linux-kernel - Re: [PATCH v2] xen: detect uninitialized xenbus in xenbus

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Date:   Mon, 22 Nov 2021 14:17:28 -0800 (PST)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Stefano Stabellini <sstabellini@...nel.org>
cc:     Jan Beulich <jbeulich@...e.com>, boris.ostrovsky@...cle.com,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Stefano Stabellini <stefano.stabellini@...inx.com>,
        stable@...r.kernel.org, jgross@...e.com
Subject: Re: [PATCH v2] xen: detect uninitialized xenbus in xenbus_init

On Mon, 22 Nov 2021, Stefano Stabellini wrote:
> On Mon, 22 Nov 2021, Jan Beulich wrote:
> > On 19.11.2021 21:29, Stefano Stabellini wrote:
> > > --- a/drivers/xen/xenbus/xenbus_probe.c
> > > +++ b/drivers/xen/xenbus/xenbus_probe.c
> > > @@ -951,6 +951,20 @@ static int __init xenbus_init(void)
> > >  		err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
> > >  		if (err)
> > >  			goto out_error;
> > > +		/* Uninitialized. */
> > > +		if (v == 0 || v == ULLONG_MAX) {
> > 
> > Didn't you have a comment in v1 here regarding the check against 0? Or was that
> > just like now only in the description? IOW I think there ought to be a code
> > comment justifying the theoretically wrong check ...
> 
> Yeah, I added all the info in the commit message and shortened the
> in-code comment this time. I am also happy to keep the details in the
> in-code comment, e.g.:
> 
> /*
>  * If the xenstore page hasn't been allocated properly, reading the
>  * value of the related hvm_param (HVM_PARAM_STORE_PFN) won't actually
>  * return error. Instead, it will succeed and return zero. Instead of
>  * attempting to xen_remap a bad guest physical address, detect this
>  * condition and return early.
>  *
>  * Note that although a guest physical address of zero for
>  * HVM_PARAM_STORE_PFN is theoretically possible, it is not a good
>  * choice and zero has never been validly used in that capacity.
>  *
>  * Also recognize the invalid value of INVALID_PFN which is ULLONG_MAX.
>  */

I sent a new version of the patch with the check below and slightly more
concise version of this comment.
 

> > Also, while I realize there are various other similar assumptions elsewhere, I
> > would generally recommend to avoid such: There's no guarantee that now and
> > forever unsigned long long and uint64_t are the same thing. And it's easy in
> > cases like this one:
> > 
> > 		if (!v || !(v + 1)) {
> 
> I am happy to use this.