cmd/link: derive GNU build ID from Go build ID

While it is possible to embed a GNU build ID into the linked executable
by passing `-B 0xBUILDID` to the linker, the build ID will need to be
precomputed by the build system somehow. This makes it unnecessarily
complex to generate a deterministic build ID as it either requires the
biuld system to hash all inputs manually or to build the binary twice,
once to compute its hash and once with the GNU build ID derived from
that hash. Despite being complex, it is also inefficient as it requires
the build system to duplicate some of the work that the Go linker
already performs anyway.

Derive the GNU build ID automatically from the Go build ID whenever the
latter has been provided. Given that the Go build ID is intended to be
deterministic, the resulting GNU build ID should be deterministic as
well, which is the desired behaviour. Users can continue to override the
derived GNU build ID by passing a custom value via the `-B` switch.

Fixes #41004

Author: pks-t

src/cmd/link/doc.go

@@ -18,6 +18,7 @@ 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.
+		Overrides the default GNU build ID derived from the Go build ID.
 	-E entry
 		Set entry symbol name.
 	-H type

src/cmd/link/elf_test.go

@@ -7,6 +7,8 @@
 package main
 
 import (
+	"cmd/internal/buildid"
+	"cmd/link/internal/ld"
 	"debug/elf"
 	"fmt"
 	"internal/platform"
@@ -199,6 +201,47 @@ func TestMinusRSymsWithSameName(t *testing.T) {
 	}
 }
 
+func TestGNUBuildIDDerivedFromGoBuildID(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	switch runtime.GOOS {
+	case "linux", "dragonfly", "openbsd", "netbsd", "freebsd":
+	default:
+		t.Skip("We should only test on elf output.")
+	}
+	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, goFile)
+	cmd.Dir = t.TempDir()
+
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Logf("%s", out)
+		t.Fatal(err)
+	}
+
+	goBuildID, err := buildid.ReadFile(outFile)
+	if err != nil {
+		t.Fatalf("can't read Go build ID")
+	}
+
+	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 goBuildID != string(gnuBuildID) {
+		t.Fatalf("GNU build ID %s does not match Go build ID %s", string(gnuBuildID), goBuildID)
+	}
+}
+
 func TestMergeNoteSections(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
 	expected := 1

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

@@ -219,14 +219,6 @@ func Main(arch *sys.Arch, theArch Arch) {
 
 	interpreter = *flagInterpreter
 
-	if *flagBuildid == "" && ctxt.Target.IsOpenbsd() {
-		// TODO(jsing): Remove once direct syscalls are no longer in use.
-		// OpenBSD 6.7 onwards will not permit direct syscalls from a
-		// dynamically linked binary unless it identifies the binary
-		// contains a .note.go.buildid ELF note. See issue #36435.
-		*flagBuildid = "go-openbsd"
-	}
-
 	// enable benchmarking
 	var bench *benchmark.Metrics
 	if len(*benchmarkFlag) != 0 {
@@ -247,6 +239,20 @@ func Main(arch *sys.Arch, theArch Arch) {
 	bench.Start("Archinit")
 	thearch.Archinit(ctxt)
 
+	// When no GNU build ID was requested explicitly we derive its value
+	// automatically from the Go build ID, if set.
+	if len(buildinfo) == 0 && *flagBuildid != "" && ctxt.IsELF {
+		buildinfo = []byte(*flagBuildid)
+	}
+
+	if *flagBuildid == "" && ctxt.Target.IsOpenbsd() {
+		// TODO(jsing): Remove once direct syscalls are no longer in use.
+		// OpenBSD 6.7 onwards will not permit direct syscalls from a
+		// dynamically linked binary unless it identifies the binary
+		// contains a .note.go.buildid ELF note. See issue #36435.
+		*flagBuildid = "go-openbsd"
+	}
+
 	if ctxt.linkShared && !ctxt.IsELF {
 		Exitf("-linkshared can only be used on elf systems")
 	}