windows: fix -d=checkptr slice failures

This CL fixes unsafe casts to slices that are missing length or capacity.

Running tests with -d=checkptr enabled may panic on casting unsafe.Pointer to a static array of large predefined length, that is most likely much bigger than the size of the actual array in memory. Checkptr check is not satisfied if slicing operator misses length and capacity arguments `(*[(1 << 30) - 1]uint16)(unsafe.Pointer(p))[:]`, or when there is no slicing at all `(*[(1 << 30) - 1]uint16)(unsafe.Pointer(p))`.

To find all potential cases I used `grep -nr ")(unsafe.Pointer(" ./windows`, then filtered out safe casts when object size is always static and known at compile time.

To reproduce the issue run tests with checkptr enabled `go test -a -gcflags=all=-d=checkptr ./windows/...`.

Updates golang/go#34972
Fixes golang/go#38355

Change-Id: I9dd2084b4f9fb7618cdb140fb2f38b56b6d6cc04
GitHub-Last-Rev: 73288ad
GitHub-Pull-Request: #65
Reviewed-on: https://go-review.googlesource.com/c/sys/+/225418
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

windows/env_windows.go

@@ -8,7 +8,6 @@ package windows
 
 import (
 	"syscall"
-	"unicode/utf16"
 	"unsafe"
 )
 
@@ -40,17 +39,11 @@ func (token Token) Environ(inheritExisting bool) (env []string, err error) {
 	defer DestroyEnvironmentBlock(block)
 	blockp := uintptr(unsafe.Pointer(block))
 	for {
-		entry := (*[(1 << 30) - 1]uint16)(unsafe.Pointer(blockp))[:]
-		for i, v := range entry {
-			if v == 0 {
-				entry = entry[:i]
-				break
-			}
-		}
+		entry := UTF16PtrToString((*uint16)(unsafe.Pointer(blockp)))
 		if len(entry) == 0 {
 			break
 		}
-		env = append(env, string(utf16.Decode(entry)))
+		env = append(env, entry)
 		blockp += 2 * (uintptr(len(entry)) + 1)
 	}
 	return env, nil

windows/security_windows.go

@@ -7,6 +7,8 @@ package windows
 import (
 	"syscall"
 	"unsafe"
+
+	"golang.org/x/sys/internal/unsafeheader"
 )
 
 const (
@@ -1229,7 +1231,7 @@ func (sd *SECURITY_DESCRIPTOR) String() string {
 		return ""
 	}
 	defer LocalFree(Handle(unsafe.Pointer(sddl)))
-	return UTF16ToString((*[(1 << 30) - 1]uint16)(unsafe.Pointer(sddl))[:])
+	return UTF16PtrToString(sddl)
 }
 
 // ToAbsolute converts a self-relative security descriptor into an absolute one.
@@ -1307,9 +1309,17 @@ func (absoluteSD *SECURITY_DESCRIPTOR) ToSelfRelative() (selfRelativeSD *SECURIT
 }
 
 func (selfRelativeSD *SECURITY_DESCRIPTOR) copySelfRelativeSecurityDescriptor() *SECURITY_DESCRIPTOR {
-	sdBytes := make([]byte, selfRelativeSD.Length())
-	copy(sdBytes, (*[(1 << 31) - 1]byte)(unsafe.Pointer(selfRelativeSD))[:len(sdBytes)])
-	return (*SECURITY_DESCRIPTOR)(unsafe.Pointer(&sdBytes[0]))
+	sdLen := (int)(selfRelativeSD.Length())
+
+	var src []byte
+	h := (*unsafeheader.Slice)(unsafe.Pointer(&src))
+	h.Data = unsafe.Pointer(selfRelativeSD)
+	h.Len = sdLen
+	h.Cap = sdLen
+
+	dst := make([]byte, sdLen)
+	copy(dst, src)
+	return (*SECURITY_DESCRIPTOR)(unsafe.Pointer(&dst[0]))
 }
 
 // SecurityDescriptorFromString converts an SDDL string describing a security descriptor into a
@@ -1391,6 +1401,6 @@ func ACLFromEntries(explicitEntries []EXPLICIT_ACCESS, mergedACL *ACL) (acl *ACL
 	}
 	defer LocalFree(Handle(unsafe.Pointer(winHeapACL)))
 	aclBytes := make([]byte, winHeapACL.aclSize)
-	copy(aclBytes, (*[(1 << 31) - 1]byte)(unsafe.Pointer(winHeapACL))[:len(aclBytes)])
+	copy(aclBytes, (*[(1 << 31) - 1]byte)(unsafe.Pointer(winHeapACL))[:len(aclBytes):len(aclBytes)])
 	return (*ACL)(unsafe.Pointer(&aclBytes[0])), nil
 }

windows/svc/mgr/config.go

@@ -8,7 +8,6 @@ package mgr
 
 import (
 	"syscall"
-	"unicode/utf16"
 	"unsafe"
 
