linux-kernel - Re: [RFC PATCH] staging, android, ashmem: invalidate pin/unpin ioctl for private map

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Message-ID: <20121203073109.GD6568@mwanda>
Date:	Mon, 3 Dec 2012 10:31:09 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	JoonSoo Kim <js1304@...il.com>
Cc:	devel@...verdev.osuosl.org, Robert Love <rlove@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Colin Cross <ccross@...roid.com>, linux-kernel@...r.kernel.org,
	Dima Zavin <dima@...roid.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	John Stultz <john.stultz@...aro.org>,
	Brian Swetland <swetland@...gle.com>,
	Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [RFC PATCH] staging, android, ashmem: invalidate pin/unpin ioctl
 for private map

On Mon, Dec 03, 2012 at 09:09:59AM +0900, JoonSoo Kim wrote:
> Hello, Dan.
> 
> 2012/12/2 Dan Carpenter <dan.carpenter@...cle.com>:
> > On Sat, Dec 01, 2012 at 02:45:57AM +0900, Joonsoo Kim wrote:
> >> @@ -614,21 +616,35 @@ static int ashmem_pin_unpin(struct ashmem_area *asma, unsigned long cmd,
> >>       pgstart = pin.offset / PAGE_SIZE;
> >>       pgend = pgstart + (pin.len / PAGE_SIZE) - 1;
> >>
> >> -     mutex_lock(&ashmem_mutex);
> >> +     if (asma->shared_mapping) {
> >> +             mutex_lock(&ashmem_mutex);
> >
> > Wouldn't we need to hold the mutex while we check the
> > ->shared_mapping?
> 
> I doesn't fully understand ashmem's lock semantic.
> Code for retrieving some value of asma instance doesn't hold the mutex, now.
> For example, in ashmem_ioctl(), asma->size, asma->prot_mask.
> And in ashmem_pin_unpin(), there is asma->file, asma->size which is
> retrieved without the mutex.
> According to this semantic, the mutex doesn't need for checking
> asma->shared_mapping.

The ashmem_ioctl() is clearly racy.  :P  asma->size can be modified
and read at the same time.  It's not an example to follow.

regards,
dan carpenter

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