188 lines
8.0 KiB
Go
188 lines
8.0 KiB
Go
// Copyright 2019 The gVisor Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// Package marshal defines the Marshallable interface for
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// serialize/deserializing go data structures to/from memory, according to the
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// Linux ABI.
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//
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// Implementations of this interface are typically automatically generated by
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// tools/go_marshal. See the go_marshal README for details.
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package marshal
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import (
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"io"
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"gvisor.dev/gvisor/pkg/usermem"
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)
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// Task provides a subset of kernel.Task, used in marshalling. We don't import
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// the kernel package directly to avoid circular dependency.
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type Task interface {
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// CopyScratchBuffer provides a task goroutine-local scratch buffer. See
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// kernel.CopyScratchBuffer.
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CopyScratchBuffer(size int) []byte
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// CopyOutBytes writes the contents of b to the task's memory. See
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// kernel.CopyOutBytes.
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CopyOutBytes(addr usermem.Addr, b []byte) (int, error)
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// CopyInBytes reads the contents of the task's memory to b. See
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// kernel.CopyInBytes.
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CopyInBytes(addr usermem.Addr, b []byte) (int, error)
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}
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// Marshallable represents operations on a type that can be marshalled to and
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// from memory.
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//
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// go-marshal automatically generates implementations for this interface for
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// types marked as '+marshal'.
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type Marshallable interface {
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io.WriterTo
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// SizeBytes is the size of the memory representation of a type in
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// marshalled form.
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//
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// SizeBytes must handle a nil receiver. Practically, this means SizeBytes
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// cannot deference any fields on the object implementing it (but will
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// likely make use of the type of these fields).
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SizeBytes() int
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// MarshalBytes serializes a copy of a type to dst. dst may be smaller than
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// SizeBytes(), which results in a part of the struct being marshalled. Note
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// that this may have unexpected results for non-packed types, as implicit
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// padding needs to be taken into account when reasoning about how much of
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// the type is serialized.
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MarshalBytes(dst []byte)
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// UnmarshalBytes deserializes a type from src. src may be smaller than
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// SizeBytes(), which results in a partially deserialized struct. Note that
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// this may have unexpected results for non-packed types, as implicit
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// padding needs to be taken into account when reasoning about how much of
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// the type is deserialized.
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UnmarshalBytes(src []byte)
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// Packed returns true if the marshalled size of the type is the same as the
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// size it occupies in memory. This happens when the type has no fields
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// starting at unaligned addresses (should always be true by default for ABI
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// structs, verified by automatically generated tests when using
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// go_marshal), and has no fields marked `marshal:"unaligned"`.
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//
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// Packed must return the same result for all possible values of the type
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// implementing it. Violating this constraint implies the type doesn't have
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// a static memory layout, and will lead to memory corruption.
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// Go-marshal-generated code reuses the result of Packed for multiple values
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// of the same type.
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Packed() bool
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// MarshalUnsafe serializes a type by bulk copying its in-memory
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// representation to the dst buffer. This is only safe to do when the type
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// has no implicit padding, see Marshallable.Packed. When Packed would
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// return false, MarshalUnsafe should fall back to the safer but slower
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// MarshalBytes. dst may be smaller than SizeBytes(), see comment for
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// MarshalBytes for implications.
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MarshalUnsafe(dst []byte)
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// UnmarshalUnsafe deserializes a type by directly copying to the underlying
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// memory allocated for the object by the runtime.
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//
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// This allows much faster unmarshalling of types which have no implicit
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// padding, see Marshallable.Packed. When Packed would return false,
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// UnmarshalUnsafe should fall back to the safer but slower unmarshal
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// mechanism implemented in UnmarshalBytes. src may be smaller than
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// SizeBytes(), see comment for UnmarshalBytes for implications.
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UnmarshalUnsafe(src []byte)
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// CopyIn deserializes a Marshallable type from a task's memory. This may
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// only be called from a task goroutine. This is more efficient than calling
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// UnmarshalUnsafe on Marshallable.Packed types, as the type being
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// marshalled does not escape. The implementation should avoid creating
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// extra copies in memory by directly deserializing to the object's
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// underlying memory.
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//
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// If the copy-in from the task memory is only partially successful, CopyIn
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// should still attempt to deserialize as much data as possible. See comment
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// for UnmarshalBytes.
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CopyIn(task Task, addr usermem.Addr) (int, error)
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// CopyOut serializes a Marshallable type to a task's memory. This may only
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// be called from a task goroutine. This is more efficient than calling
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// MarshalUnsafe on Marshallable.Packed types, as the type being serialized
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// does not escape. The implementation should avoid creating extra copies in
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// memory by directly serializing from the object's underlying memory.
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//
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// The copy-out to the task memory may be partially successful, in which
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// case CopyOut returns how much data was serialized. See comment for
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// MarshalBytes for implications.
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CopyOut(task Task, addr usermem.Addr) (int, error)
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// CopyOutN is like CopyOut, but explicitly requests a partial
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// copy-out. Note that this may yield unexpected results for non-packed
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// types and the caller may only want to allow this for packed types. See
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// comment on MarshalBytes.
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//
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// The limit must be less than or equal to SizeBytes().
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CopyOutN(task Task, addr usermem.Addr, limit int) (int, error)
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}
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// go-marshal generates additional functions for a type based on additional
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// clauses to the +marshal directive. They are documented below.
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//
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// Slice API
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// =========
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//
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// Adding a "slice" clause to the +marshal directive for structs or newtypes on
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// primitives like this:
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//
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// // +marshal slice:FooSlice
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// type Foo struct { ... }
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//
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// Generates four additional functions for marshalling slices of Foos like this:
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//
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// // MarshalUnsafeFooSlice is like Foo.MarshalUnsafe, buf for a []Foo. It's
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// // more efficient that repeatedly calling calling Foo.MarshalUnsafe over a
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// // []Foo in a loop.
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// func MarshalUnsafeFooSlice(src []Foo, dst []byte) (int, error) { ... }
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//
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// // UnmarshalUnsafeFooSlice is like Foo.UnmarshalUnsafe, buf for a []Foo. It's
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// // more efficient that repeatedly calling calling Foo.UnmarshalUnsafe over a
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// // []Foo in a loop.
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// func UnmarshalUnsafeFooSlice(dst []Foo, src []byte) (int, error) { ... }
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//
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// // CopyFooSliceIn copies in a slice of Foo objects from the task's memory.
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// func CopyFooSliceIn(task marshal.Task, addr usermem.Addr, dst []Foo) (int, error) { ... }
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//
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// // CopyFooSliceIn copies out a slice of Foo objects to the task's memory.
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// func CopyFooSliceOut(task marshal.Task, addr usermem.Addr, src []Foo) (int, error) { ... }
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//
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// The name of the functions are of the format "Copy%sIn" and "Copy%sOut", where
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// %s is the first argument to the slice clause. This directive is not supported
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// for newtypes on arrays.
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//
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// The slice clause also takes an optional second argument, which must be the
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// value "inner":
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//
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// // +marshal slice:Int32Slice:inner
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// type Int32 int32
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//
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// This is only valid on newtypes on primitives, and causes the generated
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// functions to accept slices of the inner type instead:
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//
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// func CopyInt32SliceIn(task marshal.Task, addr usermem.Addr, dst []int32) (int, error) { ... }
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//
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// Without "inner", they would instead be:
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//
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// func CopyInt32SliceIn(task marshal.Task, addr usermem.Addr, dst []Int32) (int, error) { ... }
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//
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// This may help avoid a cast depending on how the generated functions are used.
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