Geoff Goodman (Feb 26 2024 at 20:33): Geoff Goodman (Feb 26 2024 at 20:33):As someone who is trying to compile a rust crate to wasm32-wasi that has dependencies on C libs (QuickJS via rquickjs), how can I go about finding the reason that different WASI preview 1 imports are being pulled in?
This risks being something that seems trivially obvious to some folks so to add extra context: I'm at the start of my rust journey and haven't played in compiled languages since a time when I had no idea what I was doing.
Alex Crichton (Feb 26 2024 at 20:35):This unfortunately generally isn't easy AFAIK, you'd have to trace symbols backwards to some exported function or something in a table for example, and that's a pretty manual process.
If you want zero imports you can use wasm32-unknown-unknown, though, but not all crate will compile for that target
Geoff Goodman (Feb 26 2024 at 20:36):I did try targeting wasm32-unknown-unknown but quickly discovered how far out of my depth I was. :sweat_smile:
Geoff Goodman (Feb 26 2024 at 20:36):Are you aware of any tools that might expose WASM call graphs in some ergonomic form?
Alex Crichton (Feb 26 2024 at 20:37):twiggy is the closest approximation that I can think of, but that's a "theoretically this is possible" kind of thing rather than "here's the one liner to render a graphviz file"
Geoff Goodman (Feb 26 2024 at 20:39):twiggy paths looks super promising.
Joel Dice (Feb 26 2024 at 20:40):QuickJS's os module probably pulls in a lot of WASI stuff by way of wasi-libc: https://bellard.org/quickjs/quickjs.html#os-module
Geoff Goodman (Feb 26 2024 at 20:50):@Joel Dice AFAICT, rquickjs shouldn't be pulling that in unless certain crate features are being used. I had he same question w/ quickjs-wasm-rs from the Javy team and I _know_ that it isn't pulling in the libc library.
Geoff Goodman (Feb 26 2024 at 20:58):Yeah, I don't think it's pulling in the libc os module: https://github.com/DelSkayn/rquickjs/blob/7029b70b75cf9220c16a1ce2768968f3b8bef6fc/sys/build.rs#L123-L134
Geoff Goodman (Mar 20 2024 at 14:13):Well I've gone down this rabbit-hole once again and seem to have found a couple plausible explanations, as described in these issues:
As @Surma found in the first, it seems that some nuance of the way C macros are defined in the wasi-sdk is forcing NDEBUG to be unset: https://github.com/WebAssembly/wasi-sdk/issues/190#issuecomment-914682467 and then assert() resulting in this 'stong' reference to stdout that LTO isn't able to get rid of.
I was able to avoid the imports by defining some stubs like the following:
#[no_mangle]
unsafe extern "C" fn __stdio_write(ptr: *const u8, len: usize) -> usize {
0
}
(also for __stdout_write, __stdio_seek and __stdio_close)
But these all end up becoming unnecessary and unwanted exports of the WASM binary. AFAICT, it's because of this: https://github.com/rust-lang/rust/issues/73958
I'm a bit stumped for the time being on how I can put into place some of the findings from the first wasi-sdk issue.
Geoff Goodman (Mar 21 2024 at 00:47):It seems like @Sam Clegg was pointing to __stderr_used as being some sort of strong reference preventing the LTO. I can see __stderr_used listed in wasi-sdk/share/wasi-sysroot/share/wasm32-wasi/defined-symbols.txt but no other mentions in the pre-built wasi-sdk package.
Maybe that's what was meant by needing to rebuild the SDK from source. I'm out of my depth but will give that a spin next.
Geoff Goodman (Mar 21 2024 at 13:31):I've found a sneaky call to printf in the subset of QuickJS I'm consuming but something sneaky is still pulling in stdout. I see the following in the generated function table:
$__stdio_write $__stdio_close $__stdout_write $__stdio_seek
I can't figure out how to trace calls to those and perhaps nor can the linker.
Joel Dice (Mar 21 2024 at 13:33):Have you tried using e.g. wasm-tools print to inspect the generated Wasm file and find out which functions are calling those imports?
