linux-kernel - Re: [PATCH v2 3/3] dmi: Make dmi_walk and dmi_walk

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Message-ID: <CALCETrWXE1aG+q4aXx99X7kh6r7_dm6PuuUPfEiju7JZo7xgnQ@mail.gmail.com>
Date:	Tue, 19 Jan 2016 15:51:49 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Jean Delvare <jdelvare@...e.de>
Cc:	Pali Rohár <pali.rohar@...il.com>,
	Andy Lutomirski <luto@...nel.org>,
	platform-driver-x86@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] dmi: Make dmi_walk and dmi_walk_early return real
 error codes

On Tue, Jan 19, 2016 at 1:40 AM, Jean Delvare <jdelvare@...e.de> wrote:
> On Tue, 19 Jan 2016 10:07:36 +0100, Pali Rohár wrote:
>> On Tuesday 19 January 2016 10:03:03 Jean Delvare wrote:
>> > Hi Pali,
>> >
>> > On Tue, 19 Jan 2016 09:36:33 +0100, Pali Rohár wrote:
>> > > On Tuesday 19 January 2016 08:54:26 Jean Delvare wrote:
>> > > > > @@ -978,11 +978,11 @@ int dmi_walk(void (*decode)(const struct dmi_header *, void *),
>> > > > >       u8 *buf;
>> > > > >
>> > > > >       if (!dmi_available)
>> > > > > -             return -1;
>> > > > > +             return -ENOENT;
>> > > >
>> > > > -ENOSYS would seem more appropriate?
>> > >
>> > > IIRC -ENOSYS is for non implemented syscalls.
>> >
>> > I can see a lot of -ENOSYS in include/linux/*.h returned by stubs when
>> > a specific subsystem is not included. Not related to syscalls at all.
>> > This is what lead to my suggestion.
>>
>> https://lkml.org/lkml/2014/8/22/492
>
> Thanks for the pointer, I wasn't aware of that.
>
> It really should be documented. No, checkpatch.pl isn't documentation.
>
> Also the commit sadly doesn't say why using ENOSYS in other contexts is
> considered a bad thing. What actual trouble did it cause?

The trouble is that user code likes to assume that, when a syscall
returns -ENOSYS, that syscall isn't implemented.  Letting ENOSYS leak
out to userspace via a syscall that *is* implemented can confused
things.

>
> Are the current presumably incorrect uses of ENOSYS ultimately going to
> be fixed? If not, I see no point in preventing other use cases.

We at least want to prevent it from newly introduced syscalls.

I'll try to clean up the docs.

--Andy