linux-kernel - Re: [PATCH v3 4/4] parse options in the vfs level

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Date:	Sun, 14 Aug 2011 17:03:52 -0700
From:	Glauber Costa <glommer@...allels.com>
To:	Vasiliy Kulikov <segoon@...nwall.com>
CC:	<linux-kernel@...r.kernel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Nick Piggin <npiggin@...nel.dk>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	David Chinner <david@...morbit.com>,
	<containers@...ts.linux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	James Bottomley <JBottomley@...allels.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	<linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v3 4/4] parse options in the vfs level

On 08/14/2011 08:39 AM, Vasiliy Kulikov wrote:
> Hi Glauber,
>
> On Sun, Aug 14, 2011 at 19:13 +0400, Glauber Costa wrote:
>> +/**
>> + * Generic option parsing for the VFS.
>> + *
>> + * Since most of the filesystems already do their own option parsing, and with
>> + * very few code shared between them, this function strips out any options that
>> + * we succeed in parsing ourselves. Passing them forward would just give the
>> + * underlying fs an option it does not expect, leading it to fail.
>> + *
>> + * We don't yet have a pointer to the super block as well, since this is
>> + * pre-mount. We accumulate in struct vfs_options whatever data we collected,
>> + * and act on it later.
>> + */
>> +static int vfs_parse_options(char *options, struct vfs_options *ops)
>> +{
>> +	substring_t args[MAX_OPT_ARGS];
>> +	int option;
>> +	char *p;
>> +	char *opt;
>> +	char *start = NULL;
>> +	int ret;
>> +	
>> +	if (!options)
>> +		return 0;
>> +
>> +	opt = kstrdup(options, GFP_KERNEL);
>> +	if (!opt)
>> +		return 1;
>> +	
>> +	ret = 1;
>> +
>> +	start = opt;
>> +	while ((p = strsep(&opt, ",")) != NULL) {
>> +		int token;
>> +		if (!*p)
>> +			continue;
>> +
>> +		/*
>> +		 * Initialize args struct so we know whether arg was
>> +		 * found; some options take optional arguments.
>> +		 */
>> +		args[0].to = args[0].from = 0;
>> +		token = match_token(p, tokens, args);
>> +		switch (token) {
>> +		case 1:
>> +			if (!args[0].from)
>> +				break;
>> +
>> +			if (match_int(&args[0],&option))
>> +				break;
>
> What if there are 2 passed options and the second fails?
>
>     mount -o vfs_dcache_size=XXX,vfs_dcache_size=CRAP<dev>  <mntpoint>
>
> In this case you leave the second option and pass it to the fs option
> parser (as you already set ret=0), which is wrong.  I think you should
> explicitly return 1 where you know the option is related to VFS, but you
> failed to parse it.  It would look even simplier than current code.

Good point, thank you. I agree.

> (Yes, this is a rare situation, but I can imagine some program that
> automatically adds mount options to the existing list and passes it to
> mount.)
>
Absolutely.
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