pkg/cover: ensure that all PCs returned by kcov have matching callbacks

In the case some modules' addresses are off, certain kernel addresses
returned by kcov may not have corresponding coverage callbacks in the
.ko files. Keep an additional map in the backend to verify those
addresses and report an error if that is the case.

Also adjust text expectations in pkg/cover/report_test.go, as inexact
coverage will result in an error now.

pkg/cover/backend/backend.go

@@ -11,11 +11,12 @@ import (
 )
 
 type Impl struct {
-	Units     []*CompileUnit
-	Symbols   []*Symbol
-	Frames    []Frame
-	Symbolize func(pcs map[*Module][]uint64) ([]Frame, error)
-	RestorePC func(pc uint32) uint64
+	Units       []*CompileUnit
+	Symbols     []*Symbol
+	Frames      []Frame
+	Symbolize   func(pcs map[*Module][]uint64) ([]Frame, error)
+	RestorePC   func(pc uint32) uint64
+	CoverPoints map[uint64]bool
 }
 
 type Module struct {

pkg/cover/backend/dwarf.go

@@ -187,13 +187,22 @@ func makeDWARFUnsafe(params *dwarfParams) (*Impl, error) {
 		// On FreeBSD .text address in ELF is 0, but .text is actually mapped at 0xffffffff.
 		pcBase = ^uint64(0)
 	}
+	var allCoverPointsMap = make(map[uint64]bool)
+	for i := 0; i < 2; i++ {
+		for _, pc := range allCoverPoints[i] {
+			if pc != 0 {
+				allCoverPointsMap[pc] = true
+			}
+		}
+	}
 	impl := &Impl{
 		Units:   allUnits,
 		Symbols: allSymbols,
 		Symbolize: func(pcs map[*Module][]uint64) ([]Frame, error) {
 			return symbolize(target, objDir, srcDir, buildDir, pcs)
 		},
-		RestorePC: makeRestorePC(params, pcBase),
+		RestorePC:   makeRestorePC(params, pcBase),
+		CoverPoints: allCoverPointsMap,
 	}
 	return impl, nil
 }

pkg/cover/report.go

@@ -88,12 +88,18 @@ func (rg *ReportGenerator) prepareFileMap(progs []Prog) (map[string]*file, error
 		}
 	}
 	progPCs := make(map[uint64]map[int]bool)
+	allProgPCs := make(map[uint64]bool)
+	matchedProgPCs := make(map[uint64]bool)
 	for i, prog := range progs {
 		for _, pc := range prog.PCs {
 			if progPCs[pc] == nil {
 				progPCs[pc] = make(map[int]bool)
 			}
 			progPCs[pc][i] = true
+			allProgPCs[pc] = true
+			if rg.CoverPoints[pc] {
+				matchedProgPCs[pc] = true
+			}
 		}
 	}
 	matchedPC := false
@@ -123,6 +129,15 @@ func (rg *ReportGenerator) prepareFileMap(progs []Prog) (map[string]*file, error
 	if !matchedPC {
 		return nil, fmt.Errorf("coverage doesn't match any coverage callbacks")
 	}
+	if len(allProgPCs) != len(matchedProgPCs) {
+		for mismatch := range allProgPCs {
+			if !matchedProgPCs[mismatch] {
+				return nil, fmt.Errorf("%d out of %d PCs returned by kcov do not have matching "+
+					"coverage callbacks. Check the discoverModules() code, %v",
+					len(allProgPCs)-len(matchedProgPCs), len(allProgPCs), mismatch)
+			}
+		}
+	}
 	for _, unit := range rg.Units {
 		f := files[unit.Name]
 		for _, pc := range unit.PCs {

pkg/cover/report_test.go

@@ -11,6 +11,7 @@ package cover
 import (
 	"bytes"
 	"encoding/csv"
+	"fmt"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -36,8 +37,10 @@ type Test struct {
 	Progs     []Prog
 	DebugInfo bool
 	AddCover  bool
-	Result    string
-	Supports  func(target *targets.Target) bool
+	AddBadPc  bool
+	// Inexact coverage generated by AddCover=true may override empty Result.
+	Result   string
+	Supports func(target *targets.Target) bool
 }
 
 func TestReportGenerator(t *testing.T) {
@@ -73,6 +76,14 @@ func TestReportGenerator(t *testing.T) {
 			CFlags:    []string{"-fsanitize-coverage=trace-pc"},
 			DebugInfo: true,
 		},
+		{
+			Name:      "mismatch-pcs",
+			AddCover:  true,
+			AddBadPc:  true,
+			CFlags:    []string{"-fsanitize-coverage=trace-pc"},
+			DebugInfo: true,
+			Result:    `.* do not have matching coverage callbacks`,
+		},
 		{
 			Name:      "good-pie",
 			AddCover:  true,
@@ -132,7 +143,7 @@ func TestReportGenerator(t *testing.T) {
 }
 
 func testReportGenerator(t *testing.T, target *targets.Target, test Test) {
-	rep, csv, err := generateReport(t, target, test)
+	rep, csv, err := generateReport(t, target, &test)
 	if err != nil {
 		if test.Result == "" {
 			t.Fatalf("expected no error, but got:\n%v", err)
@@ -177,7 +188,7 @@ void* aslr_base() { return NULL; }
 void __sanitizer_cov_trace_pc() { printf("%llu", (long long)(__builtin_return_address(0) - aslr_base())); }
 `
 
-func buildTestBinary(t *testing.T, target *targets.Target, test Test, dir string) string {
+func buildTestBinary(t *testing.T, target *targets.Target, test *Test, dir string) string {
 	kcovSrc := filepath.Join(dir, "kcov.c")
 	kcovObj := filepath.Join(dir, "kcov.o")
 	if err := osutil.WriteFile(kcovSrc, []byte(kcovCode)); err != nil {
@@ -257,7 +268,7 @@ func buildTestBinary(t *testing.T, target *targets.Target, test Test, dir string
 	return bin
 }
 
-func generateReport(t *testing.T, target *targets.Target, test Test) ([]byte, []byte, error) {
+func generateReport(t *testing.T, target *targets.Target, test *Test) ([]byte, []byte, error) {
 	dir := t.TempDir()
 	bin := buildTestBinary(t, target, test, dir)
 	cfg := &mgrconfig.Config{
@@ -292,6 +303,7 @@ func generateReport(t *testing.T, target *targets.Target, test Test) ([]byte, []
 	}
 	if test.AddCover {
 		var pcs []uint64
+		Inexact := false
 		// Sanitizers crash when installing signal handlers with static libc.
 		const sanitizerOptions = "handle_segv=0:handle_sigbus=0:handle_sigfpe=0"
 		cmd := osutil.Command(bin)
@@ -322,6 +334,16 @@ func generateReport(t *testing.T, target *targets.Target, test Test) ([]byte, []
 				pcs = append(pcs, main.Addr+uint64(off))
 			}
 			t.Logf("using inexact coverage range 0x%x-0x%x", main.Addr, main.Addr+uint64(main.Size))
+			Inexact = true
+		}
+		// All PCs are required to have corresponding coverage callbacks. If the expected result is empty,
+		// override it with an error message.
+		if Inexact && test.Result == "" {
+			test.Result = fmt.Sprintf("%d out of %d PCs returned by kcov do not have matching coverage callbacks",
+				len(pcs)-1, len(pcs))
+		}
+		if test.AddBadPc {
+			pcs = append(pcs, 0xdeadbeef)
 		}
 		progs = append(progs, Prog{Data: "main", PCs: pcs})
 	}