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bytes/hash: TestSmhasherSmallKeys flake on android-arm-corellium builder #34834

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bcmills opened this issue Oct 11, 2019 · 2 comments
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@bcmills bcmills commented Oct 11, 2019

From the android-arm-corellium builder (https://build.golang.org/log/6d0506edef76d21049f50f3d2a77cf686d063476):

--- FAIL: TestSmhasherSmallKeys (0.15s)
    smhasher_test.go:95: unexpected number of collisions: got=256 mean=0.000000 stddev=0.000011
FAIL
FAIL	bytes/hash	0.639s

CC @randall77

@bcmills bcmills added this to the Go1.14 milestone Oct 11, 2019
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@randall77 randall77 commented Oct 13, 2019

I think what is going on here is that we're getting invalid random data on startup (_AT_RANDOM content not actually random, or perhaps zero length, or no _AT_RANDOM and /dev/urandom isn't random).
I can reproduce the behavior on 386 by setting GODEBUG=cpu.all=off (to get the same hash algorithm that arm uses) and commenting out the call to getRandomData in runtime/alg.go:alginit.

We use different parts of the startup random data to make the hash length-dependent. If the startup random data is not random (all zero, probably) then the length-dependence of the hash goes away, and that's the behavior we're seeing (the 256 length 1 hashes are colliding with some of the 256^2 length 2 hashes).

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@randall77 randall77 commented Oct 14, 2019

I guess this could happen by chance with probability ~2^-16 (top ~16 bits of two 32-bit random numbers differ by a one-bit shift).

It hasn't recurred. I'm inclined to leave this as is and see if it happens again.

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