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Subject: Re: [PATCH] x86/xen: Remove use of VLAs
To:     Juergen Gross <jgross@suse.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
        kernel-hardening@lists.openwall.com
References: <20180413221146.28476-1-labbott@redhat.com>
 <7cf66c00-74c2-62bb-81f5-4332313a5a6e@oracle.com>
 <e63f261b-9ef3-5750-20df-555291a8a0d8@suse.com>
From:   Laura Abbott <labbott@redhat.com>
Message-ID: <9fe26cf5-01dc-1fb0-25fd-29bf8615047f@redhat.com>
Date:   Tue, 17 Apr 2018 16:33:57 -0700
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On 04/17/2018 12:16 AM, Juergen Gross wrote:
> On 16/04/18 15:27, Boris Ostrovsky wrote:
>> On 04/13/2018 06:11 PM, Laura Abbott wrote:
>>> There's an ongoing effort to remove VLAs[1] from the kernel to eventually
>>> turn on -Wvla. The few VLAs in use have an upper bound based on a size
>>> of 64K. This doesn't produce an excessively large stack so just switch
>>> the upper bound.
>>>
>>> [1] https://lkml.org/lkml/2018/3/7/621
>>>
>>> Signed-off-by: Laura Abbott <labbott@redhat.com>
>>> ---
>>>   arch/x86/xen/enlighten_pv.c | 6 ++----
>>>   1 file changed, 2 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
>>> index c36d23aa6c35..d96a5a535cbb 100644
>>> --- a/arch/x86/xen/enlighten_pv.c
>>> +++ b/arch/x86/xen/enlighten_pv.c
>>> @@ -421,8 +421,7 @@ static void xen_load_gdt(const struct desc_ptr *dtr)
>>>   {
>>>   	unsigned long va = dtr->address;
>>>   	unsigned int size = dtr->size + 1;
>>> -	unsigned pages = DIV_ROUND_UP(size, PAGE_SIZE);
>>
>>
>>
>> Isn't dtr->size always either GDT_SIZE or 0?
> 
> GDT_SIZE - 1 :-)
> 
>>> -	unsigned long frames[pages];
>>> +	unsigned long frames[DIV_ROUND_UP(SZ_64K, PAGE_SIZE)];
> 
> So we could just go with one frame and modify the BUG_ON() further below
> accordingly.
> 

Do you want to just remove the loop as well since we're never going
to do more than one frame? We end up with net code deletion.

Thanks,
Laura

> 
> Juergen
>