wasm-ld crash in side module with wasm-exceptions and flto

[gl84]

I want to compile the png library and a small c++ wrapper as a loadable side module. The linking step fails with a wasm-ld crash.

Version of emscripten/emsdk:

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 5.0.0 (a7c5deabd7c88ba1c38ebe988112256775f944c6)
clang version 23.0.0git (https:/github.com/llvm/llvm-project 358db292cc6a9a8a5448a296f643312289f328d7)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /home/playground/emsdk_5.0.0/emsdk/upstream/bin

The crash can be reproduced without the png (and zlib) libraries. Simply create a file pngerror.c with the following content :

#include "setjmp.h"

extern void call_free(void* ptr);

void png_free_jmpbuf(jmp_buf *jb) {
      call_free(jb); 
      jmp_buf free_jmp_buf;
      setjmp(free_jmp_buf);
}

and compile it with

emcc -flto -O0 -fPIC -o pngerror.c.o -c pngerror.c && em++ -o libpng.so -sSIDE_MODULE=1 -fwasm-exceptions pngerror.c.o

This results in the following crash:

LLVM ERROR: Cannot select: 0x5cb09e43b200: ch = catchret 0x5cb09e402fa8, BasicBlock:ch< 0x5cb09e434528>, BasicBlock:ch< 0x5cb09e4343c8>
In function: png_free_jmpbuf
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and instructions to reproduce the bug.
Stack dump:
0.      Program arguments: /home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld -o libpng.so --import-memory --strip-debug --export-dynamic --export=__wasm_call_ctors --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=main --export-if-defined=__main_argc_argv --export-if-defined=fflush --experimental-pic --unresolved-symbols=import-dynamic -shared --stack-first --whole-archive pngerror.c.o -L/home/playground/emsdk_5.0.0/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/pic -L/home/playground/emsdk_5.0.0/emsdk/upstream/emscripten/src/lib --no-whole-archive -mllvm -combiner-global-alias-analysis=false -mllvm -wasm-enable-sjlj -mllvm -wasm-use-legacy-eh -mllvm -disable-lsr -mllvm -wasm-enable-eh -mllvm -wasm-use-legacy-eh -mllvm -exception-model=wasm
1.      Running pass 'Function Pass Manager' on module 'ld-temp.o'.
2.      Running pass 'WebAssembly Instruction Selection' on function '@png_free_jmpbuf'
 #0 0x00005cb079fbb0e8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x11560e8)
 #1 0x00005cb079fb7e25 llvm::sys::RunSignalHandlers() (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x1152e25)
 #2 0x00005cb079fbbc96 SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
 #3 0x00007352b6645330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
 #4 0x00007352b669eb2c __pthread_kill_implementation ./nptl/pthread_kill.c:44:76
 #5 0x00007352b669eb2c __pthread_kill_internal ./nptl/pthread_kill.c:78:10
 #6 0x00007352b669eb2c pthread_kill ./nptl/pthread_kill.c:89:10
 #7 0x00007352b664527e raise ./signal/../sysdeps/posix/raise.c:27:6
 #8 0x00007352b66288ff abort ./stdlib/abort.c:81:7
 #9 0x00005cb079f18216 llvm::report_fatal_error(llvm::Twine const&, bool) (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x10b3216)
#10 0x00005cb07aefe4dc llvm::SelectionDAGISel::CannotYetSelect(llvm::SDNode*) (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x20994dc)
#11 0x00005cb07aefda55 (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x2098a55)
#12 0x00005cb07a5c0f7e (anonymous namespace)::WebAssemblyDAGToDAGISel::Select(llvm::SDNode*) (.llvm.406094044484436182) WebAssemblyISelDAGToDAG.cpp:0:0
#13 0x00005cb07aef3e5c llvm::SelectionDAGISel::DoInstructionSelection() (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x208ee5c)
#14 0x00005cb07aef3330 llvm::SelectionDAGISel::CodeGenAndEmitDAG() (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x208e330)
#15 0x00005cb07aef28ad llvm::SelectionDAGISel::SelectAllBasicBlocks(llvm::Function const&) (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x208d8ad)
#16 0x00005cb07aeef1e7 llvm::SelectionDAGISel::runOnMachineFunction(llvm::MachineFunction&) (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x208a1e7)
