Unsoundness in LR register access.

[thomasw04]

If I interpret this correctly, LR can basically contain an undefined value when reading it. (see https://doc.rust-lang.org/reference/inline-assembly.html#r-asm.rules.reg-not-input) In addition, the behavior is undefined when writing to LR without declaring it as out register. (see https://doc.rust-lang.org/reference/inline-assembly.html#r-asm.rules.reg-not-output)

Furthermore, specifically with the link register (LR) in the context of function calls, (see https://github.com/ARM-software/abi-aa/blob/main/aapcs32/aapcs32.rst#subroutine-calls) this then completely relies on the compiler inlining this function, as otherwise, LR would contain the return address of the read/write function. (apart from that, by the rules above, LR does not even need to contain anything meaningful).

I am unsure how to solve this right now. If I am not mistaken, the inlining problem could be solved by using a macro.
However, regarding the undefined behavior in general, I am unsure if Rust currently has a way to make this reliably safe.

cortex-m/cortex-m/src/register/lr.rs

Lines 6 to 20 in 9177271

	/// Reads the CPU register
	#[cfg(cortex_m)]
	#[inline]
	pub fn read() -> u32 {
	let r;
	unsafe { asm!("mov {}, lr", out(reg) r, options(nomem, nostack, preserves_flags)) };
	r
	}
	/// Writes `bits` to the CPU register
	#[cfg(cortex_m)]
	#[inline]
	pub unsafe fn write(bits: u32) {
	asm!("mov lr, {}", in(reg) bits, options(nomem, nostack, preserves_flags));
	}