netfs: zero the tail of a short DIO/unbuffered readgrafted

The buffered read collector zero-fills the tail of a short read that
stops below the inode's i_size (netfs_clear_unread()), so a read that
races an extending write still returns the expected number of bytes.
The non-buffered collector path does no such thing: it just records
how much was transferred.

Add the same zero-fill for the non-buffered case, gated on
NETFS_SREQ_CLEAR_TAIL: a subreq's source sets that flag when a short
result from it is known to be safe to treat as a hole, as opposed to
NETFS_SREQ_HIT_EOF, which means the read genuinely ran off the end of
the file and should be reported short as-is. Only CLEAR_TAIL should
zero-fill here; a real EOF must stay a real short read.

No source currently sets CLEAR_TAIL on an unbuffered/DIO subrequest,
so this is inert on its own -- a following change teaches cifs to set
it in the one case that needs it.

Fixes: e2d46f2ec332 ("netfs: Change the read result collector to only use one work item")
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: stable@vger.kernel.org

1 files changed, 212 insertions, 0 deletions

diff --git a/samples/livepatch/livepatch-shadow-mod.c b/samples/livepatch/livepatch-shadow-mod.c
new file mode 100644
index 000000000..5d83ad5a8
--- /dev/null
+++ b/samples/livepatch/livepatch-shadow-mod.c
@@ -0,0 +1,212 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2017 Joe Lawrence <joe.lawrence@redhat.com>
+ */
+
+/*
+ * livepatch-shadow-mod.c - Shadow variables, buggy module demo
+ *
+ * Purpose
+ * -------
+ *
+ * As a demonstration of livepatch shadow variable API, this module
+ * introduces memory leak behavior that livepatch modules
+ * livepatch-shadow-fix1.ko and livepatch-shadow-fix2.ko correct and
+ * enhance.
+ *
+ * WARNING - even though the livepatch-shadow-fix modules patch the
+ * memory leak, please load these modules at your own risk -- some
+ * amount of memory may leaked before the bug is patched.
+ *
+ *
+ * Usage
+ * -----
+ *
+ * Step 1 - Load the buggy demonstration module:
+ *
+ *   insmod samples/livepatch/livepatch-shadow-mod.ko
+ *
+ * Watch dmesg output for a few moments to see new dummy being allocated
+ * and a periodic cleanup check.  (Note: a small amount of memory is
+ * being leaked.)
+ *
+ *
+ * Step 2 - Load livepatch fix1:
+ *
+ *   insmod samples/livepatch/livepatch-shadow-fix1.ko
+ *
+ * Continue watching dmesg and note that now livepatch_fix1_dummy_free()
+ * and livepatch_fix1_dummy_alloc() are logging messages about leaked
+ * memory and eventually leaks prevented.
+ *
+ *
+ * Step 3 - Load livepatch fix2 (on top of fix1):
+ *
+ *   insmod samples/livepatch/livepatch-shadow-fix2.ko
+ *
+ * This module extends functionality through shadow variables, as a new
+ * "check" counter is added to the dummy structure.  Periodic dmesg
+ * messages will log these as dummies are cleaned up.
+ *
+ *
+ * Step 4 - Cleanup
+ *
+ * Unwind the demonstration by disabling the livepatch fix modules, then
+ * removing them and the demo module:
+ *
+ *   echo 0 > /sys/kernel/livepatch/livepatch_shadow_fix2/enabled
+ *   echo 0 > /sys/kernel/livepatch/livepatch_shadow_fix1/enabled
+ *   rmmod livepatch-shadow-fix2
+ *   rmmod livepatch-shadow-fix1
+ *   rmmod livepatch-shadow-mod
+ */
+
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/sched.h>
+#include <linux/slab.h>
+#include <linux/stat.h>
+#include <linux/workqueue.h>
