guarantee that the kernfs entry cannot be removed. If
kernfs_get_active() succeeds that entry cannot be removed and so
anything trying to remove that entry will have to wait. It is perhaps
not obvious but since a sysfs write will trigger eventually a
kernfs_get_active() call, and *only* if this succeeds will the sysfs
op be called, this and the fact that you cannot remove the kernfs
entry while the kenfs entry is active implies that a module that
created the respective sysfs / kernfs entry *cannot* possibly be
removed during a sysfs operation. And test number 32 provides us with
proof of this. If it were not true test #32 should crash.

No null dereferences are reproduced, even though this has been observed
in some complex testing cases [0]. If this issue really exists we should
have enough tools on the sysfs_test toolbox now to try to reproduce
this easily without having to poke around other drivers. It very likley
was the case that the issue reported [0] was possibly a side issue after
the first bug which was zram specific. This is why it is important to
isolate the issue and try to reproduce it in a generic form using the
test_sysfs driver.

[0] https://lkml.kernel.org/r/20210623215007.862787-1-mcgrof@kernel.org

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
lib/Kconfig.debug | 3 +
lib/test_sysfs.c | 31 +++++
tools/testing/selftests/sysfs/config | 3 +
tools/testing/selftests/sysfs/sysfs.sh | 175 +++++++++++++++++++++++++
4 files changed, 212 insertions(+)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index a29b7d398c4e..176b822654e5 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2358,6 +2358,9 @@ config TEST_SYSFS
depends on SYSFS
depends on NET
depends on BLOCK
+ select FAULT_INJECTION
+ select FAULT_INJECTION_DEBUG_FS
+ select FAIL_KERNFS_KNOBS
help
This builds the "test_sysfs" module. This driver enables to test the
sysfs file system safely without affecting production knobs which
diff --git a/lib/test_sysfs.c b/lib/test_sysfs.c
index 2043ca494af8..c6e62de61403 100644
--- a/lib/test_sysfs.c
+++ b/lib/test_sysfs.c
@@ -38,6 +38,11 @@
#include <linux/rtnetlink.h>
#include <linux/genhd.h>
#include <linux/blkdev.h>
+#include <linux/kernfs.h>
+
+#ifdef CONFIG_FAIL_KERNFS_KNOBS
+MODULE_IMPORT_NS(KERNFS_DEBUG_PRIVATE);
+#endif

static bool enable_lock;
module_param(enable_lock, bool_enable_only, 0644);
@@ -82,6 +87,13 @@ static bool enable_verbose_rmmod;
module_param(enable_verbose_rmmod, bool_enable_only, 0644);
MODULE_PARM_DESC(enable_verbose_rmmod, "enable verbose print messages on rmmod");

+#ifdef CONFIG_FAIL_KERNFS_KNOBS
+static bool enable_completion_on_rmmod;
+module_param(enable_completion_on_rmmod, bool_enable_only, 0644);
+MODULE_PARM_DESC(enable_completion_on_rmmod,
+ "enable sending a kernfs completion on rmmod");
+#endif
+
static int sysfs_test_major;

/**
@@ -285,6 +297,12 @@ static ssize_t config_show(struct device *dev,
"enable_verbose_writes:\t%s\n",
enable_verbose_writes ? "true" : "false");

+#ifdef CONFIG_FAIL_KERNFS_KNOBS
+ len += snprintf(buf+len, PAGE_SIZE - len,
+ "enable_completion_on_rmmod:\t%s\n",
+ enable_completion_on_rmmod ? "true" : "false");
+#endif
+
test_dev_config_unlock(test_dev);

return len;
@@ -904,10 +922,23 @@ static int __init test_sysfs_init(void)
}
module_init(test_sysfs_init);

