Thread local storage link issue with Rust/wasm-ld/Emscripten

[gregbuchholz]

I am trying to narrow down the cause of a problem which occurs when trying to enable threads on a Rust program compiling to the wasm32-unknown-emscripten target. It compiles, but fails with a wasm-ld linker error. I have a small github repository with the source files and the capture of the error messages. I've extracted the failing wasm-ld command that yields the error, an placed it in the file linker_command.sh, which I've tried to ensure you can use without having to deal with Rust (or even have Rust installed). All of the compiled object files are included in the repository. The error can be seen in the snippet below, with additional information provided by the --verbose option. (Before running the script, you will have to set a couple of variable at the top of the script to point to your installation of emsdk and wasm-ld)

<snip a lot of "Processing" lines>

lld: -- createOutputSegments
lld: -- createSyntheticSections
lld: -- layoutMemory
lld: mem: global base = 1024
lld: mem: .tdata          offset=1024     size=72       align=2
lld: mem: .rodata         offset=1104     size=41484    align=4
lld: mem: .data           offset=42592    size=348      align=3
lld: mem: .bss            offset=42944    size=3184     align=4
lld: mem: __wasm_init_memory_flag offset=46128    size=4        align=4
lld: mem: static data = 45108
lld: mem: stack size  = 5242880
lld: mem: stack base  = 46144
lld: mem: stack top   = 5289024
lld: mem: heap base   = 5289024
lld: mem: total pages = 256
lld: mem: max pages   = 256
lld: -- createSyntheticSectionsPostLayout
lld: -- populateProducers
lld: -- calculateImports
lld: -- scanRelocations
lld: error: /home/greg/rust-examples/tls/target/wasm32-unknown-emscripten/debug/deps/Example.1b0wam0nd5eghoka.rcgu.o: 
    relocation R_WASM_MEMORY_ADDR_TLS_SLEB cannot be used against non-TLS symbol `Example::VAR1::__getit::__KEY::h9fd2473e7b8be3ac`

<snip more errors of the same nature>

lld: -- finalizeIndirectFunctionTable
lld: -- populateTargetFeatures
lld: -- createSyntheticInitFunctions
lld: -- assignIndexes
lld: -- calculateInitFunctions

...note the lack of a .tbss section reported above, but there is a .tdata and a .bss section. The error message indicates that the symbols in question are not marked as thread local storage, but when using wasm-objdump, it seems like there are signs that the symbols are a part of thread local storage. With the -x option (raw output in dump-x.txt and dump-x_rustfilt.txt):

$ wasm-objdump -x ./target/wasm32-unknown-emscripten/debug/deps/Example.1b0wam0nd5eghoka.rcgu.o | rustfilt

Data[14]:
<snip>
 - segment[13] <.tbss.Example::VAR1::__getit::__KEY> memory=0 size=12 - init i32=156
  - 000009c: 0000 0000 0000 0000 0000 0000            ............
<snip>
Custom:
 - name: "linking"
  - symbol table [count=52]
<snip>
   - 37: D <Example::VAR1::__getit::__KEY> segment=13 offset=0 size=12 [ binding=local vis=default ]
<snip>
  - segment info [count=14]
   - 0: .rodata..L__unnamed_8 p2align=0 [ ]
   - 1: .rodata..L__unnamed_2 p2align=2 [ ]
   - 2: .rodata..L__unnamed_3 p2align=2 [ ]
   - 3: .rodata..L__unnamed_9 p2align=0 [ ]
   - 4: .rodata..L__unnamed_10 p2align=0 [ ]
   - 5: .rodata..L__unnamed_4 p2align=2 [ ]
   - 6: .rodata..L__unnamed_1 p2align=2 [ ]
   - 7: .rodata..L__unnamed_11 p2align=0 [ ]
   - 8: .rodata..L__unnamed_5 p2align=2 [ ]
   - 9: .rodata..L__unnamed_12 p2align=0 [ ]
   - 10: .rodata..L__unnamed_6 p2align=2 [ ]
   - 11: .rodata..L__unnamed_13 p2align=0 [ ]
   - 12: .rodata..L__unnamed_7 p2align=2 [ ]
   - 13: .tbss.Example::VAR1::__getit::__KEY p2align=2 [ TLS ]
Custom:
 - name: "reloc.CODE"
  - relocations for section: 5 (Code) [70]
<snip>
   - R_WASM_MEMORY_ADDR_TLS_SLEB offset=0x000816(file=0x000e9a) symbol=37 <Example::VAR1::__getit::__KEY>
<snip>

 - name: "reloc..debug_info"
  - relocations for section: 8 (.debug_info) [242]
 <snip>
   - R_WASM_MEMORY_ADDR_I32 offset=0x000043(file=0x003910) symbol=37 <Example::VAR1::__getit::__KEY>

