Clarify seek behaviour for kernfs.GenericDirectoryFD.

- Remove comment about GenericDirectoryFD not being compatible with
  dynamic directories. It is currently being used to implement dynamic
  directories.

- Try to handle SEEK_END better than setting the offset to
  infinity. SEEK_END is poorly defined for dynamic directories
  anyways, so at least try make it work correctly for the static
  entries.

Updates #1193.

PiperOrigin-RevId: 327890128

6 files changed, 30 insertions, 12 deletions

diff --git a/pkg/sentry/fsimpl/proc/filesystem.go b/pkg/sentry/fsimpl/proc/filesystem.go
index 2463d51cd..c350ec127 100644
--- a/pkg/sentry/fsimpl/proc/filesystem.go
+++ b/pkg/sentry/fsimpl/proc/filesystem.go
@@ -110,6 +110,12 @@ func newStaticFile(data string) *staticFile {
 	return &staticFile{StaticData: vfs.StaticData{Data: data}}
 }
 
+func newStaticDir(creds *auth.Credentials, devMajor, devMinor uint32, ino uint64, perm linux.FileMode, children map[string]*kernfs.Dentry) *kernfs.Dentry {
+	return kernfs.NewStaticDir(creds, devMajor, devMinor, ino, perm, children, kernfs.GenericDirectoryFDOptions{
+		SeekEnd: kernfs.SeekEndZero,
+	})
+}
+
 // InternalData contains internal data passed in to the procfs mount via
 // vfs.GetFilesystemOptions.InternalData.
 type InternalData struct {
diff --git a/pkg/sentry/fsimpl/proc/subtasks.go b/pkg/sentry/fsimpl/proc/subtasks.go
index 79c2725f3..f25747da3 100644
--- a/pkg/sentry/fsimpl/proc/subtasks.go
+++ b/pkg/sentry/fsimpl/proc/subtasks.go
@@ -155,7 +155,9 @@ func (fd *subtasksFD) SetStat(ctx context.Context, opts vfs.SetStatOptions) erro
 // Open implements kernfs.Inode.
 func (i *subtasksInode) Open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
 	fd := &subtasksFD{task: i.task}
-	if err := fd.Init(&i.OrderedChildren, &i.locks, &opts); err != nil {
+	if err := fd.Init(&i.OrderedChildren, &i.locks, &opts, kernfs.GenericDirectoryFDOptions{
+		SeekEnd: kernfs.SeekEndZero,
+	}); err != nil {
 		return nil, err
 	}
 	if err := fd.VFSFileDescription().Init(fd, opts.Flags, rp.Mount(), vfsd, &vfs.FileDescriptionOptions{}); err != nil {
diff --git a/pkg/sentry/fsimpl/proc/task.go b/pkg/sentry/fsimpl/proc/task.go
index a5c7aa470..109b31b4c 100644
--- a/pkg/sentry/fsimpl/proc/task.go
+++ b/pkg/sentry/fsimpl/proc/task.go
@@ -105,7 +105,9 @@ func (i *taskInode) Valid(ctx context.Context) bool {
 
 // Open implements kernfs.Inode.
 func (i *taskInode) Open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &i.OrderedChildren, &i.locks, &opts)
+	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &i.OrderedChildren, &i.locks, &opts, kernfs.GenericDirectoryFDOptions{
+		SeekEnd: kernfs.SeekEndZero,
+	})
 	if err != nil {
 		return nil, err
 	}
@@ -142,7 +144,9 @@ func (fs *filesystem) newTaskOwnedDir(task *kernel.Task, ino uint64, perm linux.
 	dir := &kernfs.StaticDirectory{}
 
 	// Note: credentials are overridden by taskOwnedInode.
-	dir.Init(task.Credentials(), linux.UNNAMED_MAJOR, fs.devMinor, ino, perm)
+	dir.Init(task.Credentials(), linux.UNNAMED_MAJOR, fs.devMinor, ino, perm, kernfs.GenericDirectoryFDOptions{
+		SeekEnd: kernfs.SeekEndZero,
+	})
 
 	inode := &taskOwnedInode{Inode: dir, owner: task}
 	d := &kernfs.Dentry{}
diff --git a/pkg/sentry/fsimpl/proc/task_fds.go b/pkg/sentry/fsimpl/proc/task_fds.go
index f0d3f7f5e..e8fcb9aa1 100644
--- a/pkg/sentry/fsimpl/proc/task_fds.go
+++ b/pkg/sentry/fsimpl/proc/task_fds.go
@@ -144,7 +144,9 @@ func (i *fdDirInode) Lookup(ctx context.Context, name string) (*vfs.Dentry, erro
 
 // Open implements kernfs.Inode.
 func (i *fdDirInode) Open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &i.OrderedChildren, &i.locks, &opts)
+	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &i.OrderedChildren, &i.locks, &opts, kernfs.GenericDirectoryFDOptions{
+		SeekEnd: kernfs.SeekEndZero,
+	})
 	if err != nil {
 		return nil, err
 	}
@@ -271,7 +273,9 @@ func (i *fdInfoDirInode) Lookup(ctx context.Context, name string) (*vfs.Dentry,
 
