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

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Message-ID: <3bde9678-ac05-62f1-bfc3-434832adda11@arm.com>
Date:   Fri, 30 Nov 2018 12:20:44 +0000
From:   Dave Rodgman <dave.rodgman@....com>
To:     Yueyi Li <liyueyi@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "dsterba@...e.cz" <dsterba@...e.cz>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "w@....eu" <w@....eu>,
        "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 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).

I used this to explore the compiler output:

https://godbolt.org/z/ng2qGZ

Dave