cmd/asm: when dynamic linking, reject code that uses a clobbered R15

The assember uses R15 as scratch space when assembling global variable
references in dynamically linked code. If the assembly code uses the
clobbered value of R15, report an error. The user is probably expecting
some other value in that register.

Getting rid of the R15 use isn't very practical (we could save a
register to a field in the G maybe, but that gets cumbersome).

Fixes #43661

Change-Id: I43f848a3d8b8a28931ec733386b85e6e9a42d8ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/283474
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/asm/internal/asm/endtoend_test.go

@@ -270,7 +270,7 @@ var (
 	errQuotesRE = regexp.MustCompile(`"([^"]*)"`)
 )
 
-func testErrors(t *testing.T, goarch, file string) {
+func testErrors(t *testing.T, goarch, file string, flags ...string) {
 	input := filepath.Join("testdata", file+".s")
 	architecture, ctxt := setArch(goarch)
 	lexer := lex.NewLexer(input)
@@ -292,6 +292,14 @@ func testErrors(t *testing.T, goarch, file string) {
 		}
 		errBuf.WriteString(s)
 	}
+	for _, flag := range flags {
+		switch flag {
+		case "dynlink":
+			ctxt.Flag_dynlink = true
+		default:
+			t.Errorf("unknown flag %s", flag)
+		}
+	}
 	pList.Firstpc, ok = parser.Parse()
 	obj.Flushplist(ctxt, pList, nil, "")
 	if ok && !failed {
@@ -430,6 +438,10 @@ func TestAMD64Errors(t *testing.T) {
 	testErrors(t, "amd64", "amd64error")
 }
 
+func TestAMD64DynLinkErrors(t *testing.T) {
+	testErrors(t, "amd64", "amd64dynlinkerror", "dynlink")
+}
+
 func TestMIPSEndToEnd(t *testing.T) {
 	testEndToEnd(t, "mips", "mips")
 	testEndToEnd(t, "mips64", "mips64")

src/cmd/asm/internal/asm/testdata/amd64dynlinkerror.s

@@ -0,0 +1,68 @@
+// Copyright 2021 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.
+
+// Test to make sure that if we use R15 after it is clobbered by
+// a global variable access while dynamic linking, we get an error.
+// See issue 43661.
+
+TEXT ·a1(SB), 0, $0-0
+	CMPL runtime·writeBarrier(SB), $0
+	MOVL $0, R15
+	RET
+TEXT ·a2(SB), 0, $0-0
+	CMPL runtime·writeBarrier(SB), $0
+	MOVQ $0, R15
+	RET
+TEXT ·a3(SB), 0, $0-0
+	CMPL runtime·writeBarrier(SB), $0
+	XORL R15, R15
+	RET
+TEXT ·a4(SB), 0, $0-0
+	CMPL runtime·writeBarrier(SB), $0
+	XORQ R15, R15
+	RET
+TEXT ·a5(SB), 0, $0-0
+	CMPL runtime·writeBarrier(SB), $0
+	XORL R15, R15
+	RET
+TEXT ·a6(SB), 0, $0-0
+	CMPL runtime·writeBarrier(SB), $0
+	POPQ R15
+	PUSHQ R15
+	RET
+TEXT ·a7(SB), 0, $0-0
+	CMPL runtime·writeBarrier(SB), $0
+	MOVQ R15, AX // ERROR "when dynamic linking, R15 is clobbered by a global variable access and is used here"
+	RET
+TEXT ·a8(SB), 0, $0-0
+	CMPL runtime·writeBarrier(SB), $0
+	ADDQ AX, R15 // ERROR "when dynamic linking, R15 is clobbered by a global variable access and is used here"
+	RET
+TEXT ·a9(SB), 0, $0-0
+	CMPL runtime·writeBarrier(SB), $0
+	ORQ R15, R15 // ERROR "when dynamic linking, R15 is clobbered by a global variable access and is used here"
+	RET
+TEXT ·a10(SB), 0, $0-0
+	CMPL runtime·writeBarrier(SB), $0
+	JEQ one
+	ORQ R15, R15 // ERROR "when dynamic linking, R15 is clobbered by a global variable access and is used here"
+one:
+	RET
+TEXT ·a11(SB), 0, $0-0
+	CMPL runtime·writeBarrier(SB), $0
+	JEQ one
+	JMP two
+one:
+	ORQ R15, R15 // ERROR "when dynamic linking, R15 is clobbered by a global variable access and is used here"
+two:
+	RET
+TEXT ·a12(SB), 0, $0-0
+	CMPL runtime·writeBarrier(SB), $0
+	JMP one
+two:
+	ORQ R15, R15
+	RET
+one:
+	MOVL $0, R15
+	JMP two

src/cmd/internal/obj/link.go

@@ -949,6 +949,7 @@ func (ctxt *Link) FixedFrameSize() int64 {
 type LinkArch struct {
 	*sys.Arch
 	Init           func(*Link)
+	ErrorCheck     func(*Link, *LSym)
 	Preprocess     func(*Link, *LSym, ProgAlloc)
 	Assemble       func(*Link, *LSym, ProgAlloc)
 	Progedit       func(*Link, *Prog, ProgAlloc)

src/cmd/internal/obj/plist.go

@@ -107,6 +107,9 @@ func Flushplist(ctxt *Link, plist *Plist, newprog ProgAlloc, myimportpath string
 	// Turn functions into machine code images.
 	for _, s := range text {
 		mkfwd(s)
+		if ctxt.Arch.ErrorCheck != nil {
+			ctxt.Arch.ErrorCheck(ctxt, s)
+		}
 		linkpatch(ctxt, s, newprog)
 		ctxt.Arch.Preprocess(ctxt, s, newprog)
 		ctxt.Arch.Assemble(ctxt, s, newprog)

src/cmd/internal/obj/x86/obj6.go

@@ -37,6 +37,7 @@ import (
 	"cmd/internal/sys"
 	"log"
 	"math"
+	"path"
 	"strings"
 )
 
