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

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Date:   Mon, 22 Nov 2021 08:57:32 +0100
From:   Jan Beulich <jbeulich@...e.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     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 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 ...

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)) {

Jan