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
