Cross compile for no_std environment build is failing at linking

[isaac-asdf]

I posted this in the hal crate for the microcontroller I'm working with ESP32, but will cross post here in case it's something I'm just doing wrong with the initial set up. Am I supposed to build the C library manually, or use the crate secp256k1-sys to build the the library for use? I'm getting errors for the functions I'm calling that look like : undefined reference to 'rustsecp256k1_v0_8_1_schnorrsig_sign'

Source code for project

[apoelstra]

It's difficult to see what's happening since even cargo tree fails with error: invalid toolchain name: 'esp'. When I try moving/deleting rust-toolchain.toml I get process didn't exit successfully:rustc - --crate-name ___ --print=file-names -C link-arg=-Tlinkall.x -C link-arg=-nostartfiles -C link-arg=-Tesp32_rom_functions.x -L libsecp256k1/libsecp256k1.a -l secp256k1 --target xtensa-esp32-none-elf --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=cfg (exit status: 1).

But the short answer is that yes, you should be compiling libsecp via secp256k1-sys. If you don't want to do this, you can add RUSTFLAGS=--cfg=rust_secp_no_symbol_renaming which will forgo compiling the included version of libsecp (and avoid doing the rustsecp256k1_v0_8_1 prefixing of all the symbols); in that case you are responsible for compiling and linking a compatible version of libsecp into your project.

But having said this, your existing setup should Just Work, using secp256k1-sys (which is a dep of secp256k1), whose build.rs should build libsecp256k1 for you.

I checked my local copy of secp256k1-sys 0.8.1 in my cargo cache and it does appear to define and export that symbol..

[Kixunil]

@apoelstra IIRC you need to use a forked compiler for ESP (for now, until they upstream it).

[isaac-asdf]

@Kixunil is correct, ESP uses needs a forked compiler. Running cargo tree gave me:

├── secp256k1 v0.27.0
│   ├── rand v0.8.5
│   │   └── rand_core v0.6.4
│   ├── secp256k1-sys v0.8.1
│   │   [build-dependencies]
│   │   └── cc v1.0.79
│   └── serde v1.0.160 (*)

I'll try the step @apoelstra mentioned of compiling with RUSTFLAGS=--cfg=rust_secp_no_symbol_renaming and let y'all know if that worked

[apoelstra]

My guess is that if you compile with that flag, you'll get link errors of the form undefined reference to secp256k1_schnorrsig_sign (note no weird rust version prefixing) and the solution will be to link your own copy of libsecp in.

My guess about the original issue is that the ESP compiler is somehow incompatible with the link symbol shenanigans we do in this library ... though I have no idea who is at fault.

[Kixunil]

I'd guess this should be job for cc.

[isaac-asdf]

Yeah, the RUSTFLAGS=--cfg=rust_secp_no_symbol_renaming just gave me different errors. @Kixunil do you have any recommendations or resources I could look into for troubleshooting this on the cc side? I'm also wondering if it would just be easier to try to cut out this crate and try to right the FFI bindings for the few functions I'm actually using (basically just Schnorr signing)

[apoelstra]

@isaac-asdf if you want to do that, you could try depending on secp256k1-sys directly

[Kixunil]

@isaac-asdf maybe you could first run their tests on ESP and see if anything fails, if it does report it to them, if it doesn't then it may be actually our problem.

[isaac-asdf]

The code I have compiles and runs as long as the signature portion is commented out. I'm testing if I can get it to compile and run with just including a more basic C function to see if it's just an xtensa linking issue with cc. If that doesn't run then I think I can just close this out as it would be a linking issue

[isaac-asdf]

The issue appears to be coming from the combo of cc and the Rust fork that the xtensa chip is using. I ran the code from this repo and got the same error message of undefined reference to my_c_function. Thanks for the help @Kixunil and @apoelstra!

[0xApprentice]

@isaac-asdf

hi，Did you resolve your issue in the end?
I'm encountering compatibility issues with secp256k1 while linking a Rust static library for ESP32. and I found the following message: 'c928380082b86401-secp256k1.o: file format not recognized'.

[isaac-asdf]

Yeah, I was able to get it functional. If you check out github.com/isaac-asdf/esp32-nostr-client you can see a project I used it on. The nostr-nostd dependency is where I actually pull in secp256k1, and the key to getting it compiling is explained in the Setup on the readme. (Setting the CC and AR environment variables) Let me know if you have any questions on that

…

On Thu, Aug 29, 2024 at 4:24 AM glove ***@***.***> wrote: @isaac-asdf <https://github.com/isaac-asdf> image.png (view on web) <https://github.com/user-attachments/assets/8f3d0f08-78b7-4c30-923c-c922860edbed> hi，Did you resolve your issue in the end? I'm encountering compatibility issues with secp256k1 while linking a Rust static library for ESP32. and I found the following message: 'c928380082b86401-secp256k1.o: file format not recognized'. — Reply to this email directly, view it on GitHub <#598 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AXFPMRQTUIKP4HIXL76ZL43ZT3SGPAVCNFSM6AAAAABNJ75LWKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDGMJXGEZTGNZUGI> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[0xApprentice]

Thank you for your help, I will go and give it a try.