Add gohacks.Slice/StringHeader.

See https://github.com/golang/go/issues/19367 for rationale. Note that the
upstream decision arrived at in that thread, while useful for some of our use
cases, doesn't account for all of our SliceHeader use cases (we often use
SliceHeader to extract pointers from slices in a way that avoids bounds
checking and/or handles nil slices correctly) and also doesn't exist yet.

PiperOrigin-RevId: 358071574

3 files changed, 14 insertions, 11 deletions

diff --git a/pkg/safemem/BUILD b/pkg/safemem/BUILD
index 68ed074f8..d3b9b0ca9 100644
--- a/pkg/safemem/BUILD
+++ b/pkg/safemem/BUILD
@@ -11,7 +11,10 @@ go_library(
         "seq_unsafe.go",
     ],
     visibility = ["//:sandbox"],
-    deps = ["//pkg/safecopy"],
+    deps = [
+        "//pkg/gohacks",
+        "//pkg/safecopy",
+    ],
 )
 
 go_test(
diff --git a/pkg/safemem/block_unsafe.go b/pkg/safemem/block_unsafe.go
index 7857f5853..93879bb4f 100644
--- a/pkg/safemem/block_unsafe.go
+++ b/pkg/safemem/block_unsafe.go
@@ -16,9 +16,9 @@ package safemem
 
 import (
 	"fmt"
-	"reflect"
 	"unsafe"
 
+	"gvisor.dev/gvisor/pkg/gohacks"
 	"gvisor.dev/gvisor/pkg/safecopy"
 )
 
@@ -148,12 +148,11 @@ func (b Block) TakeFirst64(n uint64) Block {
 
 // ToSlice returns a []byte equivalent to b.
 func (b Block) ToSlice() []byte {
-	var bs []byte
-	hdr := (*reflect.SliceHeader)(unsafe.Pointer(&bs))
-	hdr.Data = uintptr(b.start)
-	hdr.Len = b.length
-	hdr.Cap = b.length
-	return bs
+	return *(*[]byte)(unsafe.Pointer(&gohacks.SliceHeader{
+		Data: b.start,
+		Len:  b.length,
+		Cap:  b.length,
+	}))
 }
 
 // Addr returns b's start address as a uintptr. It returns uintptr instead of
diff --git a/pkg/safemem/seq_unsafe.go b/pkg/safemem/seq_unsafe.go
index fc4049eeb..b315b0e5a 100644
--- a/pkg/safemem/seq_unsafe.go
+++ b/pkg/safemem/seq_unsafe.go
@@ -17,9 +17,10 @@ package safemem
 import (
 	"bytes"
 	"fmt"
-	"reflect"
 	"syscall"
 	"unsafe"
+
+	"gvisor.dev/gvisor/pkg/gohacks"
 )
 
 // A BlockSeq represents a sequence of Blocks, each of which has non-zero
@@ -184,8 +185,8 @@ func (bs BlockSeq) Tail() BlockSeq {
 		return BlockSeq{}
 	}
 	var extSlice []Block
-	extSliceHdr := (*reflect.SliceHeader)(unsafe.Pointer(&extSlice))
-	extSliceHdr.Data = uintptr(bs.data)
+	extSliceHdr := (*gohacks.SliceHeader)(unsafe.Pointer(&extSlice))
+	extSliceHdr.Data = bs.data
 	extSliceHdr.Len = bs.length
 	extSliceHdr.Cap = bs.length
 	tailSlice := skipEmpty(extSlice[1:])