Use kernel.Task.CopyScratchBuffer in syscalls/linux where possible.

PiperOrigin-RevId: 241072126
Change-Id: Ib4d9f58f550732ac4c5153d3cf159a5b1a9749da

pkg/sentry/kernel/task.go

@@ -550,17 +550,16 @@ func (t *Task) afterLoad() {
 	t.futexWaiter = futex.NewWaiter()
 }
 
-// copyScratchBufferLen is the length of the copyScratchBuffer field of the Task
-// struct.
-const copyScratchBufferLen = 52
+// copyScratchBufferLen is the length of Task.copyScratchBuffer.
+const copyScratchBufferLen = 144 // sizeof(struct stat)
 
 // CopyScratchBuffer returns a scratch buffer to be used in CopyIn/CopyOut
 // functions. It must only be used within those functions and can only be used
 // by the task goroutine; it exists to improve performance and thus
 // intentionally lacks any synchronization.
 //
-// Callers should pass a constant value as an argument, which will allow the
-// compiler to inline and optimize out the if statement below.
+// Callers should pass a constant value as an argument if possible, which will
+// allow the compiler to inline and optimize out the if statement below.
 func (t *Task) CopyScratchBuffer(size int) []byte {
 	if size > copyScratchBufferLen {
 		return make([]byte, size)

pkg/sentry/syscalls/linux/sys_prctl.go

@@ -75,7 +75,7 @@ func Prctl(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 
 	case linux.PR_GET_NAME:
 		addr := args[1].Pointer()
-		buf := make([]byte, linux.TASK_COMM_LEN)
+		buf := t.CopyScratchBuffer(linux.TASK_COMM_LEN)
 		len := copy(buf, t.Name())
 		if len < linux.TASK_COMM_LEN {
 			buf[len] = 0

pkg/sentry/syscalls/linux/sys_socket.go

@@ -516,7 +516,7 @@ func SetSockOpt(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sy
 	if optLen > maxOptLen {
 		return 0, nil, syscall.EINVAL
 	}
-	buf := make([]byte, optLen)
+	buf := t.CopyScratchBuffer(int(optLen))
 	if _, err := t.CopyIn(optValAddr, &buf); err != nil {
 		return 0, nil, err
 	}

pkg/sentry/syscalls/linux/sys_stat.go

@@ -133,7 +133,7 @@ func stat(t *kernel.Task, d *fs.Dirent, dirPath bool, statAddr usermem.Addr) err
 	// common syscall for many applications, and t.CopyObjectOut has
 	// noticeable performance impact due to its many slice allocations and
 	// use of reflection.
-	b := make([]byte, 0, linux.SizeOfStat)
+	b := t.CopyScratchBuffer(int(linux.SizeOfStat))[:0]
 
 	// Dev (uint64)
 	b = binary.AppendUint64(b, usermem.ByteOrder, uint64(d.Inode.StableAttr.DeviceID))