1 files changed, 65 insertions, 0 deletions
diff --git a/pkg/sentry/fs/offset.go b/pkg/sentry/fs/offset.go
new file mode 100644
index 000000000..3f68da149
--- /dev/null
+++ b/pkg/sentry/fs/offset.go
@@ -0,0 +1,65 @@
+// Copyright 2018 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package fs
+
+import (
+	"math"
+
+	"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
+)
+
+// OffsetPageEnd returns the file offset rounded up to the nearest
+// page boundary. OffsetPageEnd panics if rounding up causes overflow,
+// which shouldn't be possible given that offset is an int64.
+func OffsetPageEnd(offset int64) uint64 {
+	end, ok := usermem.Addr(offset).RoundUp()
+	if !ok {
+		panic("impossible overflow")
+	}
+	return uint64(end)
+}
+
+// ReadEndOffset returns an exclusive end offset for a read operation
+// so that the read does not overflow an int64 nor size.
+//
+// Parameters:
+// - offset: the starting offset of the read.
+// - length: the number of bytes to read.
+// - size:   the size of the file.
+//
+// Postconditions: The returned offset is >= offset.
+func ReadEndOffset(offset int64, length int64, size int64) int64 {
+	if offset >= size {
+		return offset
+	}
+	end := offset + length
+	// Don't overflow.
+	if end < offset || end > size {
+		end = size
+	}
+	return end
+}
+
+// WriteEndOffset returns an exclusive end offset for a write operation
+// so that the write does not overflow an int64.
+//
+// Parameters:
+// - offset: the starting offset of the write.
+// - length: the number of bytes to write.
+//
+// Postconditions: The returned offset is >= offset.
+func WriteEndOffset(offset int64, length int64) int64 {
+	return ReadEndOffset(offset, length, math.MaxInt64)
+}