Can't build with no_std on Windows because of BCryptGenRandom import

[not-matthias]

My situation: I'm building a kernel driver and decided to use this crate. When I was building my driver, I noticed a weird import: BCryptGenRandom.

I managed to track down the issue: The import comes from the getrandom crate. It's used by ahash, which is used by hashbrown which is used by iced-x86. The problem is, that ahash doesn't hide the getrandom dependency behind a feature flag, so it's always used even though you might have enabled the compile-time-rng feature. See: https://github.com/tkaitchuck/aHash/blob/e77cab8c1e15bfc9f54dfd28bd8820c2a7bb27c4/src/random_state.rs#L95-L104
They added a runtime-rng feature, but that's only available in the simplification branch.

And this is the issue. Because getrandom always detects windows as platform, it'll import BCryptGenRandom, even though we are inside a kernel driver where we don't have access to bcrypt.dll.

How to fix (in this library):

Hashbrown has a feature that enables the compile time rng. Once the next version of ahash/hashbrown has been released, you could either enable the compile time feature by default or forward it to the user.

How to fix (on your own):

Add this to your Cargo.toml to use the updated version of aHash where getrandom is disabled:

[patch.crates-io]
# Hashbrown already imports this with `default-features=true`
ahash = { git = "https://github.com/tkaitchuck/aHash", branch = "simplification", features = ["compile-time-rng"] }

You also need to clone this repository and manually enable the compile time rng for hashbrown in src\rust\iced-x86\Cargo.toml:

hashbrown = { version = ">=0.9.0, <100.0.0", optional = true, default-features = false, features = ["ahash-compile-time-rng"] }

References

• compile-time-rng on a platform with getrandom tkaitchuck/aHash#94
• feature gate random state tkaitchuck/aHash#97
• Use a runtime-rng feature flag instead of using OS detection tkaitchuck/aHash#82

[wtfsck]

The hashbrown crate is optional and only needed by serde or block_encoder features. However it's enabled in Cargo.toml if no_std is used because I don't think it's possible to say 'Use hashbrown if no_std and (serde or block_encoder)'.

I think one easy workaround, assuming you don't need serde or block_encoder, is we could add a new feature, eg. no_std2 that would be the same as no_std but wouldn't include hashbrown. Would that work?

[not-matthias]

Unfortunately, I need to use block_encoder.

And there's no need for a second feature flag, because rustc doesn't compile getrandom if it's not used. So if you don't use serde or block_encoder but have no_std enabled, you won't have the import.

[wtfsck]

I just checked and block_encoder only uses HashMap once, in a field called to_instr. I think it could be possible to convert this to a Vec and then use binary search. The init code is here:

iced/src/rust/iced-x86/src/block_enc.rs

Lines 166 to 183 in 1fb4876

	// There must not be any instructions with the same IP, except if IP = 0 (default value)
	this.to_instr = HashMap::with_capacity(instr_count);
	for info in &this.blocks {
	for instr in &info.1 {
	let orig_ip = instr.borrow().orig_ip();
	if this.to_instr.get(&orig_ip).is_some() {
	if orig_ip != 0 {
	return Err(IcedError::with_string(format!("Multiple instructions with the same IP: 0x{:X}", orig_ip)));
	}
	this.has_multiple_zero_ip_instrs = true;
	} else {
	let _ = this.to_instr.insert(orig_ip, instr.clone());
	}
	}
	}
	if this.has_multiple_zero_ip_instrs {
	let _ = this.to_instr.remove(&0);
	}

If this can be converted into an efficient Vec impl, we can remove the hashbrown dep from block_encoder.

Adding all instructions to a new vec. Sorting it. Verifying that there are no dupes (except ip==0) should work.

Block encoder perf would have to be checked as well to make sure it's still acceptable.