cgo: allow --export= in LDFLAGS

This allows people to export some functions, such as malloc. Example:

    // #cgo LDFLAGS: --export=malloc
    import "C"

This exports the function malloc.

Note that this is somewhat unsafe right now, but it is used regardless.
By using this workaround, people have some time to transition away from
using malloc/free directly or until malloc is made safe to be used in
this way.

cgo/security.go

@@ -142,6 +142,7 @@ var validLinkerFlags = []*regexp.Regexp{
 	re(`-L([^@\-].*)`),
 	re(`-O`),
 	re(`-O([^@\-].*)`),
+	re(`--export=(.+)`), // for wasm-ld
 	re(`-f(no-)?(pic|PIC|pie|PIE)`),
 	re(`-f(no-)?openmp(-simd)?`),
 	re(`-fsanitize=([^@\-].*)`),

compiler/compiler.go

@@ -1059,11 +1059,12 @@ func (b *builder) createFunctionStart(intrinsic bool) {
 	if b.info.section != "" {
 		b.llvmFn.SetSection(b.info.section)
 	}
-	if b.info.exported && strings.HasPrefix(b.Triple, "wasm") {
+	if b.info.exported && b.info.module != "" && strings.HasPrefix(b.Triple, "wasm") {
 		// Set the exported name. This is necessary for WebAssembly because
 		// otherwise the function is not exported.
-		functionAttr := b.ctx.CreateStringAttribute("wasm-export-name", b.info.linkName)
-		b.llvmFn.AddFunctionAttr(functionAttr)
+		b.llvmFn.AddFunctionAttr(b.ctx.CreateStringAttribute("wasm-export-name", b.info.linkName))
+		// Set the export module.
+		b.llvmFn.AddFunctionAttr(b.ctx.CreateStringAttribute("wasm-export-module", b.info.module))
 	}
 
 	// Some functions have a pragma controlling the inlining level.

compiler/symbol.go

@@ -210,8 +210,9 @@ func (c *compilerContext) getFunction(fn *ssa.Function) llvm.Value {
 // exported.
 func (c *compilerContext) getFunctionInfo(f *ssa.Function) functionInfo {
 	info := functionInfo{
-		// Pick the default linkName.
-		linkName: f.RelString(nil),
+		module:     "env",
+		importName: f.Name(),
+		linkName:   f.RelString(nil), // pick the default linkName
 	}
 	// Check for //go: pragmas, which may change the link name (among others).
 	info.parsePragmas(f)
@@ -225,10 +226,6 @@ func (info *functionInfo) parsePragmas(f *ssa.Function) {
 		return
 	}
 	if decl, ok := f.Syntax().(*ast.FuncDecl); ok && decl.Doc != nil {
-
-		// Our importName for a wasm module (if we are compiling to wasm), or llvm link name
-		var importName string
-
 		for _, comment := range decl.Doc.List {
 			text := comment.Text
 			if strings.HasPrefix(text, "//export ") {
@@ -246,7 +243,8 @@ func (info *functionInfo) parsePragmas(f *ssa.Function) {
 					continue
 				}
 
-				importName = parts[1]
+				info.importName = parts[1]
+				info.linkName = parts[1]
 				info.exported = true
 			case "//go:interrupt":
 				if hasUnsafeImport(f.Pkg.Pkg) {
@@ -254,10 +252,13 @@ func (info *functionInfo) parsePragmas(f *ssa.Function) {
 				}
 			case "//go:wasm-module":
 				// Alternative comment for setting the import module.
-				if len(parts) != 2 {
-					continue
+				if len(parts) == 1 {
+					// Function must not be exported outside of the WebAssembly
+					// module (but only be made available for linking).
+					info.module = ""
+				} else if len(parts) == 2 {
+					info.module = parts[1]
 				}
-				info.module = parts[1]
 			case "//go:inline":
 				info.inline = inlineHint
 			case "//go:noinline":
@@ -297,17 +298,6 @@ func (info *functionInfo) parsePragmas(f *ssa.Function) {
 				}
 			}
 		}
-
-		// Set the importName for our exported function if we have one
-		if importName != "" {
-			if info.module == "" {
-				info.linkName = importName
-			} else {
-				// WebAssembly import
-				info.importName = importName
-			}
-		}
-
 	}
 }
 

compiler/testdata/pragma.ll

@@ -62,7 +62,7 @@ declare void @main.undefinedFunctionNotInSection(i8*) #0
 
 attributes #0 = { "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" }
 attributes #1 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" }
-attributes #2 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" "wasm-export-name"="extern_func" "wasm-import-module"="env" "wasm-import-name"="extern_func" }
+attributes #2 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" "wasm-export-module"="env" "wasm-export-name"="extern_func" "wasm-import-module"="env" "wasm-import-name"="extern_func" }
 attributes #3 = { inlinehint nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" }
 attributes #4 = { noinline nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" }
-attributes #5 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" "wasm-export-name"="exportedFunctionInSection" "wasm-import-module"="env" "wasm-import-name"="exportedFunctionInSection" }
+attributes #5 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" "wasm-export-module"="env" "wasm-export-name"="exportedFunctionInSection" "wasm-import-module"="env" "wasm-import-name"="exportedFunctionInSection" }

src/runtime/arch_tinygowasm.go

@@ -68,6 +68,7 @@ func growHeap() bool {
 var allocs = make(map[uintptr][]byte)
 
 //export malloc
+//go:wasm-module
 func libc_malloc(size uintptr) unsafe.Pointer {
 	buf := make([]byte, size)
 	ptr := unsafe.Pointer(&buf[0])
@@ -76,6 +77,7 @@ func libc_malloc(size uintptr) unsafe.Pointer {
 }
 
 //export free
+//go:wasm-module
 func libc_free(ptr unsafe.Pointer) {
 	if ptr == nil {
 		return
@@ -88,12 +90,14 @@ func libc_free(ptr unsafe.Pointer) {
 }
 
 //export calloc
+//go:wasm-module
 func libc_calloc(nmemb, size uintptr) unsafe.Pointer {
 	// No difference between calloc and malloc.
 	return libc_malloc(nmemb * size)
 }
 
 //export realloc
+//go:wasm-module
 func libc_realloc(oldPtr unsafe.Pointer, size uintptr) unsafe.Pointer {
 	// It's hard to optimize this to expand the current buffer with our GC, but
 	// it is theoretically possible. For now, just always allocate fresh.