 // Open implements kernfs.Inode.
 func (i *fdInfoDirInode) Open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &i.OrderedChildren, &i.locks, &opts)
+	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &i.OrderedChildren, &i.locks, &opts, kernfs.GenericDirectoryFDOptions{
+		SeekEnd: kernfs.SeekEndZero,
+	})
 	if err != nil {
 		return nil, err
 	}
diff --git a/pkg/sentry/fsimpl/proc/tasks.go b/pkg/sentry/fsimpl/proc/tasks.go
index 6d2b90a8b..1391992b7 100644
--- a/pkg/sentry/fsimpl/proc/tasks.go
+++ b/pkg/sentry/fsimpl/proc/tasks.go
@@ -199,7 +199,9 @@ func (i *tasksInode) IterDirents(ctx context.Context, cb vfs.IterDirentsCallback
 
 // Open implements kernfs.Inode.
 func (i *tasksInode) Open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &i.OrderedChildren, &i.locks, &opts)
+	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &i.OrderedChildren, &i.locks, &opts, kernfs.GenericDirectoryFDOptions{
+		SeekEnd: kernfs.SeekEndZero,
+	})
 	if err != nil {
 		return nil, err
 	}
diff --git a/pkg/sentry/fsimpl/proc/tasks_sys.go b/pkg/sentry/fsimpl/proc/tasks_sys.go
index 6435385ef..038a194c7 100644
--- a/pkg/sentry/fsimpl/proc/tasks_sys.go
+++ b/pkg/sentry/fsimpl/proc/tasks_sys.go
@@ -39,14 +39,14 @@ const (
 
 // newSysDir returns the dentry corresponding to /proc/sys directory.
 func (fs *filesystem) newSysDir(root *auth.Credentials, k *kernel.Kernel) *kernfs.Dentry {
-	return kernfs.NewStaticDir(root, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), 0555, map[string]*kernfs.Dentry{
-		"kernel": kernfs.NewStaticDir(root, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), 0555, map[string]*kernfs.Dentry{
+	return newStaticDir(root, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), 0555, map[string]*kernfs.Dentry{
+		"kernel": newStaticDir(root, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), 0555, map[string]*kernfs.Dentry{
 			"hostname": fs.newDentry(root, fs.NextIno(), 0444, &hostnameData{}),
 			"shmall":   fs.newDentry(root, fs.NextIno(), 0444, shmData(linux.SHMALL)),
 			"shmmax":   fs.newDentry(root, fs.NextIno(), 0444, shmData(linux.SHMMAX)),
 			"shmmni":   fs.newDentry(root, fs.NextIno(), 0444, shmData(linux.SHMMNI)),
 		}),
-		"vm": kernfs.NewStaticDir(root, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), 0555, map[string]*kernfs.Dentry{
+		"vm": newStaticDir(root, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), 0555, map[string]*kernfs.Dentry{
 			"mmap_min_addr":     fs.newDentry(root, fs.NextIno(), 0444, &mmapMinAddrData{k: k}),
 			"overcommit_memory": fs.newDentry(root, fs.NextIno(), 0444, newStaticFile("0\n")),
 		}),
@@ -62,7 +62,7 @@ func (fs *filesystem) newSysNetDir(root *auth.Credentials, k *kernel.Kernel) *ke
 	// network namespace of the calling process.
 	if stack := k.RootNetworkNamespace().Stack(); stack != nil {
 		contents = map[string]*kernfs.Dentry{
-			"ipv4": kernfs.NewStaticDir(root, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), 0555, map[string]*kernfs.Dentry{
+			"ipv4": newStaticDir(root, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), 0555, map[string]*kernfs.Dentry{
 				"tcp_recovery": fs.newDentry(root, fs.NextIno(), 0644, &tcpRecoveryData{stack: stack}),
 				"tcp_rmem":     fs.newDentry(root, fs.NextIno(), 0644, &tcpMemData{stack: stack, dir: tcpRMem}),
 				"tcp_sack":     fs.newDentry(root, fs.NextIno(), 0644, &tcpSackData{stack: stack}),
@@ -109,7 +109,7 @@ func (fs *filesystem) newSysNetDir(root *auth.Credentials, k *kernel.Kernel) *ke
 				"tcp_syn_retries":           fs.newDentry(root, fs.NextIno(), 0444, newStaticFile("3")),
 				"tcp_timestamps":            fs.newDentry(root, fs.NextIno(), 0444, newStaticFile("1")),
 			}),
-			"core": kernfs.NewStaticDir(root, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), 0555, map[string]*kernfs.Dentry{
+			"core": newStaticDir(root, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), 0555, map[string]*kernfs.Dentry{
 				"default_qdisc": fs.newDentry(root, fs.NextIno(), 0444, newStaticFile("pfifo_fast")),
 				"message_burst": fs.newDentry(root, fs.NextIno(), 0444, newStaticFile("10")),
 				"message_cost":  fs.newDentry(root, fs.NextIno(), 0444, newStaticFile("5")),
@@ -123,7 +123,7 @@ func (fs *filesystem) newSysNetDir(root *auth.Credentials, k *kernel.Kernel) *ke
 		}
 	}
 
-	return kernfs.NewStaticDir(root, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), 0555, contents)
+	return newStaticDir(root, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), 0555, contents)
 }
 
 // mmapMinAddrData implements vfs.DynamicBytesSource for