 	"golang.org/x/sys/windows"
@@ -46,28 +45,21 @@ type Config struct {
 	DelayedAutoStart bool   // the service is started after other auto-start services are started plus a short delay
 }
 
-func toString(p *uint16) string {
-	if p == nil {
-		return ""
-	}
-	return syscall.UTF16ToString((*[4096]uint16)(unsafe.Pointer(p))[:])
-}
-
 func toStringSlice(ps *uint16) []string {
-	if ps == nil {
-		return nil
-	}
 	r := make([]string, 0)
-	for from, i, p := 0, 0, (*[1 << 24]uint16)(unsafe.Pointer(ps)); true; i++ {
-		if p[i] == 0 {
-			// empty string marks the end
-			if i <= from {
-				break
-			}
-			r = append(r, string(utf16.Decode(p[from:i])))
-			from = i + 1
+	p := unsafe.Pointer(ps)
+
+	for {
+		s := windows.UTF16PtrToString((*uint16)(p))
+		if len(s) == 0 {
+			break
 		}
+
+		r = append(r, s)
+		offset := unsafe.Sizeof(uint16(0)) * (uintptr)(len(s)+1)
+		p = unsafe.Pointer(uintptr(p) + offset)
 	}
+
 	return r
 }
 
@@ -110,13 +102,13 @@ func (s *Service) Config() (Config, error) {
 		ServiceType:      p.ServiceType,
 		StartType:        p.StartType,
 		ErrorControl:     p.ErrorControl,
-		BinaryPathName:   toString(p.BinaryPathName),
-		LoadOrderGroup:   toString(p.LoadOrderGroup),
+		BinaryPathName:   windows.UTF16PtrToString(p.BinaryPathName),
+		LoadOrderGroup:   windows.UTF16PtrToString(p.LoadOrderGroup),
 		TagId:            p.TagId,
 		Dependencies:     toStringSlice(p.Dependencies),
-		ServiceStartName: toString(p.ServiceStartName),
-		DisplayName:      toString(p.DisplayName),
-		Description:      toString(p2.Description),
+		ServiceStartName: windows.UTF16PtrToString(p.ServiceStartName),
+		DisplayName:      windows.UTF16PtrToString(p.DisplayName),
+		Description:      windows.UTF16PtrToString(p2.Description),
 		DelayedAutoStart: delayedStart,
 	}, nil
 }

windows/svc/mgr/mgr.go

@@ -17,6 +17,7 @@ import (
 	"unicode/utf16"
 	"unsafe"
 
+	"golang.org/x/sys/internal/unsafeheader"
 	"golang.org/x/sys/windows"
 )
 
@@ -73,7 +74,7 @@ func (m *Mgr) LockStatus() (*LockStatus, error) {
 		status := &LockStatus{
 			IsLocked: lockStatus.IsLocked != 0,
 			Age:      time.Duration(lockStatus.LockDuration) * time.Second,
-			Owner:    windows.UTF16ToString((*[(1 << 30) - 1]uint16)(unsafe.Pointer(lockStatus.LockOwner))[:]),
+			Owner:    windows.UTF16PtrToString(lockStatus.LockOwner),
 		}
 		return status, nil
 	}
@@ -201,10 +202,16 @@ func (m *Mgr) ListServices() ([]string, error) {
 	if servicesReturned == 0 {
 		return nil, nil
 	}
-	services := (*[1 << 20]windows.ENUM_SERVICE_STATUS_PROCESS)(unsafe.Pointer(&buf[0]))[:servicesReturned]
+
+	var services []windows.ENUM_SERVICE_STATUS_PROCESS
+	hdr := (*unsafeheader.Slice)(unsafe.Pointer(&services))
+	hdr.Data = unsafe.Pointer(&buf[0])
+	hdr.Len = int(servicesReturned)
+	hdr.Cap = int(servicesReturned)
+
 	var names []string
 	for _, s := range services {
-		name := syscall.UTF16ToString((*[1 << 20]uint16)(unsafe.Pointer(s.ServiceName))[:])
+		name := windows.UTF16PtrToString(s.ServiceName)
 		names = append(names, name)
 	}
 	return names, nil

windows/svc/mgr/recovery.go

@@ -12,6 +12,7 @@ import (
 	"time"
 	"unsafe"
 
+	"golang.org/x/sys/internal/unsafeheader"
 	"golang.org/x/sys/windows"
 )
 
@@ -68,8 +69,13 @@ func (s *Service) RecoveryActions() ([]RecoveryAction, error) {
 		return nil, err
 	}
 
