Re: [PATCH v2 2/2] rust: miscdevice: add base miscdevice abstraction

[Christian Brauner <brauner@kernel.org>]

On Wed, Oct 02, 2024 at 06:04:38PM GMT, Greg Kroah-Hartman wrote:
> On Wed, Oct 02, 2024 at 03:45:08PM +0000, Arnd Bergmann wrote:
> > On Wed, Oct 2, 2024, at 14:23, Christian Brauner wrote:
> > 
> > > and then copy the stuff via copy_struct_from_user() or copy back out to
> > > user via other means.
> > >
> > > This way you can safely extend ioctl()s in a backward and forward
> > > compatible manner and if we can enforce this for new drivers then I
> > > think that's what we should do.
> > 
> > I don't see much value in building generic code for ioctl around
> > this specific variant of extensibility. Extending ioctl commands
> > by having a larger structure that results in a new cmd code
> > constant is fine, but there is little difference between doing
> > this with the same or a different 'nr' value. Most drivers just
> > always use a new nr here, and I see no reason to discourage that.
> > 
> > There is actually a small risk in your example where it can
> > break if you have the same size between native and compat
> > variants of the same command, like
> > 
> > struct old {
> >     long a;
> > };
> > 
> > struct new {
> >     long a;
> >     int b;
> > };
> > 
> > Here, the 64-bit 'old' has the same size as the 32-bit 'new',
> > so if we try to handle them in a shared native/compat ioctl
> > function, this needs an extra in_conmpat_syscall() check that
> > adds complexity and is easy to forget.
> 
> Agreed, "extending" ioctls is considered a bad thing and it's just
> easier to create a new one.  Or use some flags and reserved fields, if
> you remember to add them in the beginning...

This statement misses the reality of what has been happening outside of
arbitrary drivers for years. Let alone that it simply asserts that it's
a bad thing.