Re: [musl] Re: [alsa-devel] [PATCH v7 8/9] ALSA: add new 32-bit layout for snd_pcm_mmap_status/control

[Rich Felker]

On Thu, Oct 07, 2021 at 06:18:52PM +0200, Takashi Iwai wrote:
> On Thu, 07 Oct 2021 18:06:36 +0200,
> Rich Felker wrote:
> > 
> > On Thu, Oct 07, 2021 at 05:33:19PM +0200, Takashi Iwai wrote:
> > > On Thu, 07 Oct 2021 15:11:00 +0200,
> > > Arnd Bergmann wrote:
> > > > 
> > > >  On Thu, Oct 7, 2021 at 2:43 PM Takashi Iwai <tiwai@xxxxxxx> wrote:
> > > > > On Thu, 07 Oct 2021 13:48:44 +0200, Arnd Bergmann wrote:
> > > > > > On Thu, Oct 7, 2021 at 12:53 PM Takashi Iwai <tiwai@xxxxxxx> wrote:
> > > > > > > On Wed, 06 Oct 2021 19:49:17 +0200, Michael Forney wrote:
> > > > > >
> > > > > > As far as I can tell, the broken interface will always result in
> > > > > > user space seeing a zero value for "avail_min". Can you
> > > > > > make a prediction what that would mean for actual
> > > > > > applications? Will they have no audio output, run into
> > > > > > a crash, or be able to use recover and appear to work normally
> > > > > > here?
> > > > >
> > > > > No, fortunately it's only about control->avail_min, and fiddling this
> > > > > value can't break severely (otherwise it'd be a security problem ;)
> > > > >
> > > > > In the buggy condition, it's always zero, and the kernel treated as if
> > > > > 1, i.e. wake up as soon as data is available, which is OK-ish for most
> > > > > applications.   Apps usually don't care about the wake-up condition so
> > > > > much.  There are subtle difference and may influence on the stability
> > > > > of stream processing, but the stability usually depends more strongly
> > > > > on the hardware and software configurations.
> > > > >
> > > > > That being said, the impact by this bug (from the application behavior
> > > > > POV) is likely quite small, but the contamination is large; as you
> > > > > pointed out, it's much larger than I thought.
> > > > 
> > > > Ok, got it.
> > > > 
> > > > > The definition in uapi/sound/asound.h is a bit cryptic, but IIUC,
> > > > > __snd_pcm_mmap_control64 is used for 64bit archs, right?  If so, the
> > > > > problem rather hits more widely on 64bit archs silently.  Then, the
> > > > > influence by this bug must be almost negligible, as we've had no bug
> > > > > report about the behavior change.
> > > > 
> > > > While __snd_pcm_mmap_control64 is only used on 32-bit
> > > > architectures when 64-bit time_t is used. At the moment, this
> > > > means all users of musl-1.2.x libc, but not glibc.
> > > > 
> > > > On 64-bit architectures, __snd_pcm_mmap_control and
> > > > __snd_pcm_mmap_control64 are meant to be identical,
> > > > and this is actually true regardless of the bug, since
> > > > __pad_before_uframe and __pad_after_uframe both
> > > > end up as zero-length arrays here.
> > > > 
> > > > > We may just fix it in kernel and for new library with hoping that no
> > > > > one sees the actual problem.  Or, we may provide a complete new set of
> > > > > mmap offsets and ioctl to cover both broken and fixed interfaces...
> > > > > The decision depends on how perfectly we'd like to address the bug.
> > > > > As of now, I'm inclined to go for the former, but I'm open for more
> > > > > opinions.
> > > > 
> > > > Adding the musl list to Cc for additional testers, anyone interested
> > > > please see [1] for the original report.
> > > > 
> > > > It would be good to hear from musl users that are already using
> > > > audio support with 32-bit applications on 64-bit kernels, which
> > > > is the case that has the problem today. Have you noticed any
> > > > problems with audio support here? If not, we can probably
> > > > "fix" the kernel here and make the existing binaries behave
> > > > the same way on 32-bit kernels. If there are applications that
> > > > don't work in that environment today, I think we need to instead
> > > > change the kernel to accept the currently broken format on
> > > > both 32-bit and 64-bit kernels, possibly introducing yet another
> > > > format that works as originally intended but requires a newly
> > > > built kernel.
> > > 
> > > Thanks!
> > > 
> > > And now, looking more deeply, I feel more desperate.
> > > 
> > > This bug makes the expected padding gone on little-endian.
> > > On LE 32bit, the buggy definition is:
> > > 
> > > 	char __pad1[0];
> > > 	u32 appl_ptr;
> > > 	char __pad2[0]; // this should have been [4]
> > > 	char __pad3[0];
> > > 	u32 avail_min;
> > > 	char __pad4[4];
> > > 	
> > > When an application issues SYNC_PTR64 ioctl to submit appl_ptr and
> > > avail_min updates, 64bit kernel (in compat mode) reads directly as:
> > > 
> > > 	u64 appl_ptr;
> > > 	u64 avail_min;
> > > 
> > > Hence a bogus appl_ptr would be passed if avail_min != 0.
> > > And usually application sets non-zero avail_min.
> > > That is, the bug must hit more severely if the new API were really
> > > used.  It wouldn't crash, but some weird streaming behavior can
> > > happen like noise, jumping or underruns.
> > > 
> > > (Reading back avail_min=0 to user-space is rather harmless.  Ditto for
> > >  the case of BE, then at least there is no appl_ptr corruption.)
> > > 
> > > This made me wonder which way to go:
> > > it's certainly possible to fix the new kernel to treat both buggy and
> > > sane formats (disabling compat mmap and re-define ioctls, having the
> > > code for old APIs).  The problem is, however, in the case where the
> > > application needs to run on the older kernel that expects the buggy
> > > format.  Then apps would still have to send in the old buggy format --
> > > or maybe better in the older 32bit format that won't hit the bug
> > > above.  It makes situation more complicated.
> > 
> > Can't an ioctl number just be redefined so that, on old kernels with
> > the buggy one, newly built applications get told that mmap is not
> > available and use the unaffected non-mmap fallback?
> 
> The problem is that the SYNC_PTR64 ioctl itself for non-mmap fallback
> is equally buggy due to this bug, too.  So disabling mmap doesn't help
> alone.
> 
> And, yes, we can redefine ioctl numbers.  But, then, application would
> have to be bilingual, as well as the kernel; it'll have to switch back
> to old API when running on older kernel, while the same binary would
> need to run in a new API for a newer kernel.
> 
> Maybe we can implement it in alsa-lib, if it really worth for it.

In musl we already have ioctl struct conversion for running on
time32-only kernels. So it may be practical to convert this too if
needed.

Rich