Geoff Goodman (Mar 21 2024 at 13:38):That's how I've been doing most of my manual analysis. Those functions are never called directly, but I can't figure out if they might be called indirectly through br_table (if I'm even understanding wasi instructions correctly).
Dan Gohman (Mar 21 2024 at 13:39):You could try adding --trace-symbol=__stdio_write to the linker flags; that can sometimes identify where the reference is coming from.
Geoff Goodman (Mar 21 2024 at 13:41):Hey @Dan Gohman, sorry for the noob question but where would I put this flag? I have a gut feeling that this might be in the linking of the wasi sdk against one of quickjs's c files but not sure.
Dan Gohman (Mar 21 2024 at 13:42):It depends on how the quickjs build works. If there's a Makefile involved, you might be able to add -Wl,--trace-symbol=__stdio_write to something like LDFLAGS.
Dan Gohman (Mar 21 2024 at 13:44):If Rust is doing the linking, setting the env var RUSTFLAGS='-C link-args=--trace-symbol=__stdio_write'
Geoff Goodman (Mar 21 2024 at 13:45):I'll try both! :goofy:
Dan Gohman (Mar 21 2024 at 13:46):Oh, and I always forget how to tell cargo to not swallow linker diagnostic output.
Geoff Goodman (Mar 21 2024 at 13:47):I'm running with -vv but I'm not seeing anything that looks like obvious traces
Geoff Goodman (Mar 21 2024 at 13:48):LDFLAGS="-Wl,--trace-symbol=__stdio_write" \
RUSTFLAGS="-Zlocation-detail=none -C link-args=--trace-symbol=__stdio_write" \
WASI_SDK="$(pwd)/../wasi-sdk/build/wasi-sdk-21.0.0ga50a641f4b5a+m" \
cargo +nightly build -Z build-std=std,panic_abort -Z build-std-features=panic_immediate_abort --target wasm32-wasi --release -vv
I'll try with a symbol that is legitimately being pulled
Dan Gohman (Mar 21 2024 at 13:52):Ok, the full magic is RUSTC_LOG=rustc_codegen_ssa::back::link=info RUSTFLAGS='-C link-args=-Wl,--trace-symbol=__stdio_write' cargo build
Geoff Goodman (Mar 21 2024 at 13:53):Also seems I had the wrong delimiter in RUSTFLAGS
Geoff Goodman (Mar 21 2024 at 13:55):Hrm, it's complaining with this:
- = note: rust-lld: error: unknown argument: -Wl,--trace-symbol=__stdio_write
Ah, in that case, remove just the -Wl,
Dan Gohman (Mar 21 2024 at 13:57):(that's the flag needed when using clang as the "linker", which some things do)
Geoff Goodman (Mar 21 2024 at 13:57)::boom: we have liftoff. Here's what I'm seeing:
INFO rustc_codegen_ssa::back::link linker stderr:
INFO rustc_codegen_ssa::back::link linker stdout:
./wasi-sysroot/lib/wasm32-wasi/libc.a(__stdio_write.o): lazy definition of __stdio_write
./wasi-sysroot/lib/wasm32-wasi/libc.a(__stdout_write.o): reference to __stdio_write
./wasi-sysroot/lib/wasm32-wasi/libc.a(__stdio_write.o): definition of __stdio_write
./wasi-sysroot/lib/wasm32-wasi/libc.a(stderr.o): reference to __stdio_write
So that must be indicative of it being unable to optimize it away from the wasi-libc libc.a?
Dan Gohman (Mar 21 2024 at 13:59):The reference is coming from stderr.o, so maybe something is pulling in stderr. Try changing __stdio_write to stderr
Geoff Goodman (Mar 21 2024 at 14:01):Ahh... meeting time. Will have to dive back in a bit later but big thanks @Dan Gohman and @Joel Dice.
Now that you mention stderr, maybe it's back to the conclusions drawn in this comment: https://github.com/WebAssembly/wasi-sdk/issues/190#issuecomment-916783936
Dan Gohman (Mar 21 2024 at 14:05):That particular issue is specific to C++; is there C++ code in QuickJS?
