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Date:   Wed, 4 Sep 2019 08:36:33 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     linux-kernel@vger.kernel.org
Cc:     Miklos Szeredi <miklos@szeredi.hu>, linux-fsdevel@vger.kernel.org,
        stefanha@redhat.com, dgilbert@redhat.com
Subject: [PATCH] fuse: reserve byteswapped init opcodes
Message-ID: <20190904123607.10048-1-mst@redhat.com>
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Sender: linux-kernel-owner@vger.kernel.org
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virtio fs tunnels fuse over a virtio channel.  One issue is two sides
might be speaking different endian-ness. To detects this,
host side looks at the opcode value in the FUSE_INIT command.
Works fine at the moment but might fail if a future version
of fuse will use such an opcode for initialization.
Let's reserve this opcode so we remember and don't do this.

Same for CUSE_INIT.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/uapi/linux/fuse.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index 2971d29a42e4..f042e63f4aa0 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -425,6 +425,10 @@ enum fuse_opcode {
 
 	/* CUSE specific operations */
 	CUSE_INIT		= 4096,
+
+	/* Reserved opcodes: helpful to detect structure endian-ness */
+	FUSE_INIT_BSWAP_RESERVED	= 26 << 24,
+	CUSE_INIT_BSWAP_RESERVED	= 16 << 16,
 };
 
 enum fuse_notify_code {
-- 
MST