runtime: use one more address bit for tagged pointers

We use one extra bit to placate systems which simulate amd64 binaries on
an arm64 host. Allocated arm64 addresses could be as high as 1<<48-1,
which would be invalid if we assumed 48-bit sign-extended addresses.

(Note that this does not help the other way around, simluating arm64
on amd64, but we don't have that problem at the moment.)

Fixes golang#69255

Change-Id: Iace17a5d41a65e34abf201d03d8b0ff6f7bf1150
Reviewed-on: https://go-review.googlesource.com/c/go/+/700515
Reviewed-by: Keith Randall <[email protected]>
Auto-Submit: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: randall77

src/runtime/tagptr_64bit.go

@@ -22,10 +22,17 @@ const (
 	// On AMD64, virtual addresses are 48-bit (or 57-bit) sign-extended.
 	// Other archs are 48-bit zero-extended.
 	//
+	// We use one extra bit to placate systems which simulate amd64 binaries on
+	// an arm64 host. Allocated arm64 addresses could be as high as 1<<48-1,
+	// which would be invalid if we assumed 48-bit sign-extended addresses.
+	// See issue 69255.
+	// (Note that this does not help the other way around, simluating arm64
+	// on amd64, but we don't have that problem at the moment.)
+	//
 	// On s390x, virtual addresses are 64-bit. There's not much we
 	// can do about this, so we just hope that the kernel doesn't
 	// get to really high addresses and panic if it does.
-	defaultAddrBits = 48
+	defaultAddrBits = 48 + 1
 
 	// On AIX, 64-bit addresses are split into 36-bit segment number and 28-bit
 	// offset in segment.  Segment numbers in the range 0x0A0000000-0x0AFFFFFFF(LSA)