Stream: wit-bindgen

Topic: Component failing to compile with mixed wit-bindgen versions

Landon James (Feb 18 2024 at 05:21):

I've recently started encountering a compilation with some of my components:

  = note: rust-lld: error: duplicate symbol: cabi_realloc
          >>> defined in /Volumes/workplace/smithy-rs/tools/target/wasm32-wasi/release/deps/libwit_bindgen-e0be62897f1f39ee.rlib(wit_bindgen-e0be62897f1f39ee.wit_bindgen.772b3d7561ccd03a-cgu.0.rcgu.o)
          >>> defined in /Volumes/workplace/smithy-rs/tools/target/wasm32-wasi/release/deps/libwit_bindgen-51f815a7b68edcc4.rlib(wit_bindgen-51f815a7b68edcc4.wit_bindgen.d59611476daebe1f-cgu.0.rcgu.o)

Through testing this seems to happen whenever I change my local version of wit-bindgen to anything other than 0.16.0. I am also using the wasi::http bindings from the wasi crate, and those bindings seem to be generated with wit-bindgen 16 so I think that is likely the cause of the conflicting symbols whenever I change my version.

Is the guidance that I should pin my wit-bindgen version to whatever the wasi crate uses, or is there another way of solving the duplicated symbol issue?

Note: this is happening fairly deep in a repo I'm working on at the moment, I will try to pull out a minimal repro soon.

Alex Crichton (Feb 18 2024 at 18:25):

This looks like you might be including two versions of wit-bindgen in the final binary. That's probably something we should fix at the wit-bindgen layer to support that, but in the near-term the best fix is probably to ensure everything is using the same version of wit-bindgen

Landon James (Feb 19 2024 at 02:15):

Gotcha, thankfully wasi is my only other wit-bindgen using dependency at the moment, so it is pretty easy to coordinate. I managed to get up a repro and created an issue to track this.

Alex Crichton (Feb 20 2024 at 18:40):

Thanks for the issues here.

The "best" way I can think of solving this is to switch the cabi_realloc symbol to a weak symbol. That way it's ok to have multiple definitions and they'll all be largely the same so it doesn't matter which one the linker ends up picking. The bad part about this is that there's no way to define a weak symbol in stable Rust.

@Dan Gohman you wouldn't happen to have any tricks to defining a weak symbol in stable Rust would you? (or anyone else?)

Alex Crichton (Feb 20 2024 at 18:40):

This is definitely going to be something I want to solve because I don't want to have to update the wasi crate every time wit-bindgen updates

Dan Gohman (Feb 20 2024 at 18:40):

No, last I looked into it the weak symbol attributes were nightly-only.

Alex Crichton (Feb 20 2024 at 18:41):

one idea I had was to use global_asm! perhaps with various directives there

Alex Crichton (Feb 20 2024 at 18:41):

but I'm not sure how stable that would be since I don't think LLVM provides strict stability guarantees about that layer of assembly

Alex Crichton (Feb 20 2024 at 18:41):

especially for a somewhat niche target like wasm

Alex Crichton (Feb 20 2024 at 18:42):

hm maybe I'll try going that route for now and see how it pans out, looks like the directive is .weak

Dan Gohman (Feb 20 2024 at 18:42):

We could make weak symbols using .weak inside a global_asm!. (edit: ah, you beat me too it)

Alex Crichton (Feb 20 2024 at 18:43):

bah global_asm! isn't available on wasm targets

Alex Crichton (Feb 20 2024 at 18:44):

I suppose we could do the same object-file-hackery that the adapter used to do

Alex Crichton (Feb 20 2024 at 18:44):

(still does?)

Alex Crichton (Feb 20 2024 at 18:45):

oof that's a lot to hardcode though

Dan Gohman (Feb 20 2024 at 18:46):

How hard would it be to give cabi_realloc a mangled name?

