[PATCH] D18055: ELF: Implement --build-id.

[Sean Silva via llvm-commits]

On Thu, Mar 10, 2016 at 7:56 PM, Rui Ueyama <ruiu at google.com> wrote:

> AES is for encryption, so it's not usable for secure hashing, no?
>
> On Thu, Mar 10, 2016 at 6:33 PM, Sean Silva <chisophugis at gmail.com> wrote:
>
>>
>>
>> On Thu, Mar 10, 2016 at 2:51 PM, Rui Ueyama <ruiu at google.com> wrote:
>>
>>> For LLD, it hashed 75,421,320 bytes. For Scylla, it hashed 207,703,010
>>> bytes. So the throughput of MD5 is 444 MB/s and 480 MB/s, respectively.
>>>
>>> BLAKE2 claims that it is about 1.6x faster than MD5, but it is probably
>>> the lower bound for a secure hash function.
>>>
>>
>> AESNI claims to be able to do 1.3 cycles per byte on an old Nehalem. That
>> is multiple gigabytes per second.
>>
>> https://software.intel.com/sites/default/files/m/d/4/1/d/8/10TB24_Breakthrough_AES_Performance_with_Intel_AES_New_Instructions.final.secure.pdf
>>
>> Have you tried using ADT/Hashing.h? That is probably the simplest option.
>>
>
> No, I haven't. But if CRC32 is not okay, then it is not okay as well.
>

I disagree. CRC32 has dramatically weaker characteristics than
ADT/Hashing.h. CRC polynomials are constructed to detect specific classes
of errors that are characteristic to physical communication channels. e.g.
"anything which toggles a single bit is guaranteed to change the CRC
value". They are not particularly good as hash values.
ADT/Hashing.h is designed against much more stringent tests (I believe it
is tested against SMHasher).

The fedora page does not mention any use case that requires
cryptographically strong hashing.

-- Sean Silva


>
>
>> -- Sean Silva
>>
>>
>>>
>>> On Thu, Mar 10, 2016 at 2:16 PM, Sean Silva <chisophugis at gmail.com>
>>> wrote:
>>>
>>>> silvas added a subscriber: silvas.
>>>> silvas added a comment.
>>>>
>>>> One thing that is missing from the data you provided in the OP is the
>>>> actual amount of data hashed, and in what pattern (lots of small, or a
>>>> couple large?). How fast does the hashing have to be to be "acceptable"?
>>>> E.g. ADT/Hashing.h claims 6.5GB/s hashing for large keys on Nehalem (which
>>>> is pretty old at this point, modern CPU's likely to better). That is
>>>> 6.5MB/ms. If we need to hash 100MB then the overhead should be about 15ms.
>>>>
>>>> For LLD, you quoted a number 713.78ms link time. So for 1% overhead we
>>>> need to spend <7ms. This is enough for 45MB hashed with ADT/Hashing.h
>>>> (which is similar to LLD text+data size), assuming that we can get max
>>>> memory bandwidth (like Rafael said, we can do this while we would be
>>>> copying data otherwise, so I don't see a reason we can't hit the full
>>>> performance of ADT/Hashing.h).
>>>>
>>>>
>>>> http://reviews.llvm.org/D18055
>>>>
>>>>
>>>>
>>>>
>>>
>>
>
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