Geoff Goodman (Mar 21 2024 at 15:21):Ah, gotcha. AFAICT it's 100% C.
Geoff Goodman (Mar 21 2024 at 15:22):INFO rustc_codegen_ssa::back::link linker stderr:
INFO rustc_codegen_ssa::back::link linker stdout:
.wasi-sysroot/lib/wasm32-wasi/libc.a(stderr.o): lazy definition of stderr
.wasi-sysroot/lib/wasm32-wasi/libc.a(stderr.o): definition of stderr
Hmm, maybe also try __stderr_FILE or __stderr_used
Geoff Goodman (Mar 21 2024 at 15:24):Both stderr.c and stdout.c in wasi-libc look very similar.
Geoff Goodman (Mar 21 2024 at 15:53):__stderr_used gives:
INFO rustc_codegen_ssa::back::link linker stderr:
INFO rustc_codegen_ssa::back::link linker stdout:
./wasi-sysroot/lib/wasm32-wasi/libc.a(__stdio_exit.o): lazy definition of __stderr_used
./wasi-sysroot/lib/wasm32-wasi/libc.a(__stdio_exit.o): definition of __stderr_used
./wasi-sysroot/lib/wasm32-wasi/libc.a(stderr.o): definition of __stderr_used
And __stderr_FILE:
INFO rustc_codegen_ssa::back::link linker stderr:
INFO rustc_codegen_ssa::back::link linker stdout:
./wasi-sysroot/lib/wasm32-wasi/libc.a(stderr.o): lazy definition of __stderr_FILE
./wasi-sysroot/lib/wasm32-wasi/libc.a(vfprintf.o): reference to __stderr_FILE
./wasi-sysroot/lib/wasm32-wasi/libc.a(stderr.o): definition of __stderr_FILE
./wasi-sysroot/lib/wasm32-wasi/libc.a(strtod.o): reference to __stderr_FILE
Quick Q: are there any asserts in the codebase? That can pull in stderr. Compiling with -DNDEBUG can disable that.
Dan Gohman (Mar 21 2024 at 15:56):Hrm, why does strtod.o reference __stderr_FILE?
Dan Gohman (Mar 21 2024 at 15:57):I'll be in a meeting for the next hour or so, but I'll do some investigation when I get back.
Geoff Goodman (Mar 21 2024 at 16:04):There are definitely asserts in QuickJS. I did something terrible to try to work around that here: https://github.com/ggoodman/rquickjs/commit/a95736a505d92219c2bad46664cd18ab88e81c13
Geoff Goodman (Mar 21 2024 at 16:13):I think this might be what is pulling it in: https://github.com/WebAssembly/wasi-libc/blob/6593687e25f07526c4b92a20fe5ddf507599d5b3/libc-top-half/headers/private/printscan.h#L46-L52
__attribute__((__cold__, __noreturn__))
static void long_double_not_supported(void) {
void abort(void) __attribute__((__noreturn__));
fputs("Support for formatting long double values is currently disabled.\n"
"To enable it, " __wasilibc_printscan_full_support_option ".\n", &__stderr_FILE);
abort();
}
AFAICT, the similar construct for floats is not being pulled in.
Geoff Goodman (Mar 21 2024 at 16:27):I tried ripping out the fputs calls in there in my local wasi-sdk build. Maybe holding it wrong but didn't see the reference disappear.
Geoff Goodman (Mar 21 2024 at 16:35):I think I'm either rebuilding the sdk incorrectly or I'm not correctly propagating its path to the rquickjs crate's build process.
Geoff Goodman (Mar 21 2024 at 16:53):I'm going to try a full rebuild of wasi-libc.
Geoff Goodman (Mar 21 2024 at 17:28):Almost there:
INFO rustc_codegen_ssa::back::link linker stderr:
INFO rustc_codegen_ssa::back::link linker stdout:
.wasi-sysroot/lib/wasm32-wasi/libc.a(stderr.o): lazy definition of __stderr_FILE
So AFAICT, that got rid of the two unexpected references to __stderr_FILE in vfprintf and strtod. They were both because of that pseudo-assertion around long long / long double support.
And yet, the wasi imports persist! They are stubborn.
