Re: [PATCH] fsi: Aspeed: Fix a potential double free

[Christophe JAILLET]

Le 06/01/2022 à 18:25, Guenter Roeck a écrit :
> On 1/6/22 12:14 AM, Dan Carpenter wrote:
> > On Mon, Dec 27, 2021 at 07:29:07AM +0100, Greg KH wrote:
> > > On Sun, Dec 26, 2021 at 05:56:02PM +0100, Christophe JAILLET wrote:
> > > > 'aspeed' is a devm_alloc'ed, so there is no need to free it 
> > > > explicitly or
> > > > there will be a double free().
> > >
> > > A struct device can never be devm_alloced for obvious reasons.  Perhaps
> > > that is the real problem here?
> >
> > I don't understand how "aspeed" is a struct device.
>
> -static void aspeed_master_release(struct device *dev)
> -{
> -    struct fsi_master_aspeed *aspeed =
> -        to_fsi_master_aspeed(dev_to_fsi_master(dev));
> -
> -    kfree(aspeed);
> -}
>
> So "dev" is embedded in struct fsi_master, and struct fsi_master is 
> embedded
> in struct fsi_master_aspeed. Since "struct device" is embedded, the data
> structure embedding it must be released with the release function, as is 
> done
> here. The problem is indeed that the data structure is allocated with
> devm_kzalloc(), which as Greg points out must not be devm_ allocated
> (because its lifetime does not match the lifetime of devm_ allocated
> memory).

Thanks a lot for the detailed explanation.
Crystal clear for me now.

Do you want me to send a patch to remove the devm_ or will you?

CJ

> > I've been working on understanding device managed memory recently for
> > Smatch.  It's really complicated.  There are a bunch of rules/heuristics
> > that I'm slowly creating to generate new warnings but I'm a long way
> > from understanding it well myself.
>
> A data structure embedding struct device must not be devm_ allocated,
> and it must be released with the release callback. Maybe there is
> a means to flag that somehow ?
>
> Guenter