syscall: unimplemented EpollWait fails to return an error on android/arm64

[jakubgs]

What version of Go are you using?

$ go version
go version go1.13.4 linux/amd64

Does this issue reproduce with the latest release?

Yes. Also tested it with go1.13.1.

What operating system and processor architecture are you using?

I built the binary for Android with GOARCH=arm64 on Linux with GOHOSTARCH=amd64.

What did you do?

I've built this test application that does TCP pinging using the syscall package, and found out that the EpollWait call returns an EpollEvent with only Events attribute set. The Fd attribute is always set to 0 when run on arm64.

Here's the code:
https://github.com/status-im/infra-utils/blob/113e4e50/tcp-ping/main.go

I ran it on my android device(OnePlus 6 with Android 9) with the following results:

OnePlus6:/data # ./tcp-ping-arm64                                                                                                    
TCP_PING:2019/11/09 15:43:52 PINGING: 8.8.8.8:53
TCP_PING:2019/11/09 15:43:52 Socket FD: 3
TCP_PING:2019/11/09 15:43:53 Epoll FD: 4
TCP_PING:2019/11/09 15:43:53 EpollWait #: 1
TCP_PING:2019/11/09 15:43:53 SO_ERROR: 0
TCP_PING:2019/11/09 15:43:53 EpollEvent: {4 0 0}
TCP_PING:2019/11/09 15:43:53 SUCCESS

As you can see the EpollEvent returned by EpollWait has the 2nd attribute(Fd) set to 0.

I have fixed it by switching from using syscall package to x/sys/unix:
https://github.com/status-im/infra-utils/blob/ec20ad39/tcp-ping/main.go

What did you expect to see?

I expected the event returned from EpollWait to include the number of the file descriptor for which the event was triggered.

What did you see instead?

File descriptor being set to 0, making the EpollWait call useless when used with multiple file descriptors.

Notes

I understand that the syscall was frozen as of Go 1.3 and that it won't be fixed. My intention in creating this issue is maybe helping someone else avoid hours of research by finding this issue.

I was also curious about why exactly the value for the file descriptor is always 0.

[bcmills]

Your program is missing a check of the err variable bound from the call (see also #23142).

If you check the error, I suspect you will find that that system call is not implemented at all (#25813).

[bcmills]

Duplicate of #25813

[jakubgs]

Thanks for the explanation, appreciate it.

The syscall package should not be used.

Oh, now that I look at it there is a deprecation warning:

Deprecated: this package is locked down. Callers should use the corresponding package in the golang.org/x/sys repository instead.
https://golang.org/pkg/syscall/

But I must have missed it every single time I was browsing that page. Maybe making it bold or moving it to the top of the description would be a good idea.

[jakubgs]

Your program is missing a check of the err variable bound from the call.

That was just a proof-of-concept code, I have other examples that do check it. I just verified this on my simple test code by adding a check for error from sycall.EpollWait() and there is no error. If it's not implemented it should return an error and not a partially filled out event.

[bcmills]

If you have a concrete example that demonstrates a nil error in conjunction with missing data, that aspect of it is probably worth fixing, at least.

[jakubgs]

Well, yes, I modified that test program: status-im/infra-utils@28aea97 & status-im/infra-utils@68a3244
And ran it on arm64:

 $ make go-arm64
GOARCH=arm64 go build -o tcp-ping-arm64 main.go

 $ adb push tcp-ping-arm64 /sdcard/
tcp-ping-arm64: 1 file pushed. 27.8 MB/s (3132491 bytes in 0.108s)

 $ adb shell
OnePlus6:/ $ su
OnePlus6:/ # cd /data
OnePlus6:/data # mv /sdcard/tcp-ping-arm64 ./
OnePlus6:/data # chmod +x tcp-ping-arm64
OnePlus6:/data # ./tcp-ping-arm64
TCP_PING:2019/11/11 14:29:14 PINGING: 8.8.8.8:53
TCP_PING:2019/11/11 14:29:14 Socket FD: 3
TCP_PING:2019/11/11 14:29:14 Epoll FD: 4
TCP_PING:2019/11/11 14:29:14 EpollWait err: <nil>
TCP_PING:2019/11/11 14:29:14 EpollWait #: 1
TCP_PING:2019/11/11 14:29:14 SO_ERROR: 0
TCP_PING:2019/11/11 14:29:14 EpollEvent: {4 0 3 0}
TCP_PING:2019/11/11 14:29:14 SUCCESS

See as EpollWait err is <nil> and EpollEvent has 2nd field set to 0. It's clearly broken.

[tklauser]

The implementation of syscall.EpollWait is exactly the same as unix.EpollWait. I'd rather suspect a problem with the definition of EpollEvent on arm64 missing padding before its Fd member. This was fixed in x/sys/unix (https://golang.org/cl/21971) but not in syscall. I'll send a CL.

[gopherbot]

Change https://golang.org/cl/206838 mentions this issue: syscall: fix epoll_event padding on linux/arm64

[gopherbot]

Change https://golang.org/cl/206858 mentions this issue: unix: test returned fd in TestEpoll