Geoff Goodman (Mar 21 2024 at 19:04):I notice that in stdout.c in wasi-libc, we reference __stdout_write while in stderr.c, we reference __stdio_write. Could there be some circular reference here preventing LTO?
Compare:
Geoff Goodman (Mar 21 2024 at 19:11):Rebuilding wasi-libc and heating the room a bit.
Geoff Goodman (Mar 21 2024 at 19:24):Got rid of the strong reference but still have the imports sneaking in.
Geoff Goodman (Mar 21 2024 at 19:49):Pretty confused by this asymmetry:
./wasi-sysroot/lib/wasm32-wasi/libc.a(stderr.o): lazy definition of stderr
./wasi-sysroot/lib/wasm32-wasi/libc.a(stdout.o): definition of stdout
stderr is never buffered, while stdout is buffered by default. __stdout_write has extra code for handling line-buffering if it's attached to a terminal.
Geoff Goodman (Mar 21 2024 at 19:58):Here I am coming through with an axe when a scalpel is needed. Here's my hack-job so far: https://github.com/WebAssembly/wasi-libc/compare/main...ggoodman:wasi-libc:prune_stdio
Geoff Goodman (Mar 21 2024 at 20:24):@Dan Gohman do you have a hypothesis as to why one would be lazy and the other not?
Dan Gohman (Mar 21 2024 at 20:29):Not offhand. "lazy" here is about wasm-ld's handling of archive files, where .o files are only pulled in if a symbol in them is defined, but I don't know the specifics.
Geoff Goodman (Mar 21 2024 at 21:33):Out of curiosity, is it within reason to compile against the wasi-libc source instead of built objects?
Dan Gohman (Mar 21 2024 at 21:33):There aren't Makefiles set up to work that way, unfortunately.
Dan Gohman (Mar 21 2024 at 21:33):If there were, then it'd be reasonable :-)
Geoff Goodman (Mar 21 2024 at 21:35):Gotcha. I've spent my novelty tokens and taken a line of credit already.
Dan Gohman (Mar 21 2024 at 21:36):Do you have code you could publish somewhere, so I could take a look?
Geoff Goodman (Mar 21 2024 at 21:37):Good idea. I'm not sure how to get my wasi-libc fork linked up to my rust project in a consumable way though.
Geoff Goodman (Mar 21 2024 at 21:43):I'm working from the bridge branch in https://github.com/ggoodman/quicky/tree/bridge. That should already be linked to my fork of rquickjs but the WASI_SDK reference in .cargo/config.toml is relative to my local check-out.
Geoff Goodman (Mar 21 2024 at 21:44):I've been using this script to help w/ the cycle time between shots in the dark :joy: https://github.com/ggoodman/quicky/blob/4d2d859c765bb337a9791a4e1b2dc367925d5916/scripts/build.sh#L6-L8
Geoff Goodman (Mar 21 2024 at 21:47):My fork of wasi-sdk: https://github.com/ggoodman/wasi-sdk/tree/strip-wasi-stdio
Points to wasi-libc that has these changes: https://github.com/WebAssembly/wasi-libc/compare/main...ggoodman:wasi-libc:prune_stdio
Geoff Goodman (Mar 21 2024 at 21:47):That would be a lot of work to repro. I might be able to get a setup where only quicky needs to be pulled.
