linux-kernel - Re: [PATCH v2] checkpatch.pl: New instances of ENOSYS are errors

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Date:	Mon, 25 Aug 2014 11:38:28 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Pavel Machek <pavel@....cz>
Cc:	Joe Perches <joe@...ches.com>, Andy Whitcroft <apw@...onical.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] checkpatch.pl: New instances of ENOSYS are errors

On Aug 25, 2014 3:29 AM, "Pavel Machek" <pavel@....cz> wrote:
>
> On Fri 2014-08-22 09:26:31, Andy Lutomirski wrote:
> > ENOSYS means that a nonexistent system call was called.  We have a
> > bad habit of using it for things like invalid operations on
> > otherwise valid syscalls.  We should avoid this in new code.
>
> Is it good idea? I mean, doing EINVAL for subcalls is pretty
> unhelpful.

EOPNOTSUPP?

The problem is that user code wants to do:

bool foo_wrapper(int op)
{
  if (foo_not_supported)
    return false

  if (foo(op) != 0) {
    if (errno == ENOSYS)
      foo_not_supported = true;
    return false;
  }

  return true;
}

foo_wrapper(FOO_OP_A);
foo_wrapper(FOO_OP_B);

If FOO_OP_A returns -ENOSYS but FOO_OP_B does not, then this doesn't work.

--Andy
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