+	var actions []windows.SC_ACTION
+	hdr := (*unsafeheader.Slice)(unsafe.Pointer(&actions))
+	hdr.Data = unsafe.Pointer(p.Actions)
+	hdr.Len = int(p.ActionsCount)
+	hdr.Cap = int(p.ActionsCount)
+
 	var recoveryActions []RecoveryAction
-	actions := (*[1024]windows.SC_ACTION)(unsafe.Pointer(p.Actions))[:p.ActionsCount]
 	for _, action := range actions {
 		recoveryActions = append(recoveryActions, RecoveryAction{Type: int(action.Type), Delay: time.Duration(action.Delay) * time.Millisecond})
 	}
@@ -112,7 +118,7 @@ func (s *Service) RebootMessage() (string, error) {
 		return "", err
 	}
 	p := (*windows.SERVICE_FAILURE_ACTIONS)(unsafe.Pointer(&b[0]))
-	return toString(p.RebootMsg), nil
+	return windows.UTF16PtrToString(p.RebootMsg), nil
 }
 
 // SetRecoveryCommand sets the command line of the process to execute in response to the RunCommand service controller action.
@@ -131,5 +137,5 @@ func (s *Service) RecoveryCommand() (string, error) {
 		return "", err
 	}
 	p := (*windows.SERVICE_FAILURE_ACTIONS)(unsafe.Pointer(&b[0]))
-	return toString(p.Command), nil
+	return windows.UTF16PtrToString(p.Command), nil
 }

windows/svc/security.go

@@ -7,8 +7,6 @@
 package svc
 
 import (
-	"unsafe"
-
 	"golang.org/x/sys/windows"
 )
 
@@ -48,9 +46,8 @@ func IsAnInteractiveSession() (bool, error) {
 	if err != nil {
 		return false, err
 	}
-	p := unsafe.Pointer(&gs.Groups[0])
-	groups := (*[2 << 20]windows.SIDAndAttributes)(p)[:gs.GroupCount]
-	for _, g := range groups {
+
+	for _, g := range gs.AllGroups() {
 		if windows.EqualSid(g.Sid, interSid) {
 			return true, nil
 		}

windows/svc/service.go

@@ -14,6 +14,7 @@ import (
 	"syscall"
 	"unsafe"
 
+	"golang.org/x/sys/internal/unsafeheader"
 	"golang.org/x/sys/windows"
 )
 
@@ -224,10 +225,16 @@ const (
 func (s *service) run() {
 	s.goWaits.Wait()
 	s.h = windows.Handle(ssHandle)
-	argv := (*[100]*int16)(unsafe.Pointer(sArgv))[:sArgc]
+
+	var argv []*uint16
+	hdr := (*unsafeheader.Slice)(unsafe.Pointer(&argv))
+	hdr.Data = unsafe.Pointer(sArgv)
+	hdr.Len = int(sArgc)
+	hdr.Cap = int(sArgc)
+
 	args := make([]string, len(argv))
 	for i, a := range argv {
-		args[i] = syscall.UTF16ToString((*[1 << 20]uint16)(unsafe.Pointer(a))[:])
+		args[i] = windows.UTF16PtrToString(a)
 	}
 
 	cmdsToHandler := make(chan ChangeRequest)

windows/syscall_windows.go

@@ -13,6 +13,8 @@ import (
 	"time"
 	"unicode/utf16"
 	"unsafe"
+
+	"golang.org/x/sys/internal/unsafeheader"
 )
 
 type Handle uintptr
@@ -117,6 +119,32 @@ func UTF16PtrFromString(s string) (*uint16, error) {
 	return &a[0], nil
 }
 
+// UTF16PtrToString takes a pointer to a UTF-16 sequence and returns the corresponding UTF-8 encoded string.
+// If the pointer is nil, this returns the empty string. This assumes that the UTF-16 sequence is terminated
+// at a zero word; if the zero word is not present, the program may crash.
+func UTF16PtrToString(p *uint16) string {
+	if p == nil {
+		return ""
+	}
+	if *p == 0 {
+		return ""
+	}
+
+	// Find NUL terminator.
+	n := 0
+	for ptr := unsafe.Pointer(p); *(*uint16)(ptr) != 0; n++ {
+		ptr = unsafe.Pointer(uintptr(ptr) + unsafe.Sizeof(*p))
+	}
+
+	var s []uint16
+	h := (*unsafeheader.Slice)(unsafe.Pointer(&s))
+	h.Data = unsafe.Pointer(p)
+	h.Len = n
+	h.Cap = n
+
+	return string(utf16.Decode(s))
+}
+
 func Getpagesize() int { return 4096 }
 
 // NewCallback converts a Go function to a function pointer conforming to the stdcall calling convention.
@@ -1383,7 +1411,7 @@ func (t Token) KnownFolderPath(folderID *KNOWNFOLDERID, flags uint32) (string, e
 		return "", err
 	}
 	defer CoTaskMemFree(unsafe.Pointer(p))
-	return UTF16ToString((*[(1 << 30) - 1]uint16)(unsafe.Pointer(p))[:]), nil
+	return UTF16PtrToString(p), nil
 }
 
 // RtlGetVersion returns the version of the underlying operating system, ignoring