Re: [PATCH] rust: ioctl: Add ioctl number manipulation functions

[Asahi Lina <lina@asahilina.net>]

On 25/02/2023 09.38, Gary Guo wrote:
> On Fri, 24 Feb 2023 09:43:27 +0100
> "Arnd Bergmann" <arnd@arndb.de> wrote:
> 
>> On Fri, Feb 24, 2023, at 08:36, Asahi Lina wrote:
>>> Add simple 1:1 wrappers of the C ioctl number manipulation functions.
>>> Since these are macros we cannot bindgen them directly, and since they
>>> should be usable in const context we cannot use helper wrappers, so
>>> we'll have to reimplement them in Rust. Thankfully, the C headers do
>>> declare defines for the relevant bitfield positions, so we don't need
>>> to duplicate that.
>>>
>>> Signed-off-by: Asahi Lina <lina@asahilina.net>  
>>
>> I don't know much rust yet, but it looks like a correct abstraction
>> that handles all the corner cases of architectures with unusual
>> _IOC_*MASK combinations the same way as the C version.
>>
>> There is one corner case I'm not sure about:
>>
>>> +/// Build an ioctl number, analogous to the C macro of the same name.
>>> +const fn _IOC(dir: u32, ty: u32, nr: u32, size: usize) -> u32 {
>>> +    core::assert!(dir <= bindings::_IOC_DIRMASK);
>>> +    core::assert!(ty <= bindings::_IOC_TYPEMASK);
>>> +    core::assert!(nr <= bindings::_IOC_NRMASK);
>>> +    core::assert!(size <= (bindings::_IOC_SIZEMASK as usize));
>>> +
>>> +    (dir << bindings::_IOC_DIRSHIFT)
>>> +        | (ty << bindings::_IOC_TYPESHIFT)
>>> +        | (nr << bindings::_IOC_NRSHIFT)
>>> +        | ((size as u32) << bindings::_IOC_SIZESHIFT)
>>> +}  
>>
>> This has the assertions inside of _IOC() while the C version
>> has them in the outer _IOR()/_IOW() /_IOWR() helpers. This was
>> intentional since some users of _IOC() pass a variable
>> length in rather than sizeof(type), and this would cause
>> a link failure in C.
>>
>> How is the _IOC_SIZEMASK assertion evaluated here? It's
>> probably ok if this is a compile-time assertion that prevents
>> the variable-length arguments, but it would be bad if this
>> could lead to a BUG() or panic() in case of a user-supplied
>> length that is out of range.
> 
> This is a very good point.
> 
> The code, as currently written, will cause a compile-time error if
> `_IOC` is used in const contexts (i.e. used in const generics
> arguments, or inside a `const {}` block), and it will become a runtime
> `BUG()` if used elsewhere.
> 
> We do have a facility to enforce compile-time checks, that's
> `kernel::build_assert!()`. If runtime values are used and the
> compiler can't optimise these assertions out, a link failure would
> be triggered just like how our C code does that.
> 
> Lina, could you change these `core::assert!` calls to build assert?

Thanks, I'll do that for v2!

~~ Lina