Problem using getrandom

help
#1

I need to generate random numbers. For that, I define in my Cargo.toml file the following:

[package]
name = "dice"
version = "0.1.0"
description = "Created with Anchor"
edition = "2018"

[lib]
crate-type = ["cdylib", "lib"]
name = "dice"
doctest = false

[features]
no-entrypoint = []
no-idl = []
no-log-ix-name = []
cpi = ["no-entrypoint"]
default = []

[dependencies]
rand = "0.8.5"

When I run anchor build in console, I get the following error:

BPF SDK: /home/krl/.local/share/solana/install/releases/1.9.13/solana-release/bin/sdk/bpf
cargo-build-bpf child: rustup toolchain list -v
cargo-build-bpf child: cargo +bpf build --target bpfel-unknown-unknown --release
   Compiling getrandom v0.2.6
error: target is not supported, for more information see: https://docs.rs/getrandom/#unsupported-targets
   --> ~.cargo/registry/src/github.com-1ecc6299db9ec823/getrandom-0.2.6/src/lib.rs:240:9
    |
240 | /         compile_error!("target is not supported, for more information see: \
241 | |                         https://docs.rs/getrandom/#unsupported-targets");
    | |_________________________________________________________________________^

error[E0433]: failed to resolve: use of undeclared crate or module `imp`
   --> ~/.cargo/registry/src/github.com-1ecc6299db9ec823/getrandom-0.2.6/src/lib.rs:262:5
    |
262 |     imp::getrandom_inner(dest)
    |     ^^^ use of undeclared crate or module `imp`

For more information about this error, try `rustc --explain E0433`.

After a search, I found the same problem in https://users.rust-lang.org/t/use-of-undeclared-crate-or-module-imp-on-getrandom-0-2-3-when-building-for-wasm32-unknown-unknown/70171, whose solution is to add in Cargo.toml:

getrandom = { version = "0.2", features = ["js"] }

However, it doesn't work for me.

#2

That solution is only for the wasm32-unknown-unknown target, as it's a bit special. In your case, that target is simple not supported. You may be able to use the register_custom_getrandom! macro.

I'd normally suggest making an issue or PR to support your target, but I don't see bpfel-unknown-unknown in the target list, so I'm not sure if getrandom could support it.

#3

Is there another way in rust to generate random numbers without going through the rand library?

#4

Certainly. The solution depends on what quality of randomness you are looking for.

Implementing a basic random number generator is fairly simple (cryptographic, not so much). The larger issue is seeding it randomly. Seeding from the system clock is not directly possible on wasm-unknown-unknown because that platform doesn't support one. In the past I've created a simple binding to pull in the time via javascript in order to seed the generator.

Edit: Sorry, I didn't catch that this was for bpf. I just saw the mention of wasm and assumed a web scenario.

#5

It's hard to use traditional RNGs in eBPF environment. RNGs are stateful but only state eBPF functions can have between calls are those maps. That's why the bpf-helpers provides bpf_get_prandom_u32 function. You should use it instead.

https://man7.org/linux/man-pages/man7/bpf-helpers.7.html