runtime/debug: provide Addr method for errors from SetPanicOnFault

When we're building a panic that's triggered by a memory fault when
SetPanicOnFault has been called, include an Addr method. This
method reports the address at which the fault occurred.

Fixes #37023

RELNOTE=yes

Change-Id: Idff144587d6b75070fdc861a36efec76f4ec7384
Reviewed-on: https://go-review.googlesource.com/c/go/+/249677
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Keith Randall <khr@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

2 files changed, 51 insertions, 0 deletions

diff --git a/src/runtime/debug/garbage.go b/src/runtime/debug/garbage.go
index 785e9d4598..e36e54f12d 100644
--- a/src/runtime/debug/garbage.go
+++ b/src/runtime/debug/garbage.go
@@ -139,6 +139,11 @@ func SetMaxThreads(threads int) int {
 // manipulation of memory may cause faults at non-nil addresses in less
 // dramatic situations; SetPanicOnFault allows such programs to request
 // that the runtime trigger only a panic, not a crash.
+// The runtime.Error that the runtime panics with may have an additional method:
+//     Addr() uintptr
+// If that method exists, it returns the memory address which triggered the fault.
+// The results of Addr are best-effort and the veracity of the result
+// may depend on the platform.
 // SetPanicOnFault applies only to the current goroutine.
 // It returns the previous setting.
 func SetPanicOnFault(enabled bool) bool {
diff --git a/src/runtime/debug/panic_test.go b/src/runtime/debug/panic_test.go
new file mode 100644
index 0000000000..2aad418bae
--- /dev/null
+++ b/src/runtime/debug/panic_test.go
@@ -0,0 +1,46 @@
+// Copyright 2020 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.
+
+// +build aix darwin dragonfly freebsd linux netbsd openbsd
+
+// TODO: test on Windows?
+
+package debug_test
+
+import (
+	"runtime/debug"
+	"syscall"
+	"testing"
+	"unsafe"
+)
+
+func TestPanicOnFault(t *testing.T) {
+	m, err := syscall.Mmap(-1, 0, 0x1000, syscall.PROT_READ /* Note: no PROT_WRITE */, syscall.MAP_SHARED|syscall.MAP_ANON)
+	if err != nil {
+		t.Fatalf("can't map anonymous memory: %s", err)
+	}
+	defer syscall.Munmap(m)
+	old := debug.SetPanicOnFault(true)
+	defer debug.SetPanicOnFault(old)
+	const lowBits = 0x3e7
+	defer func() {
+		r := recover()
+		if r == nil {
+			t.Fatalf("write did not fault")
+		}
+		type addressable interface {
+			Addr() uintptr
+		}
+		a, ok := r.(addressable)
+		if !ok {
+			t.Fatalf("fault does not contain address")
+		}
+		want := uintptr(unsafe.Pointer(&m[lowBits]))
+		got := a.Addr()
+		if got != want {
+			t.Fatalf("fault address %x, want %x", got, want)
+		}
+	}()
+	m[lowBits] = 1 // will fault
+}