linux-kernel - Re: autofs: make the autofsv5 packet file descriptor use a packetized pipe

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Message-ID: <4F9E30BF.4030704@msgid.tls.msk.ru>
Date:	Mon, 30 Apr 2012 10:27:11 +0400
From:	Michael Tokarev <mjt@....msk.ru>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ian Kent <raven@...maw.net>, Thomas Meyer <thomas@...3r.de>,
	autofs@...r.kernel.org
Subject: Re: autofs: make the autofsv5 packet file descriptor use a packetized
 pipe

> On 04/29/2012 01:54 PM, Linux Kernel Mailing List wrote:
>>     However, a prettier solution exists now thanks to the packetized pipe
>>     mode.  By marking the communication pipe as being packetized (by simply
>>     setting the O_DIRECT flag), we can always just write the bigger packet
>>     size, and if user-space does a smaller read, it will just get that
>>     partial end result and the extra alignment padding will simply be thrown
>>     away.

> +static inline int autofs_prepare_pipe(struct file *pipe)
> +{
> +	if (!pipe->f_op || !pipe->f_op->write)
> +		return -EINVAL;
> +	if (!S_ISFIFO(pipe->f_dentry->d_inode->i_mode))
> +		return -EINVAL;
> +	/* We want a packet pipe */
> +	pipe->f_flags |= O_DIRECT;
> +	return 0;
> +}
> +

@@ -376,7 +376,7 @@ static int autofs_dev_ioctl_setpipefd(st
 			err = -EBADF;
 			goto out;
 		}
-		if (!pipe->f_op || !pipe->f_op->write) {
+		if (autofs_prepare_pipe(pipe) < 0) {
 			err = -EPIPE;
 			fput(pipe);
 			goto out;

I've one more concern.  I'm not sure but I think there's some
risk still.  This packetizing gets applied to all VERSIONS of
the autofs PROTOCOL.  Which means it will be applied to the
lowest supported version (3) TOO, but did that version read
whole packets too?

Maybe something like the attached should be applied?

Thanks,

/mjt

View attachment "autofs-enable-workaround-for-v5-only.diff" of type "text/x-patch" (2544 bytes)