#17 0x00005cb07aeec4a4 llvm::SelectionDAGISelLegacy::runOnMachineFunction(llvm::MachineFunction&) (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x20874a4)
#18 0x00005cb07b254723 llvm::MachineFunctionPass::runOnFunction(llvm::Function&) (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x23ef723)
#19 0x00005cb07ce6f393 llvm::FPPassManager::runOnFunction(llvm::Function&) (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x400a393)
#20 0x00005cb07ce7756c llvm::FPPassManager::runOnModule(llvm::Module&) (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x401256c)
#21 0x00005cb07ce70244 llvm::legacy::PassManagerImpl::run(llvm::Module&) (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x400b244)
#22 0x00005cb07aadf2f9 codegen(llvm::lto::Config const&, llvm::TargetMachine*, std::__2::function<llvm::Expected<std::__2::unique_ptr<llvm::CachedFileStream, std::__2::default_delete<llvm::CachedFileStream>>> (unsigned int, llvm::Twine const&)>, unsigned int, llvm::Module&, llvm::ModuleSummaryIndex const&) LTOBackend.cpp:0:0
#23 0x00005cb07aaddc3d llvm::lto::backend(llvm::lto::Config const&, std::__2::function<llvm::Expected<std::__2::unique_ptr<llvm::CachedFileStream, std::__2::default_delete<llvm::CachedFileStream>>> (unsigned int, llvm::Twine const&)>, unsigned int, llvm::Module&, llvm::ModuleSummaryIndex&) (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x1c78c3d)
#24 0x00005cb07aabfe0e llvm::lto::LTO::runRegularLTO(std::__2::function<llvm::Expected<std::__2::unique_ptr<llvm::CachedFileStream, std::__2::default_delete<llvm::CachedFileStream>>> (unsigned int, llvm::Twine const&)>) (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x1c5ae0e)
#25 0x00005cb07aabeea7 llvm::lto::LTO::run(std::__2::function<llvm::Expected<std::__2::unique_ptr<llvm::CachedFileStream, std::__2::default_delete<llvm::CachedFileStream>>> (unsigned int, llvm::Twine const&)>, llvm::FileCache) (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x1c59ea7)
#26 0x00005cb07a4d6e58 lld::wasm::BitcodeCompiler::compile() (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x1671e58)
#27 0x00005cb07a4dad34 lld::wasm::SymbolTable::compileBitcodeFiles() (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x1675d34)
#28 0x00005cb07a4c0601 lld::wasm::(anonymous namespace)::LinkerDriver::linkerMain(llvm::ArrayRef<char const*>) Driver.cpp:0:0
#29 0x00005cb07a4bae7a lld::wasm::link(llvm::ArrayRef<char const*>, llvm::raw_ostream&, llvm::raw_ostream&, bool, bool) (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x1655e7a)
#30 0x00005cb079fbfc2c lld::unsafeLldMain(llvm::ArrayRef<char const*>, llvm::raw_ostream&, llvm::raw_ostream&, llvm::ArrayRef<lld::DriverDef>, bool) (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x115ac2c)
#31 0x00005cb079ef47f1 lld_main(int, char**, llvm::ToolContext const&) (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x108f7f1)
#32 0x00005cb079ef4f14 main (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0x108ff14)
#33 0x00007352b662a1ca __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:74:3
#34 0x00007352b662a28b call_init ./csu/../csu/libc-start.c:128:20
#35 0x00007352b662a28b __libc_start_main ./csu/../csu/libc-start.c:347:5
#36 0x00005cb079e5b22a _start (/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld+0xff622a)
em++: error: '/home/playground/emsdk_5.0.0/emsdk/upstream/bin/wasm-ld -o libpng.so --import-memory --strip-debug --export-dynamic --export=__wasm_call_ctors --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=main --export-if-defined=__main_argc_argv --export-if-defined=fflush --experimental-pic --unresolved-symbols=import-dynamic -shared --stack-first --whole-archive pngerror.c.o -L/home/playground/emsdk_5.0.0/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/pic -L/home/playground/emsdk_5.0.0/emsdk/upstream/emscripten/src/lib --no-whole-archive -mllvm -combiner-global-alias-analysis=false -mllvm -wasm-enable-sjlj -mllvm -wasm-use-legacy-eh -mllvm -disable-lsr -mllvm -wasm-enable-eh -mllvm -wasm-use-legacy-eh -mllvm -exception-model=wasm' failed (received SIGABRT (-6))

