[alsa-devel] [PATCH 04/39] ALSA: seq: copy ioctl data from user space to kernel stack

[Takashi Iwai]

On Sun, 07 Aug 2016 16:26:35 +0200,
Takashi Sakamoto wrote:
> 
> Hi Clemens,
> 
> On Aug 7 2016 19:15, Clemens Ladisch wrote:
> > Takashi Sakamoto wrote:
> >> Data from user space is once copied to kernel stack, then operated and
> >> copied to user space, in a consistent manner. This manner forces all ioctl
> >> operations to copy the data from/to user space, even if it's read-only or
> >> write-only. Thus, it has an overhead for simpler ioctl commands.
> > 
> > The ioctl code itself already contains information about the direction
> > and size of the data to be copied (and in theory, these values are
> > correct).  See dispatch_ioctl() in drivers/firewire/core-cdev.c for an
> > example.
> 
> A nice idea.
> 
> _IOC_SIZE macro pick up 13 or 14 bits (architecture-dependent) in ioctl
> command, which represents the size of argument. In my patch, the size of
> 'union ioctl_arg' is 188 (x86/x32) or 192 (x86_64) and there's enough
> rest of the size field. So we can pick up the size from ioctl command by
> the macro because the size represents the maximum bytes of argument for
> all of sequencer ioctls.

It's not only about the size.  It contains the r/w bits, so you can
avoid the unnecessary user-copy calls, too.


Takashi