runtime: add optional expensive check for invalid cgo pointer passing

If you set GODEBUG=cgocheck=2 the runtime package will use the write
barrier to detect cases where a Go program writes a Go pointer into
non-Go memory.  In conjunction with the existing cgo checks, and the
not-yet-implemented cgo check for exported functions, this should
reliably detect all cases (that do not import the unsafe package) in
which a Go pointer is incorrectly shared with C code.  This check is
optional because it turns on the write barrier at all times, which is
known to be expensive.

Update #12416.

Change-Id: I549d8b2956daa76eac853928e9280e615d6365f4
Reviewed-on: https://go-review.googlesource.com/16899
Reviewed-by: Russ Cox <rsc@golang.org>

1 files changed, 243 insertions, 0 deletions

diff --git a/src/runtime/cgocheck.go b/src/runtime/cgocheck.go
new file mode 100644
index 0000000000..0077e22332
--- /dev/null
+++ b/src/runtime/cgocheck.go
@@ -0,0 +1,243 @@
+// Copyright 2015 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.
+
+// Code to check that pointer writes follow the cgo rules.
+// These functions are invoked via the write barrier when debug.cgocheck > 1.
+
+package runtime
+
+import (
+	"runtime/internal/sys"
+	"unsafe"
+)
+
+const cgoWriteBarrierFail = "Go pointer stored into non-Go memory"
+
+// cgoCheckWriteBarrier is called whenever a pointer is stored into memory.
+// It throws if the program is storing a Go pointer into non-Go memory.
+//go:nosplit
+//go:nowritebarrier
+func cgoCheckWriteBarrier(dst *uintptr, src uintptr) {
+	if !cgoIsGoPointer(unsafe.Pointer(src)) {
+		return
+	}
+	if cgoIsGoPointer(unsafe.Pointer(dst)) {
+		return
+	}
+
+	// If we are running on the system stack then dst might be an
+	// address on the stack, which is OK.
+	g := getg()
+	if g == g.m.g0 || g == g.m.gsignal {
+		return
+	}
+
+	// Allocating memory can write to various mfixalloc structs
+	// that look like they are non-Go memory.
+	if g.m.mallocing != 0 {
+		return
+	}
+
+	systemstack(func() {
+		println("write of Go pointer", hex(src), "to non-Go memory", hex(uintptr(unsafe.Pointer(dst))))
+		throw(cgoWriteBarrierFail)
+	})
+}
+
+// cgoCheckMemmove is called when moving a block of memory.
+// dst and src point off bytes into the value to copy.
+// size is the number of bytes to copy.
+// It throws if the program is copying a block that contains a Go pointer
+// into non-Go memory.
+//go:nosplit
+//go:nowritebarrier
+func cgoCheckMemmove(typ *_type, dst, src unsafe.Pointer, off, size uintptr) {
+	if typ.kind&kindNoPointers != 0 {
+		return
+	}
+	if !cgoIsGoPointer(src) {
+		return
+	}
+	if cgoIsGoPointer(dst) {
+		return
+	}
+	cgoCheckTypedBlock(typ, src, off, size)
+}
+
+// cgoCheckSliceCopy is called when copying n elements of a slice from
+// src to dst.  typ is the element type of the slice.
+// It throws if the program is copying slice elements that contain Go pointers
+// into non-Go memory.
+//go:nosplit
+//go:nowritebarrier
+func cgoCheckSliceCopy(typ *_type, dst, src slice, n int) {
+	if typ.kind&kindNoPointers != 0 {
+		return
+	}
+	if !cgoIsGoPointer(src.array) {
+		return
+	}
+	if cgoIsGoPointer(dst.array) {
+		return
+	}
+	p := src.array
+	for i := 0; i < n; i++ {
+		cgoCheckTypedBlock(typ, p, 0, typ.size)
+		p = add(p, typ.size)
+	}
+}
+
+// cgoCheckTypedBlock checks the block of memory at src, for up to size bytes,
+// and throws if it finds a Go pointer.  The type of the memory is typ,
+// and src is off bytes into that type.
+//go:nosplit
+//go:nowritebarrier
+func cgoCheckTypedBlock(typ *_type, src unsafe.Pointer, off, size uintptr) {
+	if typ.kind&kindGCProg == 0 {
+		cgoCheckBits(src, typ.gcdata, off, size)
+		return
+	}
+
+	// The type has a GC program.  Try to find GC bits somewhere else.
