x/sys: Android's amd64 seccomp policy blocks unix.EpollWait() but not epoll_pwait syscall

[chenxiaolong]

Currently, golang.org/x/sys/unix only provides unix.EpollWait, which uses the epoll_wait syscall. However, this syscall is not permitted by Android's seccomp policy on amd64. This doesn't affect non-golang programs because bionic libc's epoll_wait function actually uses the epoll_pwait syscall:

https://android.googlesource.com/platform/bionic/+/refs/tags/android-17.0.0_r1/libc/bionic/sys_epoll.cpp#86

Would it be possible to define this package's EpollWait function in terms of the epoll_pwait syscall as well? I don't believe it should cause compatibility issues since it's been available since kernel 2.6.19.

[chenxiaolong]

Looks like this was previously done for arm64 in 324e137. It would be great if the same could be done for amd64.

[Jorropo]

https://go.dev/wiki/MinimumRequirements

For Go 1.24 and later: Kernel version 3.2 or later.

https://cs.opensource.google/go/x/sys/+/master:go.mod

go 1.25.0

https://man7.org/linux/man-pages/man2/epoll_wait.2.html

The sigmask argument may be specified as NULL, in which case epoll_pwait() is equivalent to epoll_wait().

All newer versions of x/sys are only compatible with linux kernels that should support epoll_pwait and epoll_pwait is a superset of epoll_wait that looks to be more widely compatible.
Unless I've missed a case where pwait is not available, we should implement unix.EpollWait using epoll_pwait(..., nil) everywhere rather than accumulating edge cases.

[gabyhelp]

Related Issues

• syscall: unimplemented EpollWait fails to return an error on android/arm64 #35479 (closed)
• runtime: use epoll_pwait2 where available for sub-millisecond timeout precision on Linux #53824
• proposal: x/sys/unix: add EpollWaitBlock function to handoff p asap. #76290

Related Code Changes

• syscall: use pselect on amd64 to satisfy Android
• syscall: fix EpollWait for arm64

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[gopherbot]

Change https://go.dev/cl/792700 mentions this issue: unix: use epoll_pwait rather than epoll_wait

[chenxiaolong]

Thanks for the quick fix!

EDIT: This was referring to https://go.dev/cl/792700, not the spam comment above.