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Joe Lawrence <joe.lawrence@redhat.com>");
+MODULE_DESCRIPTION("Buggy module for shadow variable demo");
+
+/* Allocate new dummies every second */
+#define ALLOC_PERIOD	1
+/* Check for expired dummies after a few new ones have been allocated */
+#define CLEANUP_PERIOD	(3 * ALLOC_PERIOD)
+/* Dummies expire after a few cleanup instances */
+#define EXPIRE_PERIOD	(4 * CLEANUP_PERIOD)
+
+/*
+ * Keep a list of all the dummies so we can clean up any residual ones
+ * on module exit
+ */
+static LIST_HEAD(dummy_list);
+static DEFINE_MUTEX(dummy_list_mutex);
+
+struct dummy {
+	struct list_head list;
+	unsigned long jiffies_expire;
+};
+
+static __used noinline struct dummy *dummy_alloc(void)
+{
+	struct dummy *d;
+	int *leak;
+
+	d = kzalloc(sizeof(*d), GFP_KERNEL);
+	if (!d)
+		return NULL;
+
+	d->jiffies_expire = jiffies + secs_to_jiffies(EXPIRE_PERIOD);
+
+	/* Oops, forgot to save leak! */
+	leak = kzalloc(sizeof(*leak), GFP_KERNEL);
+	if (!leak) {
+		kfree(d);
+		return NULL;
+	}
+
+	pr_info("%s: dummy @ %p, expires @ %lx\n",
+		__func__, d, d->jiffies_expire);
+
+	return d;
+}
+
+static __used noinline void dummy_free(struct dummy *d)
+{
+	pr_info("%s: dummy @ %p, expired = %lx\n",
+		__func__, d, d->jiffies_expire);
+
+	kfree(d);
+}
+
+static __used noinline bool dummy_check(struct dummy *d,
+					   unsigned long jiffies)
+{
+	return time_after(jiffies, d->jiffies_expire);
+}
+
+/*
+ * alloc_work_func: allocates new dummy structures, allocates additional
+ *                  memory, aptly named "leak", but doesn't keep
+ *                  permanent record of it.
+ */
+
+static void alloc_work_func(struct work_struct *work);
+static DECLARE_DELAYED_WORK(alloc_dwork, alloc_work_func);
+
+static void alloc_work_func(struct work_struct *work)
+{
+	struct dummy *d;
+
+	d = dummy_alloc();
+	if (!d)
+		return;
+
+	mutex_lock(&dummy_list_mutex);
+	list_add(&d->list, &dummy_list);
+	mutex_unlock(&dummy_list_mutex);
+
+	schedule_delayed_work(&alloc_dwork, secs_to_jiffies(ALLOC_PERIOD));
+}
+
+/*
+ * cleanup_work_func: frees dummy structures.  Without knownledge of
+ *                    "leak", it leaks the additional memory that
+ *                    alloc_work_func created.
+ */
+
+static void cleanup_work_func(struct work_struct *work);
+static DECLARE_DELAYED_WORK(cleanup_dwork, cleanup_work_func);
+
+static void cleanup_work_func(struct work_struct *work)
+{
+	struct dummy *d, *tmp;
+	unsigned long j;
+
+	j = jiffies;
+	pr_info("%s: jiffies = %lx\n", __func__, j);
+
+	mutex_lock(&dummy_list_mutex);
+	list_for_each_entry_safe(d, tmp, &dummy_list, list) {
+
+		/* Kick out and free any expired dummies */
+		if (dummy_check(d, j)) {
+			list_del(&d->list);
+			dummy_free(d);
+		}
+	}
+	mutex_unlock(&dummy_list_mutex);
+
+	schedule_delayed_work(&cleanup_dwork, secs_to_jiffies(CLEANUP_PERIOD));
+}
+
+static int livepatch_shadow_mod_init(void)
+{
+	schedule_delayed_work(&alloc_dwork, secs_to_jiffies(ALLOC_PERIOD));
+	schedule_delayed_work(&cleanup_dwork, secs_to_jiffies(CLEANUP_PERIOD));
+
+	return 0;
+}
+
+static void livepatch_shadow_mod_exit(void)
+{
+	struct dummy *d, *tmp;
+
+	/* Wait for any dummies at work */
+	cancel_delayed_work_sync(&alloc_dwork);
+	cancel_delayed_work_sync(&cleanup_dwork);
+
+	/* Cleanup residual dummies */
+	list_for_each_entry_safe(d, tmp, &dummy_list, list) {
+		list_del(&d->list);
+		dummy_free(d);
+	}
+}
+
+module_init(livepatch_shadow_mod_init);
+module_exit(livepatch_shadow_mod_exit);