Re: [ANNOUNCE] Git v2.19.0-rc0

From: Derrick Stolee <stolee@gmail.com>
Date: Aug 23, 2018, 10:26 UTC
Message-ID: <f854aba0-6d28-7f2b-aad2-858983c4af36@gmail.com>
In-Reply-To: <20180823050418.GB318@sigill.intra.peff.net>

On 8/23/2018 1:04 AM, Jeff King wrote:

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> On Thu, Aug 23, 2018 at 03:47:07AM +0000, brian m. carlson wrote:
>
>> I expect that's going to be the case as well.  I have patches that
>> wire up actual SHA-256 support in my hash-impl branch.
>>
>> However, having said that, I'm happy to defer to whatever everyone else
>> thinks is best for 2.19.  The assert solution would be fine with me in
>> this situation, and if we need to pull it out in the future, that's okay
>> with me.
>>
>> I don't really have a strong opinion on this either way, so if someone
>> else does, please say so.  I have somewhat more limited availability
>> over the next couple days, as I'm travelling on business, but I'm happy
>> to review a patch (and it seems like Peff has one minus the actual
>> commit message).
> I just posted the patch elsewhere in the thread.

Thank you for that!

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> I think you can safely
> ignore the rest of it if you are otherwise occupied. Even if v2.19 ships
> without some mitigation, I don't know that it's all that big a deal,
> given the numbers I generated (which for some reason are less dramatic
> than Stolee's).

My numbers may be more dramatic because my Linux environment is a virtual machine.

I was thinking that having a mitigation for 2.19 is best, and then we can focus as part of the 2.20 cycle how we can properly avoid this cost, especially when 32 is a valid option.

Around the time that my proposed approaches were getting vetoed for alignment issues, I figured I was out of my depth here. I reached out to Daniel Lemire (of EWAH bitmap fame) on Twitter [1]. His blog is full of posts of word-based approaches to different problems, so I thought he might know something off the top of his head that would be applicable. His conclusion (after looking only a short time) was to take a 'hasheq' approach [2] like Peff suggested [3]. Since that requires auditing all callers of hashcmp to see if hasheq is appropriate, it is not a good solution for 2.19 but (in my opinion) should be evaluated as part of the 2.20 cycle.

Of course, if someone with knowledge of word-alignment issues across the platforms we support knows how to enforce an alignment for object_id, then something word-based like [4] could be reconsidered.

Thanks, everyone! -Stolee

[1] https://twitter.com/stolee/status/1032312965754748930

[2] https://lemire.me/blog/2018/08/22/avoid-lexicographical-comparisons-when-testing-for-string-equality/

[3] https://public-inbox.org/git/20180822030344.GA14684@sigill.intra.peff.net/

[4] https://public-inbox.org/git/7ea416cf-b043-1274-e161-85a8780b8e1c@gmail.com/