Re: SEGFault when optininal snd_ctl_ext_callback::read_event() not set

["Wischer, Timo (ADITG/ESB)" <twischer@de.adit-jv.com>]

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Hi Takashi,

see attached the patch.
>From my opinion failing with an error is the best solution (see explanation in the commit message)


Best regards

Timo Wischer

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________________________________________
From: Takashi Iwai [tiwai@suse.de]
Sent: Thursday, October 05, 2017 3:17 PM
To: Wischer, Timo (ADITG/ESB)
Cc: alsa-devel@alsa-project.org
Subject: Re: [alsa-devel] SEGFault when optininal       snd_ctl_ext_callback::read_event() not set

On Thu, 05 Oct 2017 14:03:14 +0200,
Wischer, Timo (ADITG/ESB) wrote:
>
> Hi all,
>
> snd_ctl_ext_callback::read_event() callback is mentioned as optional
> (see http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=include/control_external.h;h=12958e70a5230de9c74029c641a395a2073c8646;hb=refs/heads/master#l239)
>
> but there is no NULL check and the NULL pointer will be called if the read_event function callback pointer is not set.
> (see http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=src/control/control_ext.c;h=56552fa1aa0ef0e6383abf4029b63944a841c2c4;hb=refs/heads/master#l419)
>
> I think a default function has to be provided which will be called when the callback is not set or the read_event() callback should not be marked as optional.
>
> What is your opinion?

It should have a NULL check there, as documentation clearly states
that it's optional.

Care to submit a fix patch?


thanks,

Takashi

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From 8cc3dbc6fad0a4de82ab4b5c95a86499c99d56d8 Mon Sep 17 00:00:00 2001
From: Timo Wischer <twischer@de.adit-jv.com>
Date: Thu, 5 Oct 2017 16:25:23 +0200
Subject: ctl: ext: Fail with error code if snd_ctl_ext_callback::read_event()
 callback is not defined

The snd_ctl_ext_callback::read_event() callback is only optional
if no poll descriptor was given via
snd_ctl_ext_t::poll_fd
or
snd_ctl_ext_callback::snd_ctl_ext_poll_descriptors().

If a poll descriptor is given the
snd_ctl_ext_callback::read_event()
callback has also to be defined
because there is no minigful default behavior.

This callback will be called when ever the poll() on
the file descriptor indicates that there is an event pending.
Therefore returning a 0 which indicates that there is no event makes no
sense.

Signed-off-by: Timo Wischer <twischer@de.adit-jv.com>

diff --git a/src/control/control_ext.c b/src/control/control_ext.c
index 56552fa..d7de8e8 100644
--- a/src/control/control_ext.c
+++ b/src/control/control_ext.c
@@ -415,8 +415,12 @@ static int snd_ctl_ext_read(snd_ctl_t *handle, snd_ctl_event_t *event)
 {
 	snd_ctl_ext_t *ext = handle->private_data;
 
-	memset(event, 0, sizeof(*event));
-	return ext->callback->read_event(ext, &event->data.elem.id, &event->data.elem.mask);
+	if (ext->callback->read_event) {
+		memset(event, 0, sizeof(*event));
+		return ext->callback->read_event(ext, &event->data.elem.id, &event->data.elem.mask);
+	}
+
+	return -EINVAL;
 }
 
 static int snd_ctl_ext_poll_descriptors_count(snd_ctl_t *handle)

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