Alex Crichton (Feb 20 2024 at 18:48):

we could relatively easily do something like cabi_realloc_wit_bindgen_0_18_0 or something like that

Alex Crichton (Feb 20 2024 at 18:48):

another attempt would be to have cabi_realloc.c in the repo and check in a precompiled libcabi_realloc.a which is distributed to crates.io

Alex Crichton (Feb 20 2024 at 18:49):

but that would go straight to malloc which bypasses Rust's allocator stuff which at least causes wit-bindgen tests to fail

Alex Crichton (Feb 20 2024 at 18:49):

since they rely on that to count allocations

Alex Crichton (Feb 20 2024 at 18:49):

we could also move all this logic into wasm-tools component new and pick any function that starts with cabi_realloc to be the allocator, but that doesn't feel great

Alex Crichton (Feb 20 2024 at 18:50):

If we could mangle the name though are you thinking we'd have C refer to that?

Dan Gohman (Feb 20 2024 at 18:50):

I think so? Like ,cabi_realloc_wit_bindgen_0_18_0 is manageable levels of ugly.

Alex Crichton (Feb 20 2024 at 18:51):

we'd have to recomiple the *.c file per-version

Alex Crichton (Feb 20 2024 at 18:51):

which isn't the end of the world

Alex Crichton (Feb 20 2024 at 18:52):

I suppose with the shiny new release process we could even automate this

Dan Gohman (Feb 20 2024 at 18:52):

Is there a context where we have a C compiler that could compile cabi_realloc? Because we can make weak symbols from C.

Alex Crichton (Feb 20 2024 at 18:53):

in CI for wit-bindgen itself for sure, but not when users get the wit-bindgen from crates.io

Alex Crichton (Feb 20 2024 at 18:54):

so I'm thinking we could add a *.c file and verify in CI that there's a checked in *.a file which "matches"

Alex Crichton (Feb 20 2024 at 18:54):

and then PRs to do a release automatically recompile and make a commit with the new version

Alex Crichton (Feb 20 2024 at 18:54):

so that way we use C to define a weak cabi_realloc and that refers to a version-specific symbol

Alex Crichton (Feb 20 2024 at 18:54):

convoluted, but could work

Dan Gohman (Feb 20 2024 at 18:54):

Yeah

Dan Gohman (Feb 20 2024 at 18:55):

How evil would it be to smuggle a .weak inside a non-global asm! inside the body of cabi_realloc?

Alex Crichton (Feb 20 2024 at 18:55):

even asm! isn't reachable :(

Dan Gohman (Feb 20 2024 at 18:56):

Ah, I didn't realize we didn't have asm

Alex Crichton (Feb 20 2024 at 18:56):

oh I was using the wrong path, asm! and global_asm! are reachable but both flagged as unstable

Alex Crichton (Feb 20 2024 at 18:57):

I was doing core::arch::wasm32::asm! instead of core::arch::asm!

Alex Crichton (Feb 20 2024 at 20:45):

Ok https://github.com/bytecodealliance/wit-bindgen/pull/851 is my attempt at all this

Alex Crichton (Feb 20 2024 at 21:15):

@Dan Gohman question for you, which I have no idea if you'll know the answer to this:

For wasi-sdk using the precompiled binaries from CI I'm seeing !1 = !{!"clang version 16.0.0"} in LLVM IR. For wasi-sdk's precompiled binaries on macOS I'm seeing !1 = !{!"clang version 16.0.0 (https://github.com/llvm/llvm-project 08d094a0e457360ad8b94b017d2dc277e697ca76)"} in the IR. This means that a macOS host and a Linux host produce a different "producers" section in the output. You wouldn't happen to know off the top of your head why that is would you?

Dan Gohman (Feb 20 2024 at 21:17):

Wow, no, I don't know. My guess is some build system logic to distinguish between "official builds" and custom builds, but I don't know what that logic is looking at.

Dan Gohman (Feb 20 2024 at 21:17):

@Andrew Brown might perhaps know?

Alex Crichton (Feb 20 2024 at 21:19):

I need to track that down, figure out how to get clang to not emit it, or strip it myself

Last updated: Dec 23 2024 at 13:07 UTC