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node.go
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node.go
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// Copyright 2022 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 lisafs
import (
"github.com/ttpreport/gvisor-ligolo/pkg/atomicbitops"
"github.com/ttpreport/gvisor-ligolo/pkg/context"
"github.com/ttpreport/gvisor-ligolo/pkg/fspath"
"github.com/ttpreport/gvisor-ligolo/pkg/sync"
)
// numStaticChildren is the number of static children tracked by each node.
// Sampling certain filesystem heavy workloads showed that a majority of
// directories store at most 5 children in their map. This should be kept low
// to minimize the memory overhead for each node. 5 is fairly low and at the
// same time helps avoid map allocations for majority of nodes. Benchmarking
// also showed that static arrays are faster than maps for lookups until n=8.
const numStaticChildren = 5
// Node is a node on the filesystem tree. A Node is shared by all the
// ControlFDs opened on that position. For a given Server, there will only be
// one Node for a given filesystem position.
//
// Reference Model:
// - Each node holds a ref on its parent for its entire lifetime.
type Node struct {
// node's ref count is protected by its parent's childrenMu.
nodeRefs
// opMu synchronizes high level operations on this path.
//
// It is used to ensure the following which are important for security:
// * This node's data is protected by opMu. So all operations that change its
// data should hold opMu for writing. For example: write, setstat, setxattr,
// etc. This entails that if this node represents a directory, creation and
// deletion operations happening directly under this directory must lock
// opMu for writing. All operations accessing data must hold opMu for
// reading. This is to avoid the can of worms that open when creation and
// deletion are allowed to race. This prevents any walks from occurring
// during creation or deletion.
// * When this node is being deleted, the deletion handler must hold opMu for
// writing. This ensures that there are no concurrent operations going on
// this node while it is being deleted and potentially being replaced with
// something hazardous.
//
// A useful consequence of the above is that holding opMu for reading
// guarantees that the Server can not change Nodes on the path until this
// Node. For instance, if the grandparent needs to be renamed or deleted,
// the client must first delete this node to avoid ENOTEMPTY error. Deleting
// this node is not possible while opMu is read locked.
opMu sync.RWMutex
// deleted indicates whether the backing file has been unlinked. This can be
// used to deny operations on FDs on this Node after deletion because it is
// not safe for FD implementations to do host walks up to this position
// anymore. This node may have been replaced with something hazardous.
// deleted is protected by opMu. deleted must only be accessed/mutated using
// atomics; see markDeletedRecursive for more details.
deleted atomicbitops.Uint32
// name is the name of the file represented by this Node in parent. If this
// FD represents the root directory, then name is an empty string. name is
// protected by the backing server's rename mutex.
name string
// parent is this parent node which tracks this node as a child. parent is
// protected by the backing server's rename mutex.
parent *Node
// controlFDs is a linked list of all the ControlFDs opened on this node.
// Prefer this over a slice to avoid additional allocations. Each ControlFD
// is an implicit linked list node so there are no additional allocations
// needed to maintain the linked list.
controlFDsMu sync.Mutex
controlFDs controlFDList
// Here is a performance hack. Past experience has shown that map allocations
// on each node for tracking children costs a lot of memory. More small
// allocations also fragment memory. To save allocations, statically track
// upto numStaticChildren children using hardcoded pointers. If more children
// are inserted then move to a map. Use dynamicChildren iff it is non-nil.
// The folowing fields are protected by childrenMu.
childrenMu sync.Mutex
staticChildren [numStaticChildren]struct {
name string
node *Node
}
dynamicChildren map[string]*Node
}
// DecRef implements refs.RefCounter.DecRef. Note that the context
// parameter should never be used. It exists solely to comply with the
// refs.RefCounter interface.
//
// Precondition: server's rename mutex must be at least read locked.
func (n *Node) DecRef(context.Context) {
if n.parent == nil {
n.nodeRefs.DecRef(nil)
return
}
// If this is the only ref on node then it will need to be destroyed.
n.parent.childrenMu.Lock()
deleted := false
n.nodeRefs.DecRef(func() {
n.parent.removeChildLocked(n.name)
deleted = true
})
n.parent.childrenMu.Unlock()
if deleted {
// Drop ref on parent. Keep Decref call lock free for scalability.
n.parent.DecRef(nil)
}
}
// InitLocked must be called before first use of fd.
//
// Precondition: parent.childrenMu is locked.
