linux-kernel - Re: [PATCH v2] lzo: fix ip overrun during compress.

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Date:   Mon, 3 Dec 2018 02:46:38 +0000
From:   Yueyi Li <liyueyi@...e.com>
To:     Dave Rodgman <dave.rodgman@....com>,
        "dsterba@...e.cz" <dsterba@...e.cz>, "w@....eu" <w@....eu>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "donb@...uritymouse.com" <donb@...uritymouse.com>,
        "markus@...rhumer.com" <markus@...rhumer.com>, nd <nd@....com>
Subject: Re: [PATCH v2] lzo: fix ip overrun during compress.



On 2018/11/30 20:20, Dave Rodgman wrote:
>> On 2018/11/30 0:49, Dave Rodgman wrote:
>>> On 28/11/2018 1:52 pm, David Sterba wrote:
>>>
>>>> The fix is adding a few branches to code that's supposed to be as fast
>>>> as possible. The branches would be evaluated all the time while
>>>> protecting against one signle bad page address. This does not look like
>>>> a good performance tradeoff.
>>> As an alternative, for all but the first case, instead of:
>>>
>>> if (unlikely(OVERFLOW_ADD_CHECK(ip, m_len) || (ip + m_len >= ip_end)))
>>>
>>> I'd suggest we do:
>>>
>>> if (unlikely((ip_end - ip) <= m_len))
>>>
>>> which will be about as efficient as what's currently there, but doesn't
>>> have issues with overflow.
>> Ooh, yes, pretty good solution to this, thanks.
> Np :-)
>
> Actually, looking more closely at the first case, something like this
> works quite well:
>
> size_t inc = 1 + ((ip - ii) >> 5);
> if (unlikely((ip_end - ip) <= inc))
> 	break;
> ip += inc;
>
> On arm64, this generates only a single branch instruction, so it's only
> two extra arithmetic operations more than the original code (using the
> macro results in an additional compare & branch).
>
How about just instead of:

   if (unlikely(ip >= ip_end))
         break;

to:

   if(unlikely((ip - ip_end) < ~in_len))
         break;

This just generates only one more arithmetic operation than original 
code, not easy to grasp but more efficient.


Thanks,
Yueyi