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, 36 insertions, 0 deletions

diff --git a/drivers/firmware/google/memconsole.h b/drivers/firmware/google/memconsole.h
new file mode 100644
index 000000000..aaff2b72b
--- /dev/null
+++ b/drivers/firmware/google/memconsole.h
@@ -0,0 +1,36 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * memconsole.h
+ *
+ * Internal headers of the memory based BIOS console.
+ *
+ * Copyright 2017 Google Inc.
+ */
+
+#ifndef __FIRMWARE_GOOGLE_MEMCONSOLE_H
+#define __FIRMWARE_GOOGLE_MEMCONSOLE_H
+
+#include <linux/types.h>
+
+/*
+ * memconsole_setup
+ *
+ * Initialize the memory console, passing the function to handle read accesses.
+ */
+void memconsole_setup(ssize_t (*read_func)(char *, loff_t, size_t));
+
+/*
+ * memconsole_sysfs_init
+ *
+ * Update memory console length and create binary file
+ * for firmware object.
+ */
+int memconsole_sysfs_init(void);
+
+/* memconsole_exit
+ *
+ * Unmap the console buffer.
+ */
+void memconsole_exit(void);
+
+#endif /* __FIRMWARE_GOOGLE_MEMCONSOLE_H */