@@ -563,6 +564,11 @@ func rewriteToPcrel(ctxt *obj.Link, p *obj.Prog, newprog obj.ProgAlloc) {
 	obj.Nopout(p)
 }
 
+// Prog.mark
+const (
+	markBit = 1 << 0 // used in errorCheck to avoid duplicate work
+)
+
 func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 	if cursym.Func().Text == nil || cursym.Func().Text.Link == nil {
 		return
@@ -1196,6 +1202,114 @@ func stacksplit(ctxt *obj.Link, cursym *obj.LSym, p *obj.Prog, newprog obj.ProgA
 	return end
 }
 
+func isR15(r int16) bool {
+	return r == REG_R15 || r == REG_R15B
+}
+func addrMentionsR15(a *obj.Addr) bool {
+	if a == nil {
+		return false
+	}
+	return isR15(a.Reg) || isR15(a.Index)
+}
+func progMentionsR15(p *obj.Prog) bool {
+	return addrMentionsR15(&p.From) || addrMentionsR15(&p.To) || isR15(p.Reg) || addrMentionsR15(p.GetFrom3())
+}
+
+// progOverwritesR15 reports whether p writes to R15 and does not depend on
+// the previous value of R15.
+func progOverwritesR15(p *obj.Prog) bool {
+	if !(p.To.Type == obj.TYPE_REG && isR15(p.To.Reg)) {
+		// Not writing to R15.
+		return false
+	}
+	if (p.As == AXORL || p.As == AXORQ) && p.From.Type == obj.TYPE_REG && isR15(p.From.Reg) {
+		// These look like uses of R15, but aren't, so we must detect these
+		// before the use check below.
+		return true
+	}
+	if addrMentionsR15(&p.From) || isR15(p.Reg) || addrMentionsR15(p.GetFrom3()) {
+		// use before overwrite
+		return false
+	}
+	if p.As == AMOVL || p.As == AMOVQ || p.As == APOPQ {
+		return true
+		// TODO: MOVB might be ok if we only ever use R15B.
+	}
+	return false
+}
+
+func addrUsesGlobal(a *obj.Addr) bool {
+	if a == nil {
+		return false
+	}
+	return a.Name == obj.NAME_EXTERN && !a.Sym.Local()
+}
+func progUsesGlobal(p *obj.Prog) bool {
+	if p.As == obj.ACALL || p.As == obj.ATEXT || p.As == obj.AFUNCDATA || p.As == obj.ARET || p.As == obj.AJMP {
+		// These opcodes don't use a GOT to access their argument (see rewriteToUseGot),
+		// or R15 would be dead at them anyway.
+		return false
+	}
+	if p.As == ALEAQ {
+		// The GOT entry is placed directly in the destination register; R15 is not used.
+		return false
+	}
+	return addrUsesGlobal(&p.From) || addrUsesGlobal(&p.To) || addrUsesGlobal(p.GetFrom3())
+}
+
+func errorCheck(ctxt *obj.Link, s *obj.LSym) {
+	// When dynamic linking, R15 is used to access globals. Reject code that
+	// uses R15 after a global variable access.
+	if !ctxt.Flag_dynlink {
+		return
+	}
+
+	// Flood fill all the instructions where R15's value is junk.
+	// If there are any uses of R15 in that set, report an error.
+	var work []*obj.Prog
+	var mentionsR15 bool
+	for p := s.Func().Text; p != nil; p = p.Link {
+		if progUsesGlobal(p) {
+			work = append(work, p)
+			p.Mark |= markBit
+		}
+		if progMentionsR15(p) {
+			mentionsR15 = true
+		}
+	}
+	if mentionsR15 {
+		for len(work) > 0 {
+			p := work[len(work)-1]
+			work = work[:len(work)-1]
+			if q := p.To.Target(); q != nil && q.Mark&markBit == 0 {
+				q.Mark |= markBit
+				work = append(work, q)
+			}
+			if p.As == obj.AJMP || p.As == obj.ARET {
+				continue // no fallthrough
+			}
+			if progMentionsR15(p) {
+				if progOverwritesR15(p) {
+					// R15 is overwritten by this instruction. Its value is not junk any more.
+					continue
+				}
+				pos := ctxt.PosTable.Pos(p.Pos)
+				ctxt.Diag("%s:%s: when dynamic linking, R15 is clobbered by a global variable access and is used here: %v", path.Base(pos.Filename()), pos.LineNumber(), p)
+				break // only report one error
+			}
+			if q := p.Link; q != nil && q.Mark&markBit == 0 {
+				q.Mark |= markBit
+				work = append(work, q)
+			}
+		}
+	}
+
+	// Clean up.
+	for p := s.Func().Text; p != nil; p = p.Link {
+		p.Mark &^= markBit
+	}
+}
+
 var unaryDst = map[obj.As]bool{
 	ABSWAPL:     true,
 	ABSWAPQ:     true,
@@ -1284,6 +1398,7 @@ var unaryDst = map[obj.As]bool{
 var Linkamd64 = obj.LinkArch{
 	Arch:           sys.ArchAMD64,
 	Init:           instinit,
+	ErrorCheck:     errorCheck,
 	Preprocess:     preprocess,
 	Assemble:       span6,
 	Progedit:       progedit,