`STACK_OVERFLOW_CHECK` does not work with pthreads

[stevenwdv]

-sSTACK_OVERFLOW_CHECK does not work in a pthread. Instead of a clear "Stack overflow!", you get a random error ("memory access out of bounds" / "table index out of bounds" / "null function or function signature mismatch" / "unreachable executed" / etc.). This makes troubleshooting way harder.

Is this a known limitation? If so, I think it should be documented.

Version of emscripten/emsdk:

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 6.0.2 (7a2d97d)

Failing command line in full:

stack-overflow.cpp:

#include <iostream>
#include <thread>

static void stackOverflow(unsigned d) {
  std::cout << 'a';
  if (!d--) return;
  stackOverflow(d);
  std::cout << 'b';
}

int main() {
  std::jthread([] {
    stackOverflow(20'000);
  });
}

We compile with STACK_OVERFLOW_CHECK (implicit without optimization):

$ em++ ./stack-overflow.cpp -o ./stack-overflow -pthread -std=c++20 -v
 "/home/swdv/emsdk/upstream/bin/clang++" -target wasm32-unknown-emscripten -fignore-exceptions -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=/home/swdv/emsdk/upstream/emscripten/cache/sysroot -D__EMSCRIPTEN_SHARED_MEMORY__=1 -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -pthread -std=c++20 -v -c ./stack-overflow.cpp -o /tmp/emscripten_temp_clixitb0/stack-overflow.o
clang version 23.0.0git (https:/github.com/llvm/llvm-project 787619a4072e0eb7887357d5d284e86c17548aed)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /home/swdv/emsdk/upstream/bin
 (in-process)
 "/home/swdv/emsdk/upstream/bin/clang-23" -cc1 -triple wasm32-unknown-emscripten -emit-obj -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name stack-overflow.cpp -mrelocation-model static -mframe-pointer=none -ffp-contract=on -fno-rounding-math -mconstructor-aliases -target-feature +atomics -target-feature +bulk-memory -target-feature +mutable-globals -target-feature +sign-ext -target-cpu generic -fvisibility=hidden -debugger-tuning=gdb -fdebug-compilation-dir=/home/swdv/Downloads -v -fcoverage-compilation-dir=/home/swdv/Downloads -resource-dir /home/swdv/emsdk/upstream/lib/clang/23 -D __EMSCRIPTEN_SHARED_MEMORY__=1 -isysroot /home/swdv/emsdk/upstream/emscripten/cache/sysroot -internal-isystem /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/noeh/c++/v1 -internal-isystem /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1 -internal-isystem /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1 -internal-isystem /home/swdv/emsdk/upstream/lib/clang/23/include -internal-isystem /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten -internal-isystem /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include -std=c++20 -fdeprecated-macro -ferror-limit 19 -fmessage-length=214 -pthread -fgnuc-version=4.2.1 -fno-implicit-modules -fskip-odr-check-in-gmf -fcxx-exceptions -fignore-exceptions -fexceptions -fcolor-diagnostics -iwithsysroot/include/fakesdl -iwithsysroot/include/compat -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -o /tmp/emscripten_temp_clixitb0/stack-overflow.o -x c++ ./stack-overflow.cpp
clang -cc1 version 23.0.0git based upon LLVM 23.0.0git default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/noeh/c++/v1"
ignoring nonexistent directory "/home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1"
ignoring nonexistent directory "/home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten"
#include "..." search starts here:
#include <...> search starts here:
 /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/fakesdl
 /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/compat
 /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1
 /home/swdv/emsdk/upstream/lib/clang/23/include
 /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include
End of search list.
 /home/swdv/emsdk/upstream/bin/clang --version
 /home/swdv/emsdk/upstream/bin/wasm-ld -o ./stack-overflow.wasm /tmp/tmphvot3yl6libemscripten_js_symbols.so -Bstatic --import-memory --shared-memory --strip-debug --export=emscripten_stack_get_end --export=emscripten_stack_get_free --export=emscripten_stack_get_base --export=emscripten_stack_get_current --export=emscripten_stack_init --export=_emscripten_stack_alloc --export=_emscripten_thread_free_data --export=_emscripten_thread_crashed --export=__wasm_call_ctors --export=_emscripten_tls_init --export=_emscripten_thread_init --export=pthread_self --export=__set_thread_state --export=_emscripten_stack_restore --export=emscripten_stack_set_limits --export=_emscripten_thread_exit --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 --export-table -z stack-size=65536 --no-growable-memory --initial-memory=16777216 --no-entry --stack-first --table-base=1 /tmp/emscripten_temp_clixitb0/stack-overflow.o -L/home/swdv/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten -L/home/swdv/emsdk/upstream/emscripten/src/lib /home/swdv/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/crtbegin-mt.o -lGL-mt-getprocaddr -lal -lhtml5 -lstubs-debug -lnoexit -lc-mt-debug -ldlmalloc-mt-debug -lclang_rt.builtins-mt -lc++-debug-mt-noexcept -lc++abi-debug-mt-noexcept -lsockets-mt -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr
 /home/swdv/emsdk/upstream/bin/llvm-objcopy ./stack-overflow.wasm ./stack-overflow.wasm '--remove-section=llvm.func_attr.annotate.*' --remove-section=producers --remove-section=name '--remove-section=.debug*'

