cmd/go: force external linking when CGO_LDFLAGS contains static-linking flags

When CGO_LDFLAGS contains flags that trigger static linking (-static,
--static, -Wl,-static, -Wl,--static, -Wl,-Bstatic), the internal
linker cannot resolve libc symbols, producing errors like "relocation
target stderr not defined".

Add checkLinkerFlagsForInternalLink, mirroring the existing
checkCompilerFlagsForInternalLink, to detect these flags in CGO_LDFLAGS
and emit the preferlinkext token to fall back to external linking.
Also check for -static/--static in compiler flags (CGO_CFLAGS etc.),
which some toolchains accept.

Fixes #77768

Change-Id: I5cbbc3cd30880ccf49dd575dd65188d9935d9488
GitHub-Last-Rev: 215efb6360
GitHub-Pull-Request: golang/go#79331
Reviewed-on: https://go-review.googlesource.com/c/go/+/776760
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/cmd/go/internal/work/exec.go

@@ -2985,6 +2985,11 @@ func (b *Builder) runCgo(ctx context.Context, a *Action) error {
 	flagSources := []string{"CGO_CFLAGS", "CGO_CXXFLAGS", "CGO_FFLAGS"}
 	flagLists := [][]string{cgoCFLAGS, cgoCXXFLAGS, cgoFFLAGS}
 	notCompatibleWithInternalLinking := flagsNotCompatibleWithInternalLinking(flagSources, flagLists)
+	if !notCompatibleWithInternalLinking {
+		if err := checkLinkerFlagsForInternalLink("CGO_LDFLAGS", "CGO_LDFLAGS", cgoLDFLAGS); err != nil {
+			notCompatibleWithInternalLinking = true
+		}
+	}
 
 	if cfg.BuildMSan {
 		cgoCFLAGS = append([]string{"-fsanitize=memory"}, cgoCFLAGS...)

src/cmd/go/internal/work/security.go

@@ -331,10 +331,34 @@ func checkCompilerFlagsForInternalLink(name, source string, list []string) error
 	}
 	// Currently the only flag on the allow list that causes problems
 	// for the linker is "-flto"; check for it manually here.
+	// Also check for -static/--static, which some toolchains accept
+	// as a compiler flag.
 	for _, fl := range list {
 		if strings.HasPrefix(fl, "-flto") {
 			return fmt.Errorf("flag %q triggers external linking", fl)
 		}
+		if fl == "-static" || fl == "--static" {
+			return fmt.Errorf("flag %q triggers external linking", fl)
+		}
+	}
+	return nil
+}
+
+// checkLinkerFlagsForInternalLink returns an error if 'list'
+// contains linker flags that are not compatible with internal linking.
+func checkLinkerFlagsForInternalLink(name, source string, list []string) error {
+	checkOverrides := false
+	if err := checkFlags(name, source, list, nil, validLinkerFlags, validLinkerFlagsWithNextArg, checkOverrides); err != nil {
+		return err
+	}
+	// Flags that force static linking require the external linker
+	// to resolve libc symbols. See #77768.
+	for _, fl := range list {
+		if fl == "-static" || fl == "--static" ||
+			fl == "-Wl,-static" || fl == "-Wl,--static" ||
+			fl == "-Wl,-Bstatic" {
+			return fmt.Errorf("flag %q triggers external linking", fl)
+		}
 	}
 	return nil
 }

src/cmd/go/testdata/script/cgo_suspect_flag_force_external.txt

@@ -91,6 +91,36 @@ go build -x -n -o dummy.exe ./usesInternalCgo
 ! stderr preferlinkext
 env CGO_CFLAGS=
 
+# CGO_LDFLAGS with -static variants are not compatible with internal linking.
+env CGO_LDFLAGS=-static
+go build -x -n -o dummy.exe ./usesInternalCgo
+stderr preferlinkext
+env CGO_LDFLAGS=--static
+go build -x -n -o dummy.exe ./usesInternalCgo
+stderr preferlinkext
+env CGO_LDFLAGS=-Wl,-static
+go build -x -n -o dummy.exe ./usesInternalCgo
+stderr preferlinkext
+env CGO_LDFLAGS=-Wl,--static
+go build -x -n -o dummy.exe ./usesInternalCgo
+stderr preferlinkext
+env CGO_LDFLAGS=-Wl,-Bstatic
+go build -x -n -o dummy.exe ./usesInternalCgo
+stderr preferlinkext
+env CGO_LDFLAGS=
+
+# CGO_LDFLAGS=-static-libgcc does NOT trigger external linking.
+env CGO_LDFLAGS=-static-libgcc
+go build -x -n -o dummy.exe ./usesInternalCgo
+! stderr preferlinkext
+env CGO_LDFLAGS=
+
+# CGO_CFLAGS=--static triggers external linking (some toolchains accept it).
+env CGO_CFLAGS=--static
+go build -x -n -o dummy.exe ./usesInternalCgo
+stderr preferlinkext
+env CGO_CFLAGS=
+
 [short] skip
 
 # In the remaining tests below we do actual builds (without -n) to