[Bug 277118] Linux Emulation (Linuxulator): accept(2) does not time out, SO_RCVTIMEO is ignored

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=277118

            Bug ID: 277118
           Summary: Linux Emulation (Linuxulator): accept(2) does not time
                    out, SO_RCVTIMEO is ignored
           Product: Base System
           Version: 14.0-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: christian@kohlschutter.com
 Attachment #248545 text/plain
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Created attachment 248545
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=248545&action=edit
Code demonstrating the bug (compile with/for Linux, then test on Linux, and use
Linuxulator to test on FreeBSD)

In Linux, setting SO_RCVTIMEO on a listening, blocking socket will cause
accept(2) to return with -1 and errno=EAGAIN.

Linuxulator unfortunately does not mimic this behavior, and instead exposes BSD
behavior (timeout is ignored). Therefore, any Linux code relying on the timeout
may fail to work correctly with FreeBSD Linux emulation.

This can be fixed in a way compatible with Linux proper and FreeBSD Linuxulator
(if source code access is available) by calling poll(2) prior to accept(2).

However, code that is optimized for Linux proper should not require the call to
poll(2) just to support the niche use case of supporting Linuxulator on
FreeBSD, as that would increase the number of syscalls when running the binary
on Linux proper.

A small test case is attached.

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