Makefile: build executor with C++ compiler

Add C++ compiler and flags to the target
and build executor with the C++ compiler.
This will be needed to merge syz-fuzzer in to syz-executor
since it will be beefier and will most likely require linking in libc++.
But also this should fix #4821 since we won't use C++ flags
when building C sources (we already had work-around in pkg/csource,
but not in syz-extract).

Fixes #4821

4 files changed, 26 insertions, 19 deletions

diff --git a/Makefile b/Makefile
index 273043d00..258eaf2e2 100644
--- a/Makefile
+++ b/Makefile
@@ -133,8 +133,8 @@ ifneq ("$(NO_CROSS_COMPILER)", "")
 	$(info ************************************************************************************)
 else
 	mkdir -p ./bin/$(TARGETOS)_$(TARGETARCH)
-	$(CC) -o ./bin/$(TARGETOS)_$(TARGETARCH)/syz-executor$(EXE) executor/executor.cc \
-		$(ADDCFLAGS) $(CFLAGS) -DGOOS_$(TARGETOS)=1 -DGOARCH_$(TARGETARCH)=1 \
+	$(CXX) -o ./bin/$(TARGETOS)_$(TARGETARCH)/syz-executor$(EXE) executor/executor.cc \
+		$(ADDCXXFLAGS) $(CFLAGS) -DGOOS_$(TARGETOS)=1 -DGOARCH_$(TARGETARCH)=1 \
 		-DHOSTGOOS_$(HOSTOS)=1 -DGIT_REVISION=\"$(REV)\"
 endif
 endif
diff --git a/pkg/csource/build.go b/pkg/csource/build.go
index 5dd1234a7..0625e9a03 100644
--- a/pkg/csource/build.go
+++ b/pkg/csource/build.go
@@ -9,7 +9,6 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
-	"slices"
 	"strings"
 	"testing"
 
@@ -46,8 +45,6 @@ func BuildExecutor(t *testing.T, target *prog.Target, rootDir string, cflags ...
 }
 
 func build(target *prog.Target, src []byte, dir, file string, cflags ...string) (string, error) {
-	sysTarget := targets.Get(target.OS, target.Arch)
-	compiler := sysTarget.CCompiler
 	// We call the binary syz-executor because it sometimes shows in bug titles,
 	// and we don't want 2 different bugs for when a crash is triggered during fuzzing and during repro.
 	bin, err := osutil.TempFile("syz-executor")
@@ -66,18 +63,17 @@ func build(target *prog.Target, src []byte, dir, file string, cflags ...string)
 	} else {
 		flags = append(flags, file)
 	}
-	flags = append(flags, sysTarget.CFlags...)
+	sysTarget := targets.Get(target.OS, target.Arch)
+	compiler, targetCFlags := sysTarget.CCompiler, sysTarget.CFlags
+	if file != "" && !strings.HasSuffix(file, ".c") {
+		compiler, targetCFlags = sysTarget.CxxCompiler, sysTarget.CxxFlags
+	}
+	flags = append(flags, targetCFlags...)
+	flags = append(flags, cflags...)
 	if sysTarget.PtrSize == 4 {
 		// We do generate uint64's for syscall arguments that overflow longs on 32-bit archs.
 		flags = append(flags, "-Wno-overflow")
 	}
-	flags = append(flags, cflags...)
-	if file == "" || strings.HasSuffix(file, ".c") {
-		// Building C source, so remove C++ flags.
-		flags = slices.DeleteFunc(flags, func(flag string) bool {
-			return strings.HasPrefix(flag, "-std=c++")
-		})
-	}
 	cmd := osutil.Command(compiler, flags...)
 	cmd.Dir = dir
 	if file == "" {
diff --git a/sys/targets/targets.go b/sys/targets/targets.go
index 8a1b80f27..f5092b8c1 100644
--- a/sys/targets/targets.go
+++ b/sys/targets/targets.go
@@ -27,8 +27,10 @@ type Target struct {
 	Int64Alignment   uint64
 	LittleEndian     bool
 	CFlags           []string
+	CxxFlags         []string
 	Triple           string
 	CCompiler        string
+	CxxCompiler      string
 	Objdump          string // name of objdump executable
 	KernelCompiler   string // override CC when running kernel make
 	KernelLinker     string // override LD when running kernel make
@@ -584,9 +586,6 @@ var oses = map[string]osCommon{
 