+#ifdef CONFIG_FAIL_KERNFS_KNOBS
+/* The goal is to race our device removal with a pending kernfs -> store call */
+static void test_sysfs_kernfs_send_completion_rmmod(void)
+{
+ if (!enable_completion_on_rmmod)
+ return;
+ complete(&kernfs_debug_wait_completion);
+}
+#else
+static inline void test_sysfs_kernfs_send_completion_rmmod(void) {}
+#endif
+
static void __exit test_sysfs_exit(void)
{
if (enable_debugfs)
debugfs_remove(debugfs_dir);
+ test_sysfs_kernfs_send_completion_rmmod();
if (delay_rmmod_ms)
msleep(delay_rmmod_ms);
unregister_test_dev_sysfs(first_test_dev);
diff --git a/tools/testing/selftests/sysfs/config b/tools/testing/selftests/sysfs/config
index 9196f452ecd5..2876a229f95b 100644
--- a/tools/testing/selftests/sysfs/config
+++ b/tools/testing/selftests/sysfs/config
@@ -1,2 +1,5 @@
CONFIG_SYSFS=m
CONFIG_TEST_SYSFS=m
+CONFIG_FAULT_INJECTION=y
+CONFIG_FAULT_INJECTION_DEBUG_FS=y
+CONFIG_FAIL_KERNFS_KNOBS=y
diff --git a/tools/testing/selftests/sysfs/sysfs.sh b/tools/testing/selftests/sysfs/sysfs.sh
index b3f4c2236c7f..f928635d0e35 100755
--- a/tools/testing/selftests/sysfs/sysfs.sh
+++ b/tools/testing/selftests/sysfs/sysfs.sh
@@ -62,6 +62,10 @@ ALL_TESTS="$ALL_TESTS 0025:1:1:test_dev_y:block"
ALL_TESTS="$ALL_TESTS 0026:1:1:test_dev_y:block"
ALL_TESTS="$ALL_TESTS 0027:1:0:test_dev_x:block" # deadlock test
ALL_TESTS="$ALL_TESTS 0028:1:0:test_dev_x:block" # deadlock test with rntl_lock
+ALL_TESTS="$ALL_TESTS 0029:1:1:test_dev_x:block" # kernfs race removal of store
+ALL_TESTS="$ALL_TESTS 0030:1:1:test_dev_x:block" # kernfs race removal before mutex
+ALL_TESTS="$ALL_TESTS 0031:1:1:test_dev_x:block" # kernfs race removal after mutex
+ALL_TESTS="$ALL_TESTS 0032:1:1:test_dev_x:block" # kernfs race removal after active

allow_user_defaults()
{
@@ -92,6 +96,9 @@ allow_user_defaults()
if [ -z $SYSFS_DEBUGFS_DIR ]; then
SYSFS_DEBUGFS_DIR="/sys/kernel/debug/test_sysfs"
fi
+ if [ -z $KERNFS_DEBUGFS_DIR ]; then
+ KERNFS_DEBUGFS_DIR="/sys/kernel/debug/kernfs"
+ fi
if [ -z $PAGE_SIZE ]; then
PAGE_SIZE=$(getconf PAGESIZE)
fi
@@ -167,6 +174,14 @@ modprobe_reset_enable_rtnl_lock_on_rmmod()
unset FIRST_MODPROBE_ARGS
}

+modprobe_reset_enable_completion()
+{
+ FIRST_MODPROBE_ARGS="enable_completion_on_rmmod=1 enable_verbose_writes=1"
+ FIRST_MODPROBE_ARGS="$FIRST_MODPROBE_ARGS enable_verbose_rmmod=1 delay_rmmod_ms=0"
+ modprobe_reset
+ unset FIRST_MODPROBE_ARGS
+}
+
load_req_mod()
{
modprobe_reset
@@ -197,6 +212,63 @@ debugfs_reset_first_test_dev_ignore_errors()
echo -n "1" >"$SYSFS_DEBUGFS_DIR"/reset_first_test_dev
}

