windows: use Go-managed pointer list for ProcThreadAttributeList

It turns out that if you write Go pointers to Go memory, the Go compiler
must be involved so that it generates various calls to the GC in the
process. Letting Windows write Go pointers to Go memory violated this.

We fix this by having all the Windows-managed memory be just a boring
[]byte blob. Then, in order to prevent the GC from prematurely cleaning
up the pointers referenced by that []byte blob, or in the future moving
memory and attempting to fix up pointers, we copy the data to the
Windows heap and then maintain a little array of pointers that have been
used. Every time the Update function is called with a new pointer, we
make a copy and append it to the list.  Then, on Delete, we free the
pointers from the Windows heap.

Updates golang/go#44900.

Change-Id: I42340a93fd9f6b8d10340634cf833fd4559a5f4f
Reviewed-on: https://go-review.googlesource.com/c/sys/+/300369
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

windows/exec_windows.go

@@ -9,6 +9,8 @@ package windows
 import (
 	errorspkg "errors"
 	"unsafe"
+
+	"golang.org/x/sys/internal/unsafeheader"
 )
 
 // EscapeArg rewrites command line argument s as prescribed
@@ -135,8 +137,8 @@ func FullPath(name string) (path string, err error) {
 	}
 }
 
-// NewProcThreadAttributeList allocates a new ProcThreadAttributeList, with the requested maximum number of attributes.
-func NewProcThreadAttributeList(maxAttrCount uint32) (*ProcThreadAttributeList, error) {
+// NewProcThreadAttributeList allocates a new ProcThreadAttributeListContainer, with the requested maximum number of attributes.
+func NewProcThreadAttributeList(maxAttrCount uint32) (*ProcThreadAttributeListContainer, error) {
 	var size uintptr
 	err := initializeProcThreadAttributeList(nil, maxAttrCount, 0, &size)
 	if err != ERROR_INSUFFICIENT_BUFFER {
@@ -145,10 +147,9 @@ func NewProcThreadAttributeList(maxAttrCount uint32) (*ProcThreadAttributeList,
 		}
 		return nil, err
 	}
-	const psize = unsafe.Sizeof(uintptr(0))
 	// size is guaranteed to be ≥1 by InitializeProcThreadAttributeList.
-	al := (*ProcThreadAttributeList)(unsafe.Pointer(&make([]unsafe.Pointer, (size+psize-1)/psize)[0]))
-	err = initializeProcThreadAttributeList(al, maxAttrCount, 0, &size)
+	al := &ProcThreadAttributeListContainer{data: (*ProcThreadAttributeList)(unsafe.Pointer(&make([]byte, size)[0]))}
+	err = initializeProcThreadAttributeList(al.data, maxAttrCount, 0, &size)
 	if err != nil {
 		return nil, err
 	}
@@ -156,11 +157,39 @@ func NewProcThreadAttributeList(maxAttrCount uint32) (*ProcThreadAttributeList,
 }
 
 // Update modifies the ProcThreadAttributeList using UpdateProcThreadAttribute.
-func (al *ProcThreadAttributeList) Update(attribute uintptr, flags uint32, value unsafe.Pointer, size uintptr, prevValue unsafe.Pointer, returnedSize *uintptr) error {
-	return updateProcThreadAttribute(al, flags, attribute, value, size, prevValue, returnedSize)
+// Note that the value passed to this function will be copied into memory
+// allocated by LocalAlloc, the contents of which should not contain any
+// Go-managed pointers, even if the passed value itself is a Go-managed
+// pointer.
+func (al *ProcThreadAttributeListContainer) Update(attribute uintptr, value unsafe.Pointer, size uintptr) error {
+	alloc, err := LocalAlloc(LMEM_FIXED, uint32(size))
+	if err != nil {
+		return err
+	}
+	var src, dst []byte
+	hdr := (*unsafeheader.Slice)(unsafe.Pointer(&src))
+	hdr.Data = value
+	hdr.Cap = int(size)
+	hdr.Len = int(size)
+	hdr = (*unsafeheader.Slice)(unsafe.Pointer(&dst))
+	hdr.Data = unsafe.Pointer(alloc)
+	hdr.Cap = int(size)
+	hdr.Len = int(size)
+	copy(dst, src)
+	al.heapAllocations = append(al.heapAllocations, alloc)
+	return updateProcThreadAttribute(al.data, 0, attribute, unsafe.Pointer(alloc), size, nil, nil)
 }
 
 // Delete frees ProcThreadAttributeList's resources.
-func (al *ProcThreadAttributeList) Delete() {
-	deleteProcThreadAttributeList(al)
+func (al *ProcThreadAttributeListContainer) Delete() {
+	deleteProcThreadAttributeList(al.data)
+	for i := range al.heapAllocations {
+		LocalFree(Handle(al.heapAllocations[i]))
+	}
+	al.heapAllocations = nil
+}
+
+// List returns the actual ProcThreadAttributeList to be passed to StartupInfoEx.
+func (al *ProcThreadAttributeListContainer) List() *ProcThreadAttributeList {
+	return al.data
 }