Provide linkable functions of PSA getters and setters

[gilles-peskine-arm]

Getters and setters in the PSA API are declared as static inline. That's great if you're programming in C or C++, but if you're writing a wrapper for a high-level language, you may well need a linkable function. This has come up with Rust wrappers for example.

Mbed TLS, and possibly the PSA Crypto API, should be able to provide a linkable version of these functions.

Initializers are also static inline, but these might not be necessary since they can officially be replaced by all-bits-zero initialization.

[egrimley-arm]

It is possible to use the static inline functions from Rust by having a shim library. I'm playing with that approach now. It means maintaining a separate list of those functions, of course.

There's a related problem with preprocessor symbols. With Rust, if the C header contains #define X (123) then bindgen turns that into a pub const u32. But if the C header contains #define X ((int)123) then bindgen just ignores it. I don't know about other high-level languages accessing preprocessor symbols, but the C preprocessor is such a bizarre thing that I can't imagine it's straightforward in all cases.

[gilles-peskine-arm]

It would be good for bindgen to support such constants. The cast is necessary to get constants that work reliably with bitwise operations, especially involving negation. It also sometimes helps with static analyzers to detect when you're putting a numerical constant of one type where the context expects a different type. This may be complicated to implement, I don't know how sophisticated bindgen is.

[daverodgman]

There is more discussion on this topic in #6400

[daverodgman]

It looks like bindgen is working to support this kind of thing - see recent activity in rust-lang/rust-bindgen#1090