...you will notice segment 13 has both .tbss and [TLS] mentioned and with wasm-objdump -d -r (dump-d.txt and dump-d_rustfilt.txt):

$ wasm-objdump -d -r ./target/wasm32-unknown-emscripten/debug/deps/Example.1b0wam0nd5eghoka.rcgu.o | rustfilt

<snip>
000e99: 41 9c 81 80 80 00          | i32.const 156
          000e9a: R_WASM_MEMORY_ADDR_TLS_SLEB 37 <Example::VAR1::__getit::__KEY>
000e9f: 21 00                      | local.set 0
000ea1: 23 81 80 80 80 00          | global.get 1 <env.__tls_base>
          000ea2: R_WASM_GLOBAL_INDEX_LEB 18 <env.__tls_base>

(where the files ending in rustfilt have been passed through the rust name demangler).

The wasm-ld error message comes from scanRelocations function in Relocations.cpp at line #120.

That function uses the isTLS() method, which uses WASM_SYMBOL_TLS. There are only a select few places where that flag appears:

/emsdk/llvm/git/src$ find . -type f \( -name "*.h" -o -name "*.cpp" \) -exec grep -H  WASM_SYMBOL_TLS {} \;
  ./lld/wasm/Symbols.cpp:bool Symbol::isTLS() const { return flags & WASM_SYMBOL_TLS; }
  ./lld/wasm/InputFiles.cpp:      flags |= WASM_SYMBOL_TLS;
  ./llvm/include/llvm/BinaryFormat/Wasm.h:const unsigned WASM_SYMBOL_TLS = 0x100;
  ./llvm/include/llvm/MC/MCSymbolWasm.h:  bool isTLS() const { return getFlags() & wasm::WASM_SYMBOL_TLS; }
  ./llvm/include/llvm/MC/MCSymbolWasm.h:    modifyFlags(wasm::WASM_SYMBOL_TLS, wasm::WASM_SYMBOL_TLS);
  ./llvm/lib/MC/WasmObjectWriter.cpp:      Flags |= wasm::WASM_SYMBOL_TLS;

...the one from InputFiles.cpp is part of the ObjFile::createDefined method which has the following:

      case WASM_SYMBOL_TYPE_DATA: {
        InputChunk *seg = segments[sym.Info.DataRef.Segment];
        auto offset = sym.Info.DataRef.Offset;
        auto size = sym.Info.DataRef.Size;
        if (seg->implicitTLS) {
          flags |= WASM_SYMBOL_TLS;
    }

...and when you look at where seg->implicitTLS gets set:

    // Older object files did not include WASM_SEG_FLAG_TLS and instead
    // relied on the naming convention.  To maintain compat with such objects
    // we still imply the TLS flag based on the name of the segment.
    if (!seg->isTLS() &&
        (seg->name.startswith(".tdata") || seg->name.startswith(".tbss"))) {
      seg->flags |= WASM_SEG_FLAG_TLS;
      seg->implicitTLS = true;
    }

...but when stepping through the code with the debugger, I see that seg->isTLS() appears to be true for the segment that the Example::VAR1::__getit::__KEY::h9fd2473e7b8be3ac symbol is in, so it doesn't set the implicitTLS flag, and thus presumably wouldn't set the flags |= WASM_SYMBOL_TLS; and therefore isTLS() wouldn't ever be true on the symbol itself, and you always would get the linker error in Relocations.cpp. The "flags" for that symbol are 2 as can be seen from the screenshot above. But the "always failing" assumption must be false, since Emscripten can work with thread local data, as shown by this C program, which does essentially the same thing as the Rust program, and uses both .tdata and .tbss sections without issue (see the example assembly below) (build by using make wasm in that directory (and see the other targets)).

    .type	variable1,@object               # @variable1
    .section	.tdata.variable1,"T",@
    .globl	variable1
    .p2align	2
variable1:
    .int32	1                               # 0x1
    .size	variable1, 4

    .type	variable2,@object               # @variable2
    .section	.tbss.variable2,"T",@
    .globl	variable2
    .p2align	2
variable2:
    .int32	0                               # 0x0
    .size	variable2, 4

...and the fact that there are other symbols that are marked as "R_WASM_MEMORY_ADDR_TLS_SLEB" in the rust originated object file, but that don't report linking problems. From the '-x' wasm-objdump file:

   - 17: D <__THREW__> [ undefined binding=global vis=default ]
   <snip>
    Custom:
     - name: "reloc.CODE"
      - relocations for section: 5 (Code) [70]
       - R_WASM_MEMORY_ADDR_TLS_SLEB offset=0x0006c5(file=0x000d49) symbol=17 <__THREW__>

I thought it might be related to the this issue
which sounds like there was a problem because the debug symbols weren't part of the thread local storage, but the same link error occurs when building without debugging symbols. (The object files for the --release build are here as well).