+debugfs_kernfs_kernfs_fop_write_iter_exists()
+{
+ KNOB_DIR="${KERNFS_DEBUGFS_DIR}/config_fail_kernfs_fop_write_iter"
+ if [[ ! -d $KNOB_DIR ]]; then
+ echo "kernfs debugfs does not exist $KNOB_DIR"
+ return 0;
+ fi
+ KNOB_DEBUGFS="${KERNFS_DEBUGFS_DIR}/fail_kernfs_fop_write_iter"
+ if [[ ! -d $KNOB_DEBUGFS ]]; then
+ echo -n "kernfs debugfs for coniguring fail_kernfs_fop_write_iter "
+ echo "does not exist $KNOB_DIR"
+ return 0;
+ fi
+ return 1
+}
+
+debugfs_kernfs_kernfs_fop_write_iter_set_fail_once()
+{
+ KNOB_DEBUGFS="${KERNFS_DEBUGFS_DIR}/fail_kernfs_fop_write_iter"
+ echo 1 > $KNOB_DEBUGFS/interval
+ echo 100 > $KNOB_DEBUGFS/probability
+ echo 0 > $KNOB_DEBUGFS/space
+ # Disable verbose messages on the kernel ring buffer which may
+ # confuse developers with a kernel panic.
+ echo 0 > $KNOB_DEBUGFS/verbose
+
+ # Fail only once
+ echo 1 > $KNOB_DEBUGFS/times
+}
+
+debugfs_kernfs_kernfs_fop_write_iter_set_fail_never()
+{
+ KNOB_DEBUGFS="${KERNFS_DEBUGFS_DIR}/fail_kernfs_fop_write_iter"
+ echo 0 > $KNOB_DEBUGFS/times
+}
+
+debugfs_kernfs_set_wait_ms()
+{
+ SLEEP_AFTER_WAIT_MS="${KERNFS_DEBUGFS_DIR}/sleep_after_wait_ms"
+ echo $1 > $SLEEP_AFTER_WAIT_MS
+}
+
+debugfs_kernfs_disable_wait_kernfs_fop_write_iter()
+{
+ ENABLE_WAIT_KNOB="${KERNFS_DEBUGFS_DIR}/config_fail_kernfs_fop_write_iter/wait_"
+ for KNOB in ${ENABLE_WAIT_KNOB}*; do
+ echo 0 > $KNOB
+ done
+}
+
+debugfs_kernfs_enable_wait_kernfs_fop_write_iter()
+{
+ ENABLE_WAIT_KNOB="${KERNFS_DEBUGFS_DIR}/config_fail_kernfs_fop_write_iter/wait_$1"
+ echo -n "1" > $ENABLE_WAIT_KNOB
+ return $?
+}
+
set_orig()
{
if [[ ! -z $TARGET ]] && [[ ! -z $ORIG ]]; then
@@ -972,6 +1044,105 @@ sysfs_test_0028()
fi
}

