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

[Arnd Bergmann]

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.

    Arnd