 var (
 	commonCFlags = []string{
-		"-std=c++11",
-		"-I.",
-		"-Iexecutor/_include",
 		"-O2",
 		"-pthread",
 		"-Wall",
@@ -614,6 +613,13 @@ var (
 	fallbackCFlags = map[string]string{
 		"-static-pie": "-static", // if an ASLR static binary is impossible, build just a static one
 	}
+	// These are used only when building executor.
+	// For C repros and syz-extract, we build C source files.
+	commonCxxFlags = []string{
+		"-std=c++14",
+		"-I.",
+		"-Iexecutor/_include",
+	}
 )
 
 func fuchsiaCFlags(arch, clangArch string) []string {
@@ -646,9 +652,6 @@ func init() {
 	}
 	arch32, arch64 := splitArch(runtime.GOARCH)
 	goos := runtime.GOOS
-	if goos == "android" {
-		goos = Linux
-	}
 	for _, target := range List[TestOS] {
 		if List[goos] == nil {
 			continue
@@ -662,6 +665,7 @@ func init() {
 			continue
 		}
 		target.CCompiler = host.CCompiler
+		target.CxxCompiler = host.CxxCompiler
 		target.CPP = host.CPP
 		target.CFlags = append(append([]string{}, host.CFlags...), target.CFlags...)
 		target.CFlags = processMergedFlags(target.CFlags)
@@ -719,6 +723,9 @@ func initTarget(target *Target, OS, arch string) {
 	if target.CCompiler == "" {
 		target.setCompiler(useClang)
 	}
+	if target.CxxCompiler == "" {
+		target.CxxCompiler = strings.TrimSuffix(strings.TrimSuffix(target.CCompiler, "cc"), "++") + "++"
+	}
 	if target.CPP == "" {
 		target.CPP = "cpp"
 	}
@@ -734,6 +741,7 @@ func initTarget(target *Target, OS, arch string) {
 	if runtime.GOOS != target.BuildOS {
 		// Spoil native binaries if they are not usable, so that nobody tries to use them later.
 		target.CCompiler = fmt.Sprintf("cant-build-%v-on-%v", target.OS, runtime.GOOS)
+		target.CxxCompiler = target.CCompiler
 		target.CPP = target.CCompiler
 	}
 	for _, flags := range [][]string{commonCFlags, target.osCommon.cflags} {
@@ -945,6 +953,7 @@ func (target *Target) lazyInit() {
 		}
 	}
 	target.CFlags = newCFlags
+	target.CxxFlags = append(target.CFlags, commonCxxFlags...)
 	// Check that the compiler is actually functioning. It may be present, but still broken.
 	// Common for Linux distros, over time we've seen:
 	//	Error: alignment too large: 15 assumed
diff --git a/tools/syz-make/make.go b/tools/syz-make/make.go
index 6a69d5dd4..728f39352 100644
--- a/tools/syz-make/make.go
+++ b/tools/syz-make/make.go
@@ -76,7 +76,9 @@ func impl() ([]Var, error) {
 		{"TARGETARCH", targetArch},
 		{"TARGETVMARCH", targetVMArch},
 		{"CC", target.CCompiler},
+		{"CXX", target.CxxCompiler},
 		{"ADDCFLAGS", strings.Join(target.CFlags, " ")},
+		{"ADDCXXFLAGS", strings.Join(target.CxxFlags, " ")},
 		{"NCORES", strconv.Itoa(parallelism)},
 		{"EXE", target.ExeExtension},
 		{"NATIVEBUILDOS", target.BuildOS},