cmd/link: allow deriving GNU build ID from Go build ID ID

src/cmd/link/doc.go

@@ -18,6 +18,8 @@ Flags:
 	-B note
 		Add an ELF_NT_GNU_BUILD_ID note when using ELF.
 		The value should start with 0x and be an even number of hex digits.
+		Alternatively, you can pass "gobuildid" in order to derive the
+		GNU build ID from the Go build ID.
 	-E entry
 		Set entry symbol name.
 	-H type

src/cmd/link/elf_test.go

@@ -7,6 +7,10 @@
 package main
 
 import (
+	"bytes"
+	"cmd/internal/buildid"
+	"cmd/internal/notsha256"
+	"cmd/link/internal/ld"
 	"debug/elf"
 	"fmt"
 	"internal/platform"
@@ -199,6 +203,39 @@ func TestMinusRSymsWithSameName(t *testing.T) {
 	}
 }
 
+func TestGNUBuildIDDerivedFromGoBuildID(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	t.Parallel()
+
+	goFile := filepath.Join(t.TempDir(), "notes.go")
+	if err := os.WriteFile(goFile, []byte(goSource), 0444); err != nil {
+		t.Fatal(err)
+	}
+	outFile := filepath.Join(t.TempDir(), "notes.exe")
+	goTool := testenv.GoToolPath(t)
+
+	cmd := testenv.Command(t, goTool, "build", "-o", outFile, "-ldflags", "-buildid 0x1234 -B gobuildid", goFile)
+	cmd.Dir = t.TempDir()
+
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Logf("%s", out)
+		t.Fatal(err)
+	}
+
+	expectedGoBuildID := notsha256.Sum256([]byte("0x1234"))
+
+	gnuBuildID, err := buildid.ReadELFNote(outFile, string(ld.ELF_NOTE_BUILDINFO_NAME), ld.ELF_NOTE_BUILDINFO_TAG)
+	if err != nil || gnuBuildID == nil {
+		t.Fatalf("can't read GNU build ID")
+	}
+
+	if !bytes.Equal(gnuBuildID, expectedGoBuildID[:20]) {
+		t.Fatalf("build id not matching")
+	}
+}
+
 func TestMergeNoteSections(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
 	expected := 1

src/cmd/link/internal/ld/elf.go

@@ -806,6 +806,18 @@ func elfwritefreebsdsig(out *OutBuf) int {
 }
 
 func addbuildinfo(val string) {
+	if val == "gobuildid" {
+		buildID := *flagBuildid
+		if buildID == "" {
+			Exitf("-B gobuildid requires a Go build ID supplied via -buildid")
+		}
+
+		hashedBuildID := notsha256.Sum256([]byte(buildID))
+		buildinfo = hashedBuildID[:20]
+
+		return
+	}
+
 	if !strings.HasPrefix(val, "0x") {
 		Exitf("-B argument must start with 0x: %s", val)
 	}

src/cmd/link/internal/ld/main.go

@@ -190,7 +190,7 @@ func Main(arch *sys.Arch, theArch Arch) {
 	flag.Var(&ctxt.LinkMode, "linkmode", "set link `mode`")
 	flag.Var(&ctxt.BuildMode, "buildmode", "set build `mode`")
 	flag.BoolVar(&ctxt.compressDWARF, "compressdwarf", true, "compress DWARF if possible")
-	objabi.Flagfn1("B", "add an ELF NT_GNU_BUILD_ID `note` when using ELF", addbuildinfo)
+	objabi.Flagfn1("B", "add an ELF NT_GNU_BUILD_ID `note` when using ELF; use \"gobuildid\" to generate it from the Go build ID", addbuildinfo)
 	objabi.Flagfn1("L", "add specified `directory` to library path", func(a string) { Lflag(ctxt, a) })
 	objabi.AddVersionFlag() // -V
 	objabi.Flagfn1("X", "add string value `definition` of the form importpath.name=value", func(s string) { addstrdata1(ctxt, s) })