src/runtime/checkptr.go

// Copyright 2019 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.

package runtime

import "unsafe"

func checkptrAlignment(p unsafe.Pointer, elem *_type, n uintptr) {
	// Check that (*[n]elem)(p) is appropriately aligned.
	// TODO(mdempsky): What about fieldAlign?
	if uintptr(p)&(uintptr(elem.align)-1) != 0 {
		throw("checkptr: unsafe pointer conversion")
	}

	// Check that (*[n]elem)(p) doesn't straddle multiple heap objects.
	if size := n * elem.size; size > 1 && checkptrBase(p) != checkptrBase(add(p, size-1)) {
		throw("checkptr: unsafe pointer conversion")
	}
}

func checkptrArithmetic(p unsafe.Pointer, originals []unsafe.Pointer) {
	if 0 < uintptr(p) && uintptr(p) < minLegalPointer {
		throw("checkptr: unsafe pointer arithmetic")
	}

	// Check that if the computed pointer p points into a heap
	// object, then one of the original pointers must have pointed
	// into the same object.
	base := checkptrBase(p)
	if base == 0 {
		return
	}

	for _, original := range originals {
		if base == checkptrBase(original) {
			return
		}
	}

	throw("checkptr: unsafe pointer arithmetic")
}

// checkptrBase returns the base address for the allocation containing
// the address p.
//
// Importantly, if p1 and p2 point into the same variable, then
// checkptrBase(p1) == checkptrBase(p2). However, the converse/inverse
// is not necessarily true as allocations can have trailing padding,
// and multiple variables may be packed into a single allocation.
func checkptrBase(p unsafe.Pointer) uintptr {
	// stack
	if gp := getg(); gp.stack.lo <= uintptr(p) && uintptr(p) < gp.stack.hi {
		// TODO(mdempsky): Walk the stack to identify the
		// specific stack frame or even stack object that p
		// points into.
		//
		// In the mean time, use "1" as a pseudo-address to
		// represent the stack. This is an invalid address on
		// all platforms, so it's guaranteed to be distinct
		// from any of the addresses we might return below.
		return 1
	}

	// heap (must check after stack because of #35068)
	if base, _, _ := findObject(uintptr(p), 0, 0); base != 0 {
		return base
	}

	// data or bss
	for _, datap := range activeModules() {
		if datap.data <= uintptr(p) && uintptr(p) < datap.edata {
			return datap.data
		}
		if datap.bss <= uintptr(p) && uintptr(p) < datap.ebss {
			return datap.bss
		}
	}

	return 0
}