problems with splice from /proc (was Linux 5.10-rc1)

[Greg KH <gregkh@linuxfoundation.org>]

On Sun, Oct 25, 2020 at 03:40:27PM -0700, Linus Torvalds wrote:
> The most interesting - to me - change here is Christoph's setf_fs()
> removal (it got merged through Al Viro, as you can see in my mergelog
> below).  It's not a _huge_ change, but it's interesting because the
> whole model of set_fs() to specify whether a userspace copy actually
> goes to user space or kernel space goes back to pretty much the
> original release of Linux, and while the name is entirely historic (it
> hasn't used the %fs segment register in a long time), the concept has
> remained. Until now.

I told Al this yesterday, but figured I would mention it here for others
to see.

Commit 36e2c7421f02 ("fs: don't allow splice read/write without explicit
ops") from this patch series, is breaking the bionic test suite that
does the following to verify that splice is working:

	int in = open("/proc/cpuinfo", O_RDONLY);
	ASSERT_NE(in, -1);

	TemporaryFile tf;
	ssize_t bytes_read = splice(in, nullptr, pipe_fds[1], nullptr, 8*1024, SPLICE_F_MORE | SPLICE_F_MOVE);
	ASSERT_NE(bytes_read, -1);

Before this change, all works well but now splice fails on /proc files
(and I'm guessing other virtual filesystems).

I'll ask the bionic developers if they can change their test to some
other file, but this is a regression and might show up in other "test
platforms" as well.  Using /proc for this is just so simple because
these files are "always there" and don't require any housekeeping for
test suites to worry about .

thanks,

greg k-h