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x/mobile: go rand crash #21112

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axet opened this issue Jul 21, 2017 · 5 comments

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@axet
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commented Jul 21, 2017

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.8.3 darwin/amd64

What operating system and processor architecture are you using (go env)?

android

What did you do?

Seems like rand.Perm() has a bug, I'm getting a lot of crashes in my torrent app:

panic: runtime error: index out of range

goroutine 5088 [running]:
math/rand.(*Rand).Perm(0x442005a2a0, 0x3c4, 0x4420c22d20, 0x3c4, 0x4420256398)
    /opt/local/lib/go/src/math/rand/rand.go:198 +0xc0
math/rand.Perm(0x3c4, 0x3c4, 0x0, 0x44206c9a60)
    /opt/local/lib/go/src/math/rand/rand.go:288 +0x30
github.com/anacrolix/torrent.(*Torrent).getConnPieceInclination(0x442077ec80, 0x79f47cb650, 0x4420080618, 0x343)

Seems like in rare conditions, function crashes while calulating 'j := r.Intn(i + 1)' which should be 'j := r.Intn(i)'

@axet axet referenced this issue Jul 21, 2017

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go rand bug #173

@bradfitz

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commented Jul 21, 2017

The code looks fine.

Does it crash on only one device of yours, or multiple Android devices? Which ones?

@bradfitz bradfitz changed the title go rand bug x/mobile: go rand crash Jul 21, 2017

@gopherbot gopherbot added this to the Unreleased milestone Jul 21, 2017

@axet

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commented Jul 21, 2017

@bradfitz crashes in Go native code not captured by the system. So, it does only my device.

My project is:

https://gitlab.com/axet/android-torrent-client

Device Nexus 9, arm

It is hard to debug, but I can add some debug prints and say which numbers are causing the crash. But seems like 'j' out of index, only possible when 'r.Intn(i) <= i' and it should not crash when it is 0 or -1.

I do not know what to do, but just ignore Perm() calls at all in my app...

@bradfitz

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commented Jul 21, 2017

Nexus 9 uses https://en.wikipedia.org/wiki/Project_Denver and has known problems with certain instruction sequences. Go's compiler generates instruction sequences different from what Nexus 9 was tested with, so we apparently confuse the Project Denver JIT.

See similar bug #19809 and all the bugs referenced from it in other non-Go projects.

Nexus 9 just has a buggy CPU. It only works for Android because that's what was tested on it.

Please try Go 1.9. Go 1.8 didn't include the 9d5987d Nexus 9 workaround. So you might be better off with that. Or maybe not.

In any case, there's little we can do here.

@bradfitz bradfitz closed this Jul 21, 2017

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commented Jul 21, 2017

Seems works!

@cherrymui

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commented Jul 21, 2017

@dr2chase has wanted to has the workaround backported to Go 1.8. Here is another case.

@golang golang locked and limited conversation to collaborators Jul 21, 2018

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