cmd/link: fix bug in -strictdups checking of BSS symbols

The linker's -strictdups debugging option was not properly checking
for cases where you have two dupok BSS symbols with different length
(the check examined data length and content, but not symbol size).

Updates #46653.

Change-Id: I3844f25ef76dd6e4a84ffd5caed5d19a1b1a57c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/326210


Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/cmd/link/internal/loader/loader.go

@@ -699,12 +699,18 @@ func (l *Loader) checkdup(name string, r *oReader, li uint32, dup Sym) {
 	p := r.Data(li)
 	rdup, ldup := l.toLocal(dup)
 	pdup := rdup.Data(ldup)
-	if bytes.Equal(p, pdup) {
-		return
-	}
 	reason := "same length but different contents"
 	if len(p) != len(pdup) {
 		reason = fmt.Sprintf("new length %d != old length %d", len(p), len(pdup))
+	} else if bytes.Equal(p, pdup) {
+		// For BSS symbols, we need to check size as well, see issue 46653.
+		szdup := l.SymSize(dup)
+		sz := int64(r.Sym(li).Siz())
+		if szdup == sz {
+			return
+		}
+		reason = fmt.Sprintf("different sizes: new size %d != old size %d",
+			sz, szdup)
 	}
 	fmt.Fprintf(os.Stderr, "cmd/link: while reading object for '%v': duplicate symbol '%s', previous def at '%v', with mismatched payload: %s\n", r.unit.Lib, name, rdup.unit.Lib, reason)
 

src/cmd/link/link_test.go

@@ -470,10 +470,30 @@ TEXT	·f(SB), NOSPLIT|DUPOK, $0-0
 	JMP	0(PC)
 `
 
+const testStrictDupAsmSrc3 = `
+#include "textflag.h"
+GLOBL ·rcon(SB), RODATA|DUPOK, $64
+`
+
+const testStrictDupAsmSrc4 = `
+#include "textflag.h"
+GLOBL ·rcon(SB), RODATA|DUPOK, $32
+`
+
 func TestStrictDup(t *testing.T) {
 	// Check that -strictdups flag works.
 	testenv.MustHaveGoBuild(t)
 
+	asmfiles := []struct {
+		fname   string
+		payload string
+	}{
+		{"a", testStrictDupAsmSrc1},
+		{"b", testStrictDupAsmSrc2},
+		{"c", testStrictDupAsmSrc3},
+		{"d", testStrictDupAsmSrc4},
+	}
+
 	t.Parallel()
 
 	tmpdir := t.TempDir()
@@ -483,15 +503,12 @@ func TestStrictDup(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	src = filepath.Join(tmpdir, "a.s")
-	err = ioutil.WriteFile(src, []byte(testStrictDupAsmSrc1), 0666)
-	if err != nil {
-		t.Fatal(err)
-	}
-	src = filepath.Join(tmpdir, "b.s")
-	err = ioutil.WriteFile(src, []byte(testStrictDupAsmSrc2), 0666)
-	if err != nil {
-		t.Fatal(err)
+	for _, af := range asmfiles {
+		src = filepath.Join(tmpdir, af.fname+".s")
+		err = ioutil.WriteFile(src, []byte(af.payload), 0666)
+		if err != nil {
+			t.Fatal(err)
+		}
 	}
 	src = filepath.Join(tmpdir, "go.mod")
 	err = ioutil.WriteFile(src, []byte("module teststrictdup\n"), 0666)
@@ -503,7 +520,7 @@ func TestStrictDup(t *testing.T) {
 	cmd.Dir = tmpdir
 	out, err := cmd.CombinedOutput()
 	if err != nil {
-		t.Errorf("linking with -strictdups=1 failed: %v", err)
+		t.Errorf("linking with -strictdups=1 failed: %v\n%s", err, string(out))
 	}
 	if !bytes.Contains(out, []byte("mismatched payload")) {
 		t.Errorf("unexpected output:\n%s", out)
@@ -515,7 +532,11 @@ func TestStrictDup(t *testing.T) {
 	if err == nil {
 		t.Errorf("linking with -strictdups=2 did not fail")
 	}
-	if !bytes.Contains(out, []byte("mismatched payload")) {
+	// NB: on amd64 we get the 'new length' error, on arm64 the 'different
+	// contents' error.
+	if !(bytes.Contains(out, []byte("mismatched payload: new length")) ||
+		bytes.Contains(out, []byte("mismatched payload: same length but different contents"))) ||
+		!bytes.Contains(out, []byte("mismatched payload: different sizes")) {
 		t.Errorf("unexpected output:\n%s", out)
 	}
 }