//
// Postconditions: A ref on n is transferred to the caller.
func (n *Node) InitLocked(name string, parent *Node) {
n.nodeRefs.InitRefs()
n.name = name
n.parent = parent
if parent != nil {
parent.IncRef()
parent.insertChildLocked(name, n)
}
}
// LookupChildLocked looks up for a child with given name. Returns nil if child
// does not exist.
//
// Preconditions: childrenMu is locked.
func (n *Node) LookupChildLocked(name string) *Node {
if n.dynamicChildren != nil {
return n.dynamicChildren[name]
}
for i := 0; i < numStaticChildren; i++ {
if n.staticChildren[i].name == name {
return n.staticChildren[i].node
}
}
return nil
}
// WithChildrenMu executes fn with n.childrenMu locked.
func (n *Node) WithChildrenMu(fn func()) {
n.childrenMu.Lock()
defer n.childrenMu.Unlock()
fn()
}
// FilePath returns the absolute path of the backing file. This is an expensive
// operation. The returned path should be free of any intermediate symlinks
// because all internal (non-leaf) nodes are directories.
//
// Precondition:
// - server's rename mutex must be at least read locked. Calling handlers must
// at least have read concurrency guarantee from the server.
func (n *Node) FilePath() string {
// Walk upwards and prepend name to res.
var res fspath.Builder
for n.parent != nil {
res.PrependComponent(n.name)
n = n.parent
}
// n is the root node.
res.PrependByte('/')
return res.String()
}
func (n *Node) isDeleted() bool {
return n.deleted.Load() != 0
}
func (n *Node) removeFD(fd *ControlFD) {
n.controlFDsMu.Lock()
defer n.controlFDsMu.Unlock()
n.controlFDs.Remove(fd)
}
func (n *Node) insertFD(fd *ControlFD) {
n.controlFDsMu.Lock()
defer n.controlFDsMu.Unlock()
n.controlFDs.PushBack(fd)
}
func (n *Node) forEachFD(fn func(*ControlFD)) {
n.controlFDsMu.Lock()
defer n.controlFDsMu.Unlock()
for fd := n.controlFDs.Front(); fd != nil; fd = fd.Next() {
fn(fd)
}
}
// removeChildLocked removes child with given name from n and returns the
// removed child. Returns nil if no such child existed.
//
// Precondition: childrenMu is locked.
func (n *Node) removeChildLocked(name string) *Node {
if n.dynamicChildren != nil {
toRemove := n.dynamicChildren[name]
delete(n.dynamicChildren, name)
return toRemove
}
for i := 0; i < numStaticChildren; i++ {
if n.staticChildren[i].name == name {
toRemove := n.staticChildren[i].node
n.staticChildren[i].name = ""
n.staticChildren[i].node = nil
return toRemove
}
}
return nil
}
// insertChildLocked inserts child into n. It does not check for duplicates.
//
// Precondition: childrenMu is locked.
func (n *Node) insertChildLocked(name string, child *Node) {
// Try to insert statically first if staticChildren is still being used.
if n.dynamicChildren == nil {
for i := 0; i < numStaticChildren; i++ {
if n.staticChildren[i].node == nil {
n.staticChildren[i].node = child
n.staticChildren[i].name = name
return
}
}
// Ran out of static space. Need to start inserting dynamically.
// Shift everything to the map.
n.dynamicChildren = make(map[string]*Node)
for i := 0; i < numStaticChildren; i++ {
// From above loop we know all staticChildren entries are non-nil.
n.dynamicChildren[n.staticChildren[i].name] = n.staticChildren[i].node
n.staticChildren[i].name = ""
n.staticChildren[i].node = nil
}
}
n.dynamicChildren[name] = child
}
func (n *Node) forEachChild(fn func(*Node)) {
n.childrenMu.Lock()
defer n.childrenMu.Unlock()
if n.dynamicChildren != nil {
for _, child := range n.dynamicChildren {
fn(child)
}
return
}
for i := 0; i < numStaticChildren; i++ {
if n.staticChildren[i].node != nil {
fn(n.staticChildren[i].node)
}
}
}
// Precondition: opMu must be locked for writing on the root node being marked
// as deleted.
func (n *Node) markDeletedRecursive() {
n.deleted.Store(1)
// No need to hold opMu for children as it introduces lock ordering issues
// because forEachChild locks childrenMu. Locking opMu after childrenMu
// violates the lock ordering. Anyway if a directory is being deleted, it
// must not have children. The client must have already deleted the entire
// subtree. If the client did not delete this subtree nodes, then the subtree
// was deleted externally and there is not much we can do. This is best
// effort work to mark the subtree as deleted.
n.forEachChild(func(child *Node) {
child.markDeletedRecursive()
})
}