$ ./stack-overflow
worker: onmessage() captured an uncaught exception: RuntimeError: memory access out of bounds
Aborted(native code called abort())
RuntimeError: memory access out of bounds
    at wasm://wasm/000caaea:wasm-function[600]:0x115e1
    at wasm://wasm/000caaea:wasm-function[599]:0x11505
    at wasm://wasm/000caaea:wasm-function[166]:0x4683
    at wasm://wasm/000caaea:wasm-function[162]:0x455d
    at wasm://wasm/000caaea:wasm-function[157]:0x420a
    at wasm://wasm/000caaea:wasm-function[156]:0x40e7
    at wasm://wasm/000caaea:wasm-function[155]:0x4069
    at wasm://wasm/000caaea:wasm-function[155]:0x4091
    at wasm://wasm/000caaea:wasm-function[155]:0x4091
    at wasm://wasm/000caaea:wasm-function[155]:0x4091
/home/swdv/Downloads/stack-overflow:834
  var e = new WebAssembly.RuntimeError(what);
          ^

RuntimeError: Aborted(native code called abort())
    at abort (/home/swdv/Downloads/stack-overflow:834:11)
    at __abort_js (/home/swdv/Downloads/stack-overflow:1716:7)
    at wasm://wasm/000caaea:wasm-function[386]:0x8d50
    at wasm://wasm/000caaea:wasm-function[429]:0xd564
    at wasm://wasm/000caaea:wasm-function[318]:0x7a40
    at /home/swdv/Downloads/stack-overflow:848:12
    at Object.returnWorkerToPool (/home/swdv/Downloads/stack-overflow:1282:9)
    at cleanupThread (/home/swdv/Downloads/stack-overflow:1063:15)
    at __emscripten_thread_cleanup (/home/swdv/Downloads/stack-overflow:1894:36)
    at wasm://wasm/000caaea:wasm-function[1270]:0x2d1be

Node.js v22.16.0

This also happens with -sPROXY_TO_PTHREAD.

Compare with what happens without the thread:

./stack-overflow
Aborted(Stack overflow! Stack cookie has been overwritten at 0x00000004, expected hex dwords 0x89BACDFE and 0x2135467, but received 0x00000000 0x00000000)
/home/swdv/Downloads/stack-overflow:834
  var e = new WebAssembly.RuntimeError(what);
          ^

RuntimeError: Aborted(Stack overflow! Stack cookie has been overwritten at 0x00000004, expected hex dwords 0x89BACDFE and 0x2135467, but received 0x00000000 0x00000000)
    at abort (/home/swdv/Downloads/stack-overflow:834:11)
    at checkStackCookie (/home/swdv/Downloads/stack-overflow:364:5)
    at handleException (/home/swdv/Downloads/stack-overflow:1691:7)
    at callMain (/home/swdv/Downloads/stack-overflow:5539:12)
    at run (/home/swdv/Downloads/stack-overflow:5590:22)
    at /home/swdv/Downloads/stack-overflow:5642:25

Node.js v22.16.0

[sbc100]

Building with -sSTACK_OVERFLOW_CHECK only injects checks at key points, such as after main run in the above stack trace involving callMain.

If you want to synchronously detect stack overflows as they happen you can build with more expensive -sSTACK_OVERFLOW_CHECK=2 which annotates all stack manipulation ops in binary and does catch the stack overflow on the thread in the above program.

[sbc100]

See https://emscripten.org/docs/tools_reference/settings_reference.html#stack-overflow-check

[sbc100]

The reason that -sSTACK_OVERFLOW_CHECK=1 doesn't really work in your case is that in this most the stack cookie/marker is only checked when the thread starts and then it ends. However, because the threads stack itself is stored on the heap, when it does this kind of egregious overflow is likely the corrupt other heap structures before the thread get a chance to return from its main entry point (i.e. before the check is performed).

We could improve this situation by increasing the buffer zone around each pthread's stack space (especially when building in -sSTACK_OVERFLOW_CHECK mode).