linux-kernel - Re: [PATCH] VFS: Sanity check mount flags passed to change_mnt

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Message-ID: <20100828131516.GA5053@hash.localnet>
Date:	Sat, 28 Aug 2010 09:15:16 -0400
From:	Bob Copeland <me@...copeland.com>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Valerie Aurora <vaurora@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Karel Zak <kzak@...hat.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] VFS: Sanity check mount flags passed to
	change_mnt_propagation()

On Sat, Aug 28, 2010 at 04:57:07AM -0600, Matthew Wilcox wrote:
> > Didn't read surrounding code, but is that supposed to be '||'?
> > 
> > Otherwise the case where only a single non-propagation flag is
> > set no longer returns 0...
> 
> Val's original code returned 0 as failure.  So a single non-propagation
> flag set shouldn't return 0.

Not to belabor the point -- but her original code did return zero in
that case (2nd assertion fails):

#include <linux/fs.h>
#include <stdio.h>
#include <assert.h>

#define hweight32 dumb_hweight32

int dumb_hweight32(int a)
{
	int i;
	int weight = 0;

	for (i=0; i < 32; i++)
		weight += !!(a & (1 << i));

	return weight;
}

static int flags_to_propagation_type(int flags) {
	int type = flags & ~MS_REC;

	/* Fail if any non-propagation flags are set */
 	if (type & ~(MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
		return 0;
	/* Only one propagation flag should be set */
	if (((type & (MS_SHARED)) && (type & ~MS_SHARED)) ||
	   ((type & (MS_PRIVATE)) && (type & ~MS_PRIVATE)) ||
	   ((type & (MS_SLAVE)) && (type & ~MS_SLAVE)) ||
	   ((type & (MS_UNBINDABLE)) && (type & ~MS_UNBINDABLE)))
	       return 0;
	return type;
}

static int flags_to_propagation_type_2(int flags) {
 	int type = flags & ~MS_REC;

	/* Only one propagation flag should be set, and no others */
	if (hweight32(type) != 1 &&
	    (type & ~(MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE)))
		return 0;

	return type;
}

int main(int argc, char *argv[])
{
	int mytype = MS_RDONLY;

	assert(flags_to_propagation_type(mytype) == 0);
	assert(flags_to_propagation_type_2(mytype) == 0);
}

-- 
Bob Copeland %% www.bobcopeland.com

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