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Date:   Wed, 4 Sep 2019 15:22:47 +0100
From:   "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     linux-kernel@vger.kernel.org, Miklos Szeredi <miklos@szeredi.hu>,
        linux-fsdevel@vger.kernel.org, stefanha@redhat.com
Subject: Re: [PATCH] fuse: reserve byteswapped init opcodes
Message-ID: <20190904142247.GM2828@work-vm>
References: <20190904123607.10048-1-mst@redhat.com>
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* Michael S. Tsirkin (mst@redhat.com) wrote:
> 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.

I think in theory that works even for normal fuse.

> 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,

FUSE_INIT << 24 probably works?

> +	CUSE_INIT_BSWAP_RESERVED	= 16 << 16,

Dave

>  };
>  
>  enum fuse_notify_code {
> -- 
> MST
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK