/dev/sequencer descriptions

[Andrey Konovalov]

Hi Takashi,

We're working on syzkaller descriptions for /dev/sequencer [1], but
have some troubles understanding its functionality. The main question
is: is sound/synth/emux/ code reachable when tested in a VM (we're
using QEMU with -soundhw all), or does it require some specific
hardware to be present?

In our setup (with Debian Stretch userspace image) we have
/dev/sequencer with SNDRV_MINOR_OSS_SEQUENCER and /dev/sequencer2 with
SNDRV_MINOR_OSS_MUSIC. For the former, there are no synth devices as
reported by SNDCTL_SEQ_NRSYNTHS. For the latter, synth devices are
there, but the load_patch() callback is not reachable as this check
[2] fails.

Is that code impossible to test in a VM at all? Or are there some
specific kernel options/drivers that need to be enabled?

Thanks!

[1] https://github.com/google/syzkaller/pull/1952

[2] https://elixir.bootlin.com/linux/v5.7.8/source/sound/core/seq/oss/seq_oss_synth.c#L452