Backward compatibility for systems that doesn't support accept4

[zonyitoo]

When running on some old systems, like an old Android, they doesn't have the accept4 syscall. When running programs on these platforms, we will get an error: Function not implemented (os error 38).

It would be better to automatically fallback to accept when accept4 returns this errno ENOSYS.

[Thomasdezeeuw]

What Android version are you running. Because accept4(2) was added to Linux in 2.6.28. According to https://android.stackexchange.com/questions/51651/which-android-runs-which-linux-kernel Android 1.6 is based on 2.6.29. Which means this wouldn't work on Android 1.5, that way too old to support.

[jshearer]

So it's not just restricted to x86 android then. I encountered this same issue, and fixed it for my use case: #1446. Maybe the conditional just needs to be expanded to match all of android, or somehow detect at runtime whether the syscall is supported? (though that would introduce some overhead...)

[zonyitoo]

I was running on an Android Emulator (which is a x86 Android I believe).

@jshearer Your fixes are perfectly working on .apks that are built with x86 toolchain, but what if a .apk built with ARM toolchain and then ran on a x86 Android? :)

Maybe the conditional just needs to be expanded to match all of android, or somehow detect at runtime whether the syscall is supported?

Yup. See my PR.

But those Android that running on ARM platform do have the accept4 syscall. And not all x86 Android are shipped without the accept4 syscall.

[jshearer]

but what if a .apk built with ARM toolchain and then ran on a x86 Android?

Is this with the new android 11 emulation layer? Because otherwise that shouldn't work, right? Different architectures and all that

[zonyitoo]

I have no idea. I found this on an Emulator called MuMu. But accept4 works well on another x86 Emulator called BlueStacks.

Kernel version: 4.0.9-android-x86+luoweiqiao@a11-gz02-test.i.nease.net #1 Thu Jan 21 21:03:25 HKT 2021
Android version: 6.0.1

Actually most of the x86 Emulators can run ARM apks.

[jshearer]

Actually most of the x86 Emulators can run ARM apks.

I did not know that! Neat

[Thomasdezeeuw]

I currently see this as more of a problem of the emulator than Mio. The Linux kernel has accept4(2) since version 2.6.28, so if the emulator reports 4.0.9 it should it. Furthermore Android 5.0 (API version 21), released in 2014, should also have accept4(2) according to android/ndk#1372. So this leads me to believe it's a problem with the emulator(s), not actual Android.

Also @zonyitoo I'm not going to accept #1466 in its current form as it has needless overhead for non-Android/emulator platforms.

[zonyitoo]

Hmm, alright. It is reasonable.

[Thomasdezeeuw]

This was fixed mostly by #1446 for emulator. I don't there is anything actionable here.