+sysfs_race_kernfs_kernfs_fop_write_iter()
+{
+ TARGET="${DIR}/$(get_test_target $1)"
+ WAIT_AT=$2
+ EXPECT_WRITE_RETURNS=$3
+ MSDELAY=$4
+
+ modprobe_reset_enable_completion
+ ORIG=$(cat "${TARGET}")
+ TEST_STR=$(( $ORIG + 1 ))
+
+ echo -n "Test racing removal of sysfs store op with kernfs $WAIT_AT ... "
+
+ if debugfs_kernfs_kernfs_fop_write_iter_exists; then
+ echo -n "skipping test as CONFIG_FAIL_KERNFS_KNOBS "
+ echo " or CONFIG_FAULT_INJECTION_DEBUG_FS is disabled"
+ return $ksft_skip
+ fi
+
+ # Allow for failing the kernfs_kernfs_fop_write_iter call once,
+ # we'll provide exact context shortly afterwards.
+ debugfs_kernfs_kernfs_fop_write_iter_set_fail_once
+
+ # First disable all waits
+ debugfs_kernfs_disable_wait_kernfs_fop_write_iter
+
+ # Enable a wait_for_completion(&kernfs_debug_wait_completion) at the
+ # specified location inside the kernfs_fop_write_iter() routine
+ debugfs_kernfs_enable_wait_kernfs_fop_write_iter $WAIT_AT
+
+ # Configure kernfs so that after its wait_for_completion() it
+ # will msleep() this amount of time and schedule(). We figure this
+ # will be sufficient time to allow for our module removal to complete.
+ debugfs_kernfs_set_wait_ms $MSDELAY
+
+ # Now we trigger a kernfs write op, which will run kernfs_fop_write_iter,
+ # but will wait until our driver sends a respective completion
+ set_test_ignore_errors &
+ write_pid=$!
+
+ # At this point kernfs_fop_write_iter() hasn't run our op, its
+ # waiting for our completion at the specified time $WAIT_AT.
+ # We now remove our module which will send a
+ # complete(&kernfs_debug_wait_completion) right before we deregister
+ # our device and the sysfs device attributes are removed.
+ #
+ # After the completion is sent, the test_sysfs driver races with
+ # kernfs to do the device deregistration with the kernfs msleep
+ # and schedule(). This should mean we've forced trying to remove the
+ # module prior to allowing kernfs to run our store operation. If the
+ # race did happen we'll panic with a null dereference on the store op.
+ #
+ # If no race happens we should see no write operation triggered.
+ modprobe -r $TEST_DRIVER > /dev/null 2>&1
+
+ debugfs_kernfs_kernfs_fop_write_iter_set_fail_never
+
+ wait $write_pid
+ if [[ $? -eq $EXPECT_WRITE_RETURNS ]]; then
+ echo "ok"
+ else
+ echo "FAIL" >&2
+ fi
+}
+
+sysfs_test_0029()
+{
+ for delay in 0 2 4 8 16 32 64 128 246 512 1024; do
+ echo "Using delay-after-completion: $delay"
+ sysfs_race_kernfs_kernfs_fop_write_iter 0029 at_start 1 $delay
+ done
+}
+
+sysfs_test_0030()
+{
+ for delay in 0 2 4 8 16 32 64 128 246 512 1024; do
+ echo "Using delay-after-completion: $delay"
+ sysfs_race_kernfs_kernfs_fop_write_iter 0030 before_mutex 1 $delay
+ done
+}
+
+sysfs_test_0031()
+{
+ for delay in 0 2 4 8 16 32 64 128 246 512 1024; do
+ echo "Using delay-after-completion: $delay"
+ sysfs_race_kernfs_kernfs_fop_write_iter 0031 after_mutex 1 $delay
+ done
+}
+
+# A write only succeeds *iff* a module removal happens *after* the
+# kernfs active reference is obtained with kernfs_get_active().
+sysfs_test_0032()
+{
+ for delay in 0 2 4 8 16 32 64 128 246 512 1024; do
+ echo "Using delay-after-completion: $delay"
+ sysfs_race_kernfs_kernfs_fop_write_iter 0032 after_active 0 $delay
+ done
+}
+
test_gen_desc()
{
echo -n "$1 x $(get_test_count $1)"
@@ -1013,6 +1184,10 @@ list_tests()
echo "$(test_gen_desc 0026) - block test writing y larger delay and resetting device"
echo "$(test_gen_desc 0027) - test rmmod deadlock while writing x ... "
echo "$(test_gen_desc 0028) - test rmmod deadlock using rtnl_lock while writing x ..."
+ echo "$(test_gen_desc 0029) - racing removal of store op with kernfs at start"
+ echo "$(test_gen_desc 0030) - racing removal of store op with kernfs before mutex"
+ echo "$(test_gen_desc 0031) - racing removal of store op with kernfs after mutex"
+ echo "$(test_gen_desc 0032) - racing removal of store op with kernfs after active"
}

usage()
--
2.30.2