From: Matthew Dempsky <mdempsky@google.com>
Date: Tue, 12 Oct 2021 22:31:25 +0000 (-0700)
Subject: unsafe: allow unsafe.Slice up to end of address space
X-Git-Tag: go1.18beta1~914
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=4efa216c9d753c0853aa96a8c54ed5014fbc12e6;p=gostls13.git

unsafe: allow unsafe.Slice up to end of address space

Allow the user to construct slices that are larger than the Go heap as
long as they don't overflow the address space.

Updates #48798.

Change-Id: I659c8334d04676e1f253b9c3cd499eab9b9f989a
Reviewed-on: https://go-review.googlesource.com/c/go/+/355489
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
---

diff --git a/src/runtime/slice.go b/src/runtime/slice.go
index cfa862e047..66bb7d9d93 100644
--- a/src/runtime/slice.go
+++ b/src/runtime/slice.go
@@ -124,7 +124,7 @@ func unsafeslice(et *_type, ptr unsafe.Pointer, len int) {
 	}
 
 	mem, overflow := math.MulUintptr(et.size, uintptr(len))
-	if overflow || mem > maxAlloc || len < 0 {
+	if overflow || mem > -uintptr(ptr) || len < 0 {
 		panicunsafeslicelen()
 	}
 }
diff --git a/test/unsafebuiltins.go b/test/unsafebuiltins.go
index 4c940aa855..d04bcbdc7d 100644
--- a/test/unsafebuiltins.go
+++ b/test/unsafebuiltins.go
@@ -47,6 +47,11 @@ func main() {
 		// size overflows address space
 		mustPanic(func() { _ = unsafe.Slice(new(uint64), maxUintptr/8) })
 		mustPanic(func() { _ = unsafe.Slice(new(uint64), maxUintptr/8+1) })
+
+		// sliced memory overflows address space
+		last := (*byte)(unsafe.Pointer(^uintptr(0)))
+		_ = unsafe.Slice(last, 1)
+		mustPanic(func() { _ = unsafe.Slice(last, 2) })
 	}
 }