Anyone have thoughts on a good place to start diving into this issue more? Would it be more productive to try and determine why the --verbose output of wasm-ld doesn't have the .tbss section listed? If there is another more appropriate place to ask about this issue, please let me know. It seems like it may be sort of a cross-cutting issue.

The versions of the various programs:

$ wasm-ld --version
LLD 14.0.0

$ wasm-objdump --version
1.0.25

$ emcc -v
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.1-git (f8b200a67ebaae3cfdf26600d05a5ab90f14557b)
clang version 14.0.0 (https://github.com/llvm/llvm-project.git eec312ee7f97638508679169cbf4b5183d0b1112)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /home/greg/Extras/emsdk/llvm/git/build_main_64/bin

$ rustc +nightly --version
rustc 1.59.0-nightly (efec54529 2021-12-04)

...although I've tried with earlier emscripten 2.0.34 and 3.0.0, along with earlier versions of rust, all with the same results.

To generate the object files from the rust source, compile the stub:

cd src
em++ -c gxx_personality_v0_stub.cpp -pthread

...and then invoke the command:

cargo +nightly build --target=wasm32-unknown-emscripten -Z build-std=panic_abort,std

(If anyone is interested stepping through wasm-ld, there is also a vscode launch configuration file in the repository that will help with setting the arguments to lld, but you'll need to edit it to reflect the location the object files on your computer.)

[sbc100]

I think the idea is that modern object files that mark the tls segment with WASM_SEG_FLAG_TLS should also mark all the symbols within that segment as WASM_SYMBOL_TLS. So the linker should not need to inject this flag since it should already exist on the symbol.

Perhaps we should be asserting that all symbols in bss segments are marked as WASM_SYMBOL_TLS.

As to how this symbol came to not have that flag, we would need to look into how and when that flag gets added in llvm.

In short, I think the bug here is in the object file generation rather than in the linker (assume the symbol is indeed missing the WASM_SYMBOL_TLS flag).

[gregbuchholz]

Thanks for the input. I've opened an issue on this over at: rust-lang/rust#92676 so I suppose we can close it here.

[kettle11]

I bumped into the error relocation R_WASM_MEMORY_ADDR_TLS_SLEB cannot be used against non-TLS symbol today trying to build one of my shared-memory Wasm Rust projects with LLD 14.0.

Note: I don't encounter this when building the same project with Rust's standard linker. Perhaps this hints at a miscommunication between Rust and wasm-ld 14.0 when shared memory is involved?

[sbc100]

CC @tlively and @RReverser who have worked on rust/emscripten integration

[kettle11]

To be clear this is how I reproduce this issue:

I'm using nightly Rust and I built wasm-ld yesterday from LLVM main.

1. cargo init to create a regular Rust project.
2. Build with the following:

RUSTFLAGS='-C target-feature=+atomics,+bulk-memory,+mutable-globals -C linker=/Users/ian/.cargo/wasm-ld' \
  cargo +nightly build --target wasm32-unknown-unknown -Z build-std=std,panic_abort

(Replace /Users/ian/.cargo/wasm-ld with a path to your wasm-ld)

This produces a bunch of errors like the following:

wasm-ld: error: /Users/ian/Desktop/simple_rust_wasm/target/wasm32-unknown-unknown/debug/deps/libstd-0c1be36ffa0fda20.rlib(std-0c1be36ffa0fda20.std.d3c659ec-cgu.11.rcgu.o): relocation R_WASM_MEMORY_ADDR_TLS_SLEB cannot be used against non-TLS symbol _ZN3std9panicking11panic_count17LOCAL_PANIC_COUNT7__getit5__KEY17haf0108542bcac8daE

Edit: My reproduction doesn't involve Emscripten, but this seems like the most relevant place to discuss for now.

[tlively]

I wonder if Rust's version of LLVM predates the R_WASM_MEMORY_ADDR_TLS_SLEB relocation type. @sbc100, is that something you added somewhat recently?

[sbc100]

The problem here is that rust is (I suppose correctly) using R_WASM_MEMORY_ADDR_TLS_SLEB but that the symbol its being used against is not being detected as being a TLS symbol. We added explicit TLS symbol flags fairly recently (About 5 months ago: https://reviews.llvm.org/D109426), but that change was supposed to include support for older object files:

[sbc100]

I've now got the example repro case failing locally and am looking into it. @gregbuchholz if you are on discord perhaps we can work together on this?

[sbc100]

Fix is (hopefully) in https://reviews.llvm.org/D118414

[gregbuchholz]

That is awesome! After applying that patch, it did indeed fix the problem. Thanks @sbc100 (and everyone else) for all of your effort on this issue, but more so on all the work that you have done on emscripten, it is truly appreciated!