dlmalloc: require __heap_end (#394)

This commit effectively drops the support of older wasm-ld. (LLVM <15.0.7).

We have two relevant use cases:

* `memory.grow` use outside of malloc
  (eg. used by polyfill preview1 binaries)

* `--init-memory` to somehow preallocate heap
  (eg. avoid dynamic allocations, especially on small environments)

While https://github.com/WebAssembly/wasi-libc/pull/377
fixed the former, it broke the latter if you are using
an older LLVM, which doesn't provide the `__heap_end` symbol,
to link your module.

As we couldn't come up with a solution which satisfies all parties,
this commit simply makes it require new enough LLVM which provides
`__heap_end`. After all, a link-time failure is more friendly to users
than failing later in a subtle way.

.github/workflows/main.yml

@@ -8,8 +8,26 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        # oldest and newest supported LLVM version
-        clang_version: [10.0.0, 14.0.0]
+        clang_version: [10.0.0]
+        # use different LLVM versions among oses because of the lack of
+        # official assets on github.
+        include:
+          - os: ubuntu-latest
+            clang_version: 10.0.0
+            llvm_asset_suffix: x86_64-linux-gnu-ubuntu-18.04
+          - os: macos-latest
+            clang_version: 10.0.0
+            llvm_asset_suffix: x86_64-apple-darwin
+          - os: windows-latest
+            clang_version: 10.0.0
+          - os: ubuntu-latest
+            clang_version: 16.0.0
+            llvm_asset_suffix: x86_64-linux-gnu-ubuntu-18.04
+          - os: macos-latest
+            clang_version: 15.0.7
+            llvm_asset_suffix: x86_64-apple-darwin21.0
+          - os: windows-latest
+            clang_version: 16.0.0
     steps:
     - uses: actions/checkout@v1
       with:
@@ -44,8 +62,8 @@ jobs:
     - name: Install LLVM tools (MacOS)
       shell: bash
       run: |
-        curl -sSfL https://github.com/llvm/llvm-project/releases/download/llvmorg-${{ matrix.clang_version }}/clang+llvm-${{ matrix.clang_version }}-x86_64-apple-darwin.tar.xz | tar xJf -
-        export CLANG_DIR=`pwd`/clang+llvm-${{ matrix.clang_version }}-x86_64-apple-darwin/bin
+        curl -sSfL https://github.com/llvm/llvm-project/releases/download/llvmorg-${{ matrix.clang_version }}/clang+llvm-${{ matrix.clang_version }}-${{ matrix.llvm_asset_suffix }}.tar.xz | tar xJf -
+        export CLANG_DIR=`pwd`/clang+llvm-${{ matrix.clang_version }}-${{ matrix.llvm_asset_suffix }}/bin
         echo "$CLANG_DIR" >> $GITHUB_PATH
         echo "CC=$CLANG_DIR/clang" >> $GITHUB_ENV
         echo "AR=$CLANG_DIR/llvm-ar" >> $GITHUB_ENV
@@ -55,8 +73,8 @@ jobs:
     - name: Install LLVM tools (Linux)
       shell: bash
       run: |
-        curl -sSfL https://github.com/llvm/llvm-project/releases/download/llvmorg-${{ matrix.clang_version }}/clang+llvm-${{ matrix.clang_version }}-x86_64-linux-gnu-ubuntu-18.04.tar.xz | tar xJf -
-        export CLANG_DIR=`pwd`/clang+llvm-${{ matrix.clang_version }}-x86_64-linux-gnu-ubuntu-18.04/bin
+        curl -sSfL https://github.com/llvm/llvm-project/releases/download/llvmorg-${{ matrix.clang_version }}/clang+llvm-${{ matrix.clang_version }}-${{ matrix.llvm_asset_suffix }}.tar.xz | tar xJf -
+        export CLANG_DIR=`pwd`/clang+llvm-${{ matrix.clang_version }}-${{ matrix.llvm_asset_suffix }}/bin
         echo "$CLANG_DIR" >> $GITHUB_PATH
         echo "CLANG_DIR=$CLANG_DIR" >> $GITHUB_ENV
         echo "CC=$CLANG_DIR/clang" >> $GITHUB_ENV
@@ -76,12 +94,14 @@ jobs:
       run: |
         cd test
         make download
-        export WASI_DIR=$(realpath $CLANG_DIR/../lib/clang/${{ matrix.clang_version }}/lib/wasi/)
+        export WASI_DIR=$(realpath $($CLANG_DIR/clang -print-resource-dir)/lib/wasi/)
         mkdir -p $WASI_DIR
         cp download/lib/wasi/libclang_rt.builtins-wasm32.a $WASI_DIR
         make test
       # The older version of Clang does not provide the expected symbol for the
       # test entrypoints: `undefined symbol: __main_argc_argv`.
+      # The older (<15.0.7) version of wasm-ld does not provide `__heap_end`,
+      # which is required by our malloc implementation.
       if: matrix.os == 'ubuntu-latest' && matrix.clang_version != '10.0.0'
 