Removing one of -flto, -fPIC or -fwasm-exceptions doesn't result in a crash...

[sbc100]

Can confirm this crash. Looks like some kind bug in instruction selections, but its strange that it only happens with LTO.

Would you mind open an LLVM issue about this (seems like it must be an LLVM bug)?

@aheejin @dschuff any ideas?

[aheejin]

You need to use -fwasm-exceptions in both compile and link time:

emcc -fwasm-exceptions -flto -O0 -fPIC -o pngerror.c.o -c pngerror.c && em++ -o libpng.so -sSIDE_MODULE=1 -fwasm-exceptions pngerror.c.o

[sbc100]

Is there some way the -fwasm-exceptions should be stored in the bitcode? Or at least some way to give a better error message here?

Crash in isel is quite a confusing failure mode I think.

[aheejin]

I agree, but I'm not sure what would be an easy solution for this. The reason this fails is, unless you give the right option to clang, the target features attribute is not gonna set up right. For example, if you compile with -fwasm-exceptions, functions in the bitcode will have +exception-handling in its target-features attribute:

attributes #0 = { noinline nounwind optnone "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="generic" "target-features"="+bulk-memory,+bulk-memory-opt,+call-indirect-overlong,+exception-handling,+multivalue,+mutable-globals,+nontrapping-fptoint,+reference-types,+sign-ext" }

But if you don't compile with -fwasm-exceptions the attribute will not be there.

Giving -fwasm-exceptions at linking time will run the SjLj transformation using Wasm EH instructions in the LLVM backend. (The pass name is LowerEmscriptenEHSjLj for a historical reason, which is confusing, but it does Wasm SjLj transformation too) And this generates EH IR instructions like catchret, which need exception-handling target feature to be instruction-selected.

One thought. Maybe, in that transformation, we can check if the function has the exception-handling attribute, and if not, we emit a warning and don't do the transformation, even if -fwasm-exception is given at link time. In this case the resulting code won't have the SjLj handling, but at least the warning can be helpful.

[aheejin]

One thought. Maybe, in that transformation, we can check if the function has the exception-handling attribute, and if not, we emit a warning and don't do the transformation, even if -fwasm-exception is given at link time. In this case the resulting code won't have the SjLj handling, but at least the warning can be helpful.

Tried it: llvm/llvm-project#178377

[gl84]

A small note for myself: Linking C and C++ (with -fwasm-exceptions) code requires the C part to be compiled with SUPPORT_LONGJMP=wasm, which isn't the default.

[dschuff]

A small note for myself: Linking C and C++ (with -fwasm-exceptions) code requires the C part to be compiled with SUPPORT_LONGJMP=wasm, which isn't the default.

Yes, this is documented in the page on SJLJ. (The page on EH has a link to it. Do let us know if you have suggestions for improving the docs.

[gl84]

The documentation is clear, I simply had no idea that the png library was using setjmp internally...