+	for datap := &firstmoduledata; datap != nil; datap = datap.next {
+		if cgoInRange(src, datap.data, datap.edata) {
+			doff := uintptr(src) - datap.data
+			cgoCheckBits(add(src, -doff), datap.gcdatamask.bytedata, off+doff, size)
+			return
+		}
+		if cgoInRange(src, datap.bss, datap.ebss) {
+			boff := uintptr(src) - datap.bss
+			cgoCheckBits(add(src, -boff), datap.gcbssmask.bytedata, off+boff, size)
+			return
+		}
+	}
+
+	aoff := uintptr(src) - mheap_.arena_start
+	idx := aoff >> _PageShift
+	s := h_spans[idx]
+	if s.state == _MSpanStack {
+		// There are no heap bits for value stored on the stack.
+		// For a channel receive src might be on the stack of some
+		// other goroutine, so we can't unwind the stack even if
+		// we wanted to.
+		// We can't expand the GC program without extra storage
+		// space we can't easily get.
+		// Fortunately we have the type information.
+		systemstack(func() {
+			cgoCheckUsingType(typ, src, off, size)
+		})
+		return
+	}
+
+	// src must be in the regular heap.
+
+	hbits := heapBitsForAddr(uintptr(src))
+	for i := uintptr(0); i < off+size; i += sys.PtrSize {
+		bits := hbits.bits()
+		if bits != 0 {
+			println(i, bits)
+		}
+		if i >= off && bits&bitPointer != 0 {
+			v := *(*unsafe.Pointer)(add(src, i))
+			if cgoIsGoPointer(v) {
+				systemstack(func() {
+					throw(cgoWriteBarrierFail)
+				})
+			}
+		}
+		hbits = hbits.next()
+	}
+}
+
+// cgoCheckBits checks the block of memory at src, for up to size
+// bytes, and throws if it finds a Go pointer.  The gcbits mark each
+// pointer value.  The src pointer is off bytes into the gcbits.
+//go:nosplit
+//go:nowritebarrier
+func cgoCheckBits(src unsafe.Pointer, gcbits *byte, off, size uintptr) {
+	skipMask := off / sys.PtrSize / 8
+	skipBytes := skipMask * sys.PtrSize * 8
+	ptrmask := addb(gcbits, skipMask)
+	src = add(src, skipBytes)
+	off -= skipBytes
+	size += off
+	var bits uint32
+	for i := uintptr(0); i < size; i += sys.PtrSize {
+		if i&(sys.PtrSize*8-1) == 0 {
+			bits = uint32(*ptrmask)
+			ptrmask = addb(ptrmask, 1)
+		} else {
+			bits >>= 1
+		}
+		if off > 0 {
+			off -= sys.PtrSize
+		} else {
+			if bits&1 != 0 {
+				v := *(*unsafe.Pointer)(add(src, i))
+				if cgoIsGoPointer(v) {
+					systemstack(func() {
+						throw(cgoWriteBarrierFail)
+					})
+				}
+			}
+		}
+	}
+}
+
+// cgoCheckUsingType is like cgoCheckTypedBlock, but is a last ditch
+// fall back to look for pointers in src using the type information.
+// We only this when looking at a value on the stack when the type
+// uses a GC program, because otherwise it's more efficient to use the
+// GC bits.  This is called on the system stack.
+//go:nowritebarrier
+//go:systemstack
+func cgoCheckUsingType(typ *_type, src unsafe.Pointer, off, size uintptr) {
+	if typ.kind&kindNoPointers != 0 {
+		return
+	}
+	if typ.kind&kindGCProg == 0 {
+		cgoCheckBits(src, typ.gcdata, off, size)
+		return
+	}
+	switch typ.kind & kindMask {
+	default:
+		throw("can't happen")
+	case kindArray:
+		at := (*arraytype)(unsafe.Pointer(typ))
+		for i := uintptr(0); i < at.len; i++ {
+			if off < at.elem.size {
+				cgoCheckUsingType(at.elem, src, off, size)
+			}
+			src = add(src, at.elem.size)
+			skipped := off
+			if skipped > at.elem.size {
+				skipped = at.elem.size
+			}
+			checked := at.elem.size - skipped
+			off -= skipped
+			if size <= checked {
+				return
+			}
+			size -= checked
+		}
+	case kindStruct:
+		st := (*structtype)(unsafe.Pointer(typ))
+		for _, f := range st.fields {
+			if off < f.typ.size {
+				cgoCheckUsingType(f.typ, src, off, size)
+			}
+			src = add(src, f.typ.size)
+			skipped := off
+			if skipped > f.typ.size {
+				skipped = f.typ.size
+			}
+			checked := f.typ.size - skipped
+			off -= skipped
+			if size <= checked {
+				return
+			}
+			size -= checked
+		}
+	}
+}