     - uses: actions/upload-artifact@v1

dlmalloc/src/malloc.c

@@ -5215,7 +5215,7 @@ static void internal_inspect_all(mstate m,
 
 /* Symbol marking the end of data, bss and explicit stack, provided by wasm-ld. */
 extern char __heap_base;
-extern char __heap_end __attribute__((__weak__));
+extern char __heap_end;
 
 /* Initialize the initial state of dlmalloc to be able to use free memory between __heap_base and initial. */
 static void try_init_allocator(void) {
@@ -5227,23 +5227,18 @@ static void try_init_allocator(void) {
 
   char *base = &__heap_base;
   // Try to use the linker pseudo-symbol `__heap_end` for the initial size of
-  // the heap, but if that's not defined due to LLVM being too old perhaps then
-  // round up `base` to the nearest `PAGESIZE`. The initial size of linear
-  // memory will be at least the heap base to this page boundary, and it's then
-  // assumed that the initial linear memory image was truncated at that point.
-  // While this reflects the default behavior of `wasm-ld` it is also possible
-  // for users to craft larger linear memories by passing options to extend
-  // beyond this threshold. In this situation the memory will not be used for
-  // dlmalloc.
-  //
-  // Note that `sbrk(0)`, or in dlmalloc-ese `CALL_MORECORE(0)`, is specifically
-  // not used here. That captures the current size of the heap but is only
-  // correct if the we're the first to try to grow the heap. If the heap has
-  // grown elsewhere, such as a different allocator in place, then this would
-  // incorrectly claim such memroy as our own.
+  // the heap.
   char *end = &__heap_end;
-  if (end == NULL)
-    end = (char*) page_align((size_t) base);
+  if (end < base) {
+    // "end" can be NULL when 1. you are using an old wasm-ld which doesn't
+    // provide `__heap_end` (< 15.0.7) and 2. something (other libraries
+    // or maybe your app?) includes a weak reference to `__heap_end` and
+    // 3. the weak reference is found by the linker before this strong
+    // reference.
+    //
+    // Note: This is a linker bug: https://github.com/llvm/llvm-project/issues/60829
+    __builtin_trap();
+  }
   size_t initial_heap_size = end - base;
 
   /* Check that initial heap is long enough to serve a minimal allocation request. */

expected/wasm32-wasi-threads/undefined-symbols.txt

@@ -13,6 +13,7 @@ __getf2
 __global_base
 __gttf2
 __heap_base
+__heap_end
 __imported_wasi_snapshot_preview1_args_get
 __imported_wasi_snapshot_preview1_args_sizes_get
 __imported_wasi_snapshot_preview1_clock_res_get

expected/wasm32-wasi/undefined-symbols.txt

@@ -11,6 +11,7 @@ __floatunsitf
 __getf2
 __gttf2
 __heap_base
+__heap_end
 __imported_wasi_snapshot_preview1_args_get
 __imported_wasi_snapshot_preview1_args_sizes_get
 __imported_wasi_snapshot_preview1_clock_res_get