runtime: initialise debug settings much earlier in startup process

This is necessary specifically to set the value of `debug.decoratemappings`
sufficiently early in the startup sequence that all memory ranges allocated
can be named appropriately using the new Linux-specific naming API
introduced in #71546.

Example output (on ARM64):
https://gist.github.com/9muir/3667654b9c3f52e8be92756219371672

Fixes: #75324

Change-Id: Ic0b16233f54a45adef1660c4d0df59af2f5af86a
Reviewed-on: https://go-review.googlesource.com/c/go/+/703476
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/decoratemappings_test.go

@@ -0,0 +1,72 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime_test
+
+import (
+	"os"
+	"regexp"
+	"runtime"
+	"testing"
+)
+
+func validateMapLabels(t *testing.T, labels []string) {
+	// These are the specific region labels that need get added during the
+	// runtime phase. Hence they are the ones that need to be confirmed as
+	// present at the time the test reads its own region labels, which
+	// is sufficient to validate that the default `decoratemappings` value
+	// (enabled) was set early enough in the init process.
+	regions := map[string]bool{
+		"allspans array":    false,
+		"gc bits":           false,
+		"heap":              false,
+		"heap index":        false,
+		"heap reservation":  false,
+		"immortal metadata": false,
+		"page alloc":        false,
+		"page alloc index":  false,
+		"page summary":      false,
+		"scavenge index":    false,
+	}
+	for _, label := range labels {
+		if _, ok := regions[label]; !ok {
+			t.Logf("unexpected region label found: \"%s\"", label)
+		}
+		regions[label] = true
+	}
+	for label, found := range regions {
+		if !found {
+			t.Logf("region label missing: \"%s\"", label)
+		}
+	}
+}
+
+func TestDecorateMappings(t *testing.T) {
+	if runtime.GOOS != "linux" {
+		t.Skip("decoratemappings is only supported on Linux")
+		// /proc/self/maps is also Linux-specific
+	}
+
+	var labels []string
+	if rawMaps, err := os.ReadFile("/proc/self/maps"); err != nil {
+		t.Fatalf("failed to read /proc/self/maps: %v", err)
+	} else {
+		t.Logf("maps:%s\n", string(rawMaps))
+		matches := regexp.MustCompile("[^[]+ \\[anon: Go: (.+)\\]\n").FindAllSubmatch(rawMaps, -1)
+		for _, match_pair := range matches {
+			// match_pair consists of the matching substring and the parenthesized group
+			labels = append(labels, string(match_pair[1]))
+		}
+	}
+	t.Logf("DebugDecorateMappings: %v", *runtime.DebugDecorateMappings)
+	if *runtime.DebugDecorateMappings != 0 && runtime.SetVMANameSupported() {
+		validateMapLabels(t, labels)
+	} else {
+		if len(labels) > 0 {
+			t.Errorf("unexpected mapping labels present: %v", labels)
+		} else {
+			t.Skip("mapping labels absent as expected")
+		}
+	}
+}

src/runtime/export_test.go

@@ -1936,3 +1936,7 @@ func (t *TraceStackTable) Reset() {
 func TraceStack(gp *G, tab *TraceStackTable) {
 	traceStack(0, gp, (*traceStackTable)(tab))
 }
+
+var DebugDecorateMappings = &debug.decoratemappings
+
+func SetVMANameSupported() bool { return setVMANameSupported() }

src/runtime/proc.go

@@ -789,7 +789,9 @@ func cpuinit(env string) {
 // getGodebugEarly extracts the environment variable GODEBUG from the environment on
 // Unix-like operating systems and returns it. This function exists to extract GODEBUG
 // early before much of the runtime is initialized.
-func getGodebugEarly() string {
+//
+// Returns nil, false if OS doesn't provide env vars early in the init sequence.
+func getGodebugEarly() (string, bool) {
 	const prefix = "GODEBUG="
 	var env string
 	switch GOOS {
@@ -807,12 +809,16 @@ func getGodebugEarly() string {
 			s := unsafe.String(p, findnull(p))
 
 			if stringslite.HasPrefix(s, prefix) {
-				env = gostring(p)[len(prefix):]
+				env = gostringnocopy(p)[len(prefix):]
 				break
 			}
 		}
+		break
+
+	default:
+		return "", false
 	}
-	return env
+	return env, true
 }
 
