[PATCH] ALSA: seq: fix function cast warnings

[Takashi Iwai]

clang-16 points out a control flow integrity (kcfi) issue when event
callbacks get converted to incompatible types:

sound/core/seq/seq_midi.c:135:30: error: cast from 'int (*)(struct snd_rawmidi_substream *, const char *, int)' to 'snd_seq_dump_func_t' (aka 'int (*)(void *, void *, int)') converts to incompatible function type [-Werror,-Wcast-function-type-strict]
  135 |                 snd_seq_dump_var_event(ev, (snd_seq_dump_func_t)dump_midi, substream);
      |                                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
sound/core/seq/seq_virmidi.c:83:31: error: cast from 'int (*)(struct snd_rawmidi_substream *, const unsigned char *, int)' to 'snd_seq_dump_func_t' (aka 'int (*)(void *, void *, int)') converts to incompatible function type [-Werror,-Wcast-function-type-strict]
   83 |                         snd_seq_dump_var_event(ev, (snd_seq_dump_func_t)snd_rawmidi_receive, vmidi->substream);
      |                                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

For addressing those errors, introduce wrapper functions that are used
for callbacks and bridge to the actual function call with pointer
cast.

The code was originally added with the initial ALSA merge in linux-2.5.4.

[ the patch description shamelessly copied from Arnd's original patch
  -- tiwai ]

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
Link: https://lore.kernel.org/r/20240213101020.459183-1-arnd@xxxxxxxxxx
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
---
 sound/core/seq/seq_midi.c    | 8 +++++++-
 sound/core/seq/seq_virmidi.c | 9 ++++++++-
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/sound/core/seq/seq_midi.c b/sound/core/seq/seq_midi.c
index 18320a248aa7..78dcb0ea1558 100644
--- a/sound/core/seq/seq_midi.c
+++ b/sound/core/seq/seq_midi.c
@@ -113,6 +113,12 @@ static int dump_midi(struct snd_rawmidi_substream *substream, const char *buf, i
 	return 0;
 }
 
+/* callback for snd_seq_dump_var_event(), bridging to dump_midi() */
+static int __dump_midi(void *ptr, void *buf, int count)
+{
+	return dump_midi(ptr, buf, count);
+}
+
 static int event_process_midi(struct snd_seq_event *ev, int direct,
 			      void *private_data, int atomic, int hop)
 {
@@ -132,7 +138,7 @@ static int event_process_midi(struct snd_seq_event *ev, int direct,
 			pr_debug("ALSA: seq_midi: invalid sysex event flags = 0x%x\n", ev->flags);
 			return 0;
 		}
-		snd_seq_dump_var_event(ev, (snd_seq_dump_func_t)dump_midi, substream);
+		snd_seq_dump_var_event(ev, __dump_midi, substream);
 		snd_midi_event_reset_decode(msynth->parser);
 	} else {
 		if (msynth->parser == NULL)
diff --git a/sound/core/seq/seq_virmidi.c b/sound/core/seq/seq_virmidi.c
index 1b9260108e48..1678737f11be 100644
--- a/sound/core/seq/seq_virmidi.c
+++ b/sound/core/seq/seq_virmidi.c
@@ -62,6 +62,13 @@ static void snd_virmidi_init_event(struct snd_virmidi *vmidi,
 /*
  * decode input event and put to read buffer of each opened file
  */
+
+/* callback for snd_seq_dump_var_event(), bridging to snd_rawmidi_receive() */
+static int dump_to_rawmidi(void *ptr, void *buf, int count)
+{
+	return snd_rawmidi_receive(ptr, buf, count);
+}
+
 static int snd_virmidi_dev_receive_event(struct snd_virmidi_dev *rdev,
 					 struct snd_seq_event *ev,
 					 bool atomic)
@@ -80,7 +87,7 @@ static int snd_virmidi_dev_receive_event(struct snd_virmidi_dev *rdev,
 		if (ev->type == SNDRV_SEQ_EVENT_SYSEX) {
 			if ((ev->flags & SNDRV_SEQ_EVENT_LENGTH_MASK) != SNDRV_SEQ_EVENT_LENGTH_VARIABLE)
 				continue;
-			snd_seq_dump_var_event(ev, (snd_seq_dump_func_t)snd_rawmidi_receive, vmidi->substream);
+			snd_seq_dump_var_event(ev, dump_to_rawmidi, vmidi->substream);
 			snd_midi_event_reset_decode(vmidi->parser);
 		} else {
 			len = snd_midi_event_decode(vmidi->parser, msg, sizeof(msg), ev);
-- 
2.35.3