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

[Rich Felker]

On Fri, Oct 08, 2021 at 02:37:12PM +0200, Arnd Bergmann wrote:
> On Fri, Oct 8, 2021 at 2:06 PM Rich Felker <dalias at libc.org> wrote:
> > On Fri, Oct 08, 2021 at 11:24:39AM +0200, Arnd Bergmann wrote:
> > >
> > > I've tried to understand this part of musl's convert_ioctl_struct(), but I just
> > > can't figure out whether it does the conversion based the on the layout that
> > > is currently used in the kernel, or based on the layout we should have been
> > > using, and would use with the above fix. Rich, can you help me here?
> >
> > If the attempted 64-bit ioctl is missing (ENOTTY), it does the
> > conversion to the legacy 32-bit one and retries with that, then
> > converts the results back to the 64-bit form.
> 
> I understand that it tries to do that.
> 
> The part that I'm not sure about is which of the two possible
> 64-bit forms it's using -- the broken one we have defined in the
> kernel headers, or the one we were trying to define but failed.

It's attempting to convert the intended format, not the one that the
uapi headers defined. That is, it's taking padded-to-64-bit values at
offsets 0 and 8 in __snd_pcm_mmap_control64, putting them at offsets 0
and 4 in the 32-bit struct, and padding them back to 64-bit in the
result.

Since applications would have been compiled with the buggy
(unintended) version of the uapi headers, this will not match the
application's layout of the struct. I haven't worked through what all
the consequences of that are, but I think some fix is needed here in
musl regardless of what happens on the kernel side.

Rich