 // The bootstrap sequence is:
@@ -859,12 +865,15 @@ func schedinit() {
 	// The world starts stopped.
 	worldStopped()
 
+	godebug, parsedGodebug := getGodebugEarly()
+	if parsedGodebug {
+		parseRuntimeDebugVars(godebug)
+	}
 	ticks.init() // run as early as possible
 	moduledataverify()
 	stackinit()
 	randinit() // must run before mallocinit, alginit, mcommoninit
 	mallocinit()
-	godebug := getGodebugEarly()
 	cpuinit(godebug) // must run before alginit
 	alginit()        // maps, hash, rand must not be used before this call
 	mcommoninit(gp.m, -1)
@@ -880,7 +889,12 @@ func schedinit() {
 	goenvs()
 	secure()
 	checkfds()
-	parsedebugvars()
+	if !parsedGodebug {
+		// Some platforms, e.g., Windows, didn't make env vars available "early",
+		// so try again now.
+		parseRuntimeDebugVars(gogetenv("GODEBUG"))
+	}
+	finishDebugVarsSetup()
 	gcinit()
 
 	// Allocate stack space that can be used when crashing due to bad stack

src/runtime/runtime1.go

@@ -402,7 +402,7 @@ var dbgvars = []*dbgVar{
 	{name: "updatemaxprocs", value: &debug.updatemaxprocs, def: 1},
 }
 
-func parsedebugvars() {
+func parseRuntimeDebugVars(godebug string) {
 	// defaults
 	debug.cgocheck = 1
 	debug.invalidptr = 1
@@ -420,12 +420,6 @@ func parsedebugvars() {
 	}
 	debug.traceadvanceperiod = defaultTraceAdvancePeriod
 
-	godebug := gogetenv("GODEBUG")
-
-	p := new(string)
-	*p = godebug
-	godebugEnv.Store(p)
-
 	// apply runtime defaults, if any
 	for _, v := range dbgvars {
 		if v.def != 0 {
@@ -437,7 +431,6 @@ func parsedebugvars() {
 			}
 		}
 	}
-
 	// apply compile-time GODEBUG settings
 	parsegodebug(godebugDefault, nil)
 
@@ -463,6 +456,12 @@ func parsedebugvars() {
 	if debug.gccheckmark > 0 {
 		debug.asyncpreemptoff = 1
 	}
+}
+
+func finishDebugVarsSetup() {
+	p := new(string)
+	*p = gogetenv("GODEBUG")
+	godebugEnv.Store(p)
 
 	setTraceback(gogetenv("GOTRACEBACK"))
 	traceback_env = traceback_cache

src/runtime/set_vma_name_linux.go

@@ -14,9 +14,13 @@ import (
 
 var prSetVMAUnsupported atomic.Bool
 
+func setVMANameSupported() bool {
+	return !prSetVMAUnsupported.Load()
+}
+
 // setVMAName calls prctl(PR_SET_VMA, PR_SET_VMA_ANON_NAME, start, len, name)
 func setVMAName(start unsafe.Pointer, length uintptr, name string) {
-	if debug.decoratemappings == 0 || prSetVMAUnsupported.Load() {
+	if debug.decoratemappings == 0 || !setVMANameSupported() {
 		return
 	}
 

src/runtime/set_vma_name_stub.go

@@ -10,3 +10,5 @@ import "unsafe"
 
 // setVMAName isn’t implemented
 func setVMAName(start unsafe.Pointer, len uintptr, name string) {}
+
+func setVMANameSupported() bool { return false }