Geoff Goodman (Mar 22 2024 at 01:21):The workflow is now described and show be reproducible here: https://github.com/ggoodman/quicky/tree/bridge
Geoff Goodman (Mar 22 2024 at 02:14):A new potential lead is __stdio_exit that I'm seeing getting linked. It seems like __toread and __towrite might both cause those weak references to become strong references:
/* atexit.c and __stdio_exit.c override these. the latter is linked
* as a consequence of linking either __toread.c or __towrite.c. */
weak_alias(dummy, __funcs_on_exit);
weak_alias(dummy, __stdio_exit);
I'm seeing this:
.../wasm32-wasi/libc.a(__stdio_close.o): lazy definition of __stdio_close
.../wasm32-wasi/libc.a(__stdio_seek.o): lazy definition of __stdio_seek
.../wasm32-wasi/libc.a(__stdio_write.o): lazy definition of __stdio_write
.../wasm32-wasi/libc.a(__toread.o): lazy definition of __toread
.../wasm32-wasi/libc.a(__towrite.o): lazy definition of __towrite
.../wasm32-wasi/libc.a(__overflow.o): reference to __towrite
.../wasm32-wasi/libc.a(__towrite.o): definition of __towrite
.../wasm32-wasi/libc.a(fwrite.o): reference to __towrite
.../wasm32-wasi/libc.a(stderr.o): lazy definition of stderr
.../wasm32-wasi/libc.a(stdout.o): reference to __stdio_close
.../wasm32-wasi/libc.a(__stdio_close.o): definition of __stdio_close
.../wasm32-wasi/libc.a(stdout.o): reference to __stdio_write
.../wasm32-wasi/libc.a(__stdio_write.o): definition of __stdio_write
.../wasm32-wasi/libc.a(stdout.o): reference to __stdio_seek
.../wasm32-wasi/libc.a(__stdio_seek.o): definition of __stdio_seek
.../wasm32-wasi/libc.a(stdout.o): definition of stdout
.../wasm32-wasi/libc.a(vfprintf.o): reference to __towrite
.../wasm32-wasi/libc.a(__uflow.o): reference to __toread
.../wasm32-wasi/libc.a(__toread.o): definition of __toread
One nuance is that this project is designed to be run through wizer. I only see an obvious reference to __stdio_exit here:
(func $__wasm_call_dtors (;630;) (type 72)
call $#func629<dummy>
call $__stdio_exit
)
(func $init.command_export (;631;) (type 31) (result i32)
call $init
call $__wasm_call_dtors
)
Is there a way to elect out of calling destructors?
Geoff Goodman (Mar 22 2024 at 13:42):Lots of learning for me going down this rabbit hole but now I'm hoping to rein it back in. I think it's probably worth trying to come up with a minimal repro here. Any hypotheses on the shape of a minimal c library I can wrap in rust bindings to produce what we're seeing here?
Geoff Goodman (Mar 22 2024 at 17:24):OK @Dan Gohman I think I have a pretty sweet, mostly minimal repro here: https://github.com/ggoodman/wasi-import-repro
The key finding is that the very presence of a printf call in a c library results in those fd_* imports getting pulled along even if we _know_ that the code calling printf is being eliminated by the linker.
Geoff Goodman (Mar 22 2024 at 17:33):This is showing that a rust function that references a c binding referencing a call to printf is enough, even if the rust function is omitted by a feature flag.
Geoff Goodman (Mar 22 2024 at 17:52):Filed an issue here: https://github.com/WebAssembly/wasi-sdk/issues/401
Geoff Goodman (May 09 2024 at 15:46):Thinking more about this, I have the hypothesis that it's the finalizers that could be holding onto strong references.
That made me wonder (at least for my use-case) if I actually _cared_ about finalizers and clean-up. The main use-case I have at work uses a WASM instance exactly once and throws it away. All work after producing a 'result' is essentially wasted cycles as far as I'm concerned.
So I wonder if tooling could be parametrized for this sort of use-case... Could we tell the wasi-sdk / wasi-libc that we don't actually care about clean-up? In other words, the host and runtime can handle that for us. I understand that this wouldn't be a general-purpose strategy which is why I'm wondering about some opt-in behaviour. Does it sound feasible?
Joel Dice (May 09 2024 at 15:53):AFAIK, wasi-libc generally won't clean anything up unless you tell it to (e.g. by closing a file descriptor). So I think the answer in your case is: "don't call close" (which at the Rust level means e.g. wrapping a File in ManuallyDrop and never dropping it).
Joel Dice (May 09 2024 at 15:55):But maybe you're asking for an optional build of wasi-libc where close is a no-op?
Geoff Goodman (May 10 2024 at 19:09):I think the solution when using rust is a bit easier but the use-case that confounds me is when building a c library against wasi-libc--and especially one whose source code I don't control.
Last updated: Oct 23 2024 at 20:03 UTC