[Bug 262535] Linuxulator sendfile does not support all file descriptors

From: <bugzilla-noreply_at_freebsd.org>
Date: Sun, 13 Mar 2022 23:17:07 UTC

            Bug ID: 262535
           Summary: Linuxulator sendfile does not support all file
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: james@mclgh.net

Created attachment 232438
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=232438&action=edit
Patch to add a fallback to Linuxulator sendfile to support any FDs

In Linux < 2.6.33, sendfile could only be used to send from a file to a socket.
This is the behaviour implemented by the FreeBSD Linuxulator. However, since
2.6.33, sendfile can send from any FD to any other FD, which is not implemented
by FreeBSD sendfile or by the Linuxulator.

I ran into this problem trying to run the game "Factorio" using the
Linuxulator, which relies on sendfile to copy files on disk. Here is another
thread on freebsd-emulation describing the same problem:


Attached is a patch which changes the Linuxulator sendfile such that it will
use FreeBSD sendfile where possible, but otherwise fallback on a simple
read/write loop for compatibility.

I would very much appreciate some feedback on the patch, as I have never
contributed to the FreeBSD kernel before. In particular, any suggestion for a
better way to choose the buffer size than the arbitrary 8 KB I have hardcoded
would be welcome.

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