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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@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