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, 145 insertions, 0 deletions

diff --git a/samples/kobject/kobject-example.c b/samples/kobject/kobject-example.c
new file mode 100644
index 000000000..36f3fbc47
--- /dev/null
+++ b/samples/kobject/kobject-example.c
@@ -0,0 +1,145 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Sample kobject implementation
+ *
+ * Copyright (C) 2004-2007 Greg Kroah-Hartman <greg@kroah.com>
+ * Copyright (C) 2007 Novell Inc.
+ */
+#include <linux/kobject.h>
+#include <linux/string.h>
+#include <linux/sysfs.h>
+#include <linux/module.h>
+#include <linux/init.h>
+
+/*
+ * This module shows how to create a simple subdirectory in sysfs called
+ * /sys/kernel/kobject_example  In that directory, 3 files are created:
+ * "foo", "baz", and "bar".  If an integer is written to these files, it can be
+ * later read out of it.
+ */
+
+static int foo;
+static int baz;
+static int bar;
+
+/*
+ * The "foo" file where a static variable is read from and written to.
+ */
+static ssize_t foo_show(struct kobject *kobj, const struct kobj_attribute *attr,
+			char *buf)
+{
+	return sysfs_emit(buf, "%d\n", foo);
+}
+
+static ssize_t foo_store(struct kobject *kobj, const struct kobj_attribute *attr,
+			 const char *buf, size_t count)
+{
+	int ret;
+
+	ret = kstrtoint(buf, 10, &foo);
+	if (ret < 0)
+		return ret;
+
+	return count;
+}
+
+/* Sysfs attributes cannot be world-writable. */
+static const struct kobj_attribute foo_attribute =
+	__KOBJ_ATTR(foo, 0664, foo_show, foo_store);
+
+/*
+ * More complex function where we determine which variable is being accessed by
+ * looking at the attribute for the "baz" and "bar" files.
+ */
+static ssize_t b_show(struct kobject *kobj, const struct kobj_attribute *attr,
+		      char *buf)
+{
+	int var;
+
+	if (strcmp(attr->attr.name, "baz") == 0)
+		var = baz;
+	else
+		var = bar;
+	return sysfs_emit(buf, "%d\n", var);
+}
+
+static ssize_t b_store(struct kobject *kobj, const struct kobj_attribute *attr,
+		       const char *buf, size_t count)
+{
+	int var, ret;
+
+	ret = kstrtoint(buf, 10, &var);
+	if (ret < 0)
+		return ret;
+
+	if (strcmp(attr->attr.name, "baz") == 0)
+		baz = var;
+	else
+		bar = var;
+	return count;
+}
+
+static const struct kobj_attribute baz_attribute =
+	__KOBJ_ATTR(baz, 0664, b_show, b_store);
+static const struct kobj_attribute bar_attribute =
+	__KOBJ_ATTR(bar, 0664, b_show, b_store);
+
+
+/*
+ * Create a group of attributes so that we can create and destroy them all
+ * at once.
+ */
+static const struct attribute *const attrs[] = {
+	&foo_attribute.attr,
+	&baz_attribute.attr,
+	&bar_attribute.attr,
+	NULL,	/* need to NULL terminate the list of attributes */
+};
+
+/*
+ * An unnamed attribute group will put all of the attributes directly in
+ * the kobject directory.  If we specify a name, a subdirectory will be
+ * created for the attributes with the directory being the name of the
+ * attribute group.
+ */
+static const struct attribute_group attr_group = {
+	.attrs_const = attrs,
+};
+
+static struct kobject *example_kobj;
+
+static int __init example_init(void)
+{
+	int retval;
+
+	/*
+	 * Create a simple kobject with the name of "kobject_example",
+	 * located under /sys/kernel/
+	 *
+	 * As this is a simple directory, no uevent will be sent to
+	 * userspace.  That is why this function should not be used for
+	 * any type of dynamic kobjects, where the name and number are
+	 * not known ahead of time.
+	 */
+	example_kobj = kobject_create_and_add("kobject_example", kernel_kobj);
+	if (!example_kobj)
+		return -ENOMEM;
+
+	/* Create the files associated with this kobject */
+	retval = sysfs_create_group(example_kobj, &attr_group);
+	if (retval)
+		kobject_put(example_kobj);
+
+	return retval;
+}
+
+static void __exit example_exit(void)
+{
+	kobject_put(example_kobj);
+}
+
+module_init(example_init);
+module_exit(example_exit);
+MODULE_DESCRIPTION("Sample kobject implementation");
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("Greg Kroah-Hartman <greg@kroah.com>");