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

3 files changed, 50 insertions, 16 deletions

diff --git a/pkg/sentry/fsimpl/kernfs/fd_impl_util.go b/pkg/sentry/fsimpl/kernfs/fd_impl_util.go
index fcee6200a..6518ff5cd 100644
--- a/pkg/sentry/fsimpl/kernfs/fd_impl_util.go
+++ b/pkg/sentry/fsimpl/kernfs/fd_impl_util.go
@@ -15,7 +15,7 @@
 package kernfs
 
 import (
-	"math"
+	"fmt"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
@@ -28,9 +28,25 @@ import (
 	"gvisor.dev/gvisor/pkg/usermem"
 )
 
+// SeekEndConfig describes the SEEK_END behaviour for FDs.
+type SeekEndConfig int
+
+// Constants related to SEEK_END behaviour for FDs.
+const (
+	// Consider the end of the file to be after the final static entry. This is
+	// the default option.
+	SeekEndStaticEntries = iota
+	// Consider the end of the file to be at offset 0.
+	SeekEndZero
+)
+
+// GenericDirectoryFDOptions contains configuration for a GenericDirectoryFD.
+type GenericDirectoryFDOptions struct {
+	SeekEnd SeekEndConfig
+}
+
 // GenericDirectoryFD implements vfs.FileDescriptionImpl for a generic directory
-// inode that uses OrderChildren to track child nodes. GenericDirectoryFD is not
-// compatible with dynamic directories.
+// inode that uses OrderChildren to track child nodes.
 //
 // Note that GenericDirectoryFD holds a lock over OrderedChildren while calling
 // IterDirents callback. The IterDirents callback therefore cannot hash or
@@ -45,6 +61,9 @@ type GenericDirectoryFD struct {
 	vfs.DirectoryFileDescriptionDefaultImpl
 	vfs.LockFD
 
+	// Immutable.
+	seekEnd SeekEndConfig
+
 	vfsfd    vfs.FileDescription
 	children *OrderedChildren
 
@@ -57,9 +76,9 @@ type GenericDirectoryFD struct {
 
 // NewGenericDirectoryFD creates a new GenericDirectoryFD and returns its
 // dentry.
-func NewGenericDirectoryFD(m *vfs.Mount, d *vfs.Dentry, children *OrderedChildren, locks *vfs.FileLocks, opts *vfs.OpenOptions) (*GenericDirectoryFD, error) {
+func NewGenericDirectoryFD(m *vfs.Mount, d *vfs.Dentry, children *OrderedChildren, locks *vfs.FileLocks, opts *vfs.OpenOptions, fdOpts GenericDirectoryFDOptions) (*GenericDirectoryFD, error) {
 	fd := &GenericDirectoryFD{}
-	if err := fd.Init(children, locks, opts); err != nil {
+	if err := fd.Init(children, locks, opts, fdOpts); err != nil {
 		return nil, err
 	}
 	if err := fd.vfsfd.Init(fd, opts.Flags, m, d, &vfs.FileDescriptionOptions{}); err != nil {
@@ -71,12 +90,13 @@ func NewGenericDirectoryFD(m *vfs.Mount, d *vfs.Dentry, children *OrderedChildre
 // Init initializes a GenericDirectoryFD. Use it when overriding
 // GenericDirectoryFD. Caller must call fd.VFSFileDescription.Init() with the
 // correct implementation.
-func (fd *GenericDirectoryFD) Init(children *OrderedChildren, locks *vfs.FileLocks, opts *vfs.OpenOptions) error {
+func (fd *GenericDirectoryFD) Init(children *OrderedChildren, locks *vfs.FileLocks, opts *vfs.OpenOptions, fdOpts GenericDirectoryFDOptions) error {
 	if vfs.AccessTypesForOpenFlags(opts)&vfs.MayWrite != 0 {
 		// Can't open directories for writing.
 		return syserror.EISDIR
 	}
 	fd.LockFD.Init(locks)
+	fd.seekEnd = fdOpts.SeekEnd
 	fd.children = children
 	return nil
 }
@@ -209,9 +229,17 @@ func (fd *GenericDirectoryFD) Seek(ctx context.Context, offset int64, whence int
 	case linux.SEEK_CUR:
 		offset += fd.off
 	case linux.SEEK_END:
-		// TODO(gvisor.dev/issue/1193): This can prevent new files from showing up
-		// if they are added after SEEK_END.
-		offset = math.MaxInt64
+		switch fd.seekEnd {
+		case SeekEndStaticEntries:
+			fd.children.mu.RLock()
+			offset += int64(len(fd.children.set))
+			offset += 2 // '.' and '..' aren't tracked in children.
+			fd.children.mu.RUnlock()
+		case SeekEndZero:
+			// No-op: offset += 0.
+		default:
+			panic(fmt.Sprintf("Invalid GenericDirectoryFD.seekEnd = %v", fd.seekEnd))
+		}
 	default:
 		return 0, syserror.EINVAL
 	}
diff --git a/pkg/sentry/fsimpl/kernfs/inode_impl_util.go b/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
index fe8a1e710..885856868 100644
--- a/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
+++ b/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
@@ -555,15 +555,16 @@ type StaticDirectory struct {
 	InodeNoDynamicLookup
 	OrderedChildren
 
-	locks vfs.FileLocks
+	locks  vfs.FileLocks
+	fdOpts GenericDirectoryFDOptions
 }
 
 var _ Inode = (*StaticDirectory)(nil)
 
 // NewStaticDir creates a new static directory and returns its dentry.
-func NewStaticDir(creds *auth.Credentials, devMajor, devMinor uint32, ino uint64, perm linux.FileMode, children map[string]*Dentry) *Dentry {
+func NewStaticDir(creds *auth.Credentials, devMajor, devMinor uint32, ino uint64, perm linux.FileMode, children map[string]*Dentry, fdOpts GenericDirectoryFDOptions) *Dentry {
 	inode := &StaticDirectory{}
-	inode.Init(creds, devMajor, devMinor, ino, perm)
+	inode.Init(creds, devMajor, devMinor, ino, perm, fdOpts)
 
 	dentry := &Dentry{}
 	dentry.Init(inode)
@@ -576,16 +577,17 @@ func NewStaticDir(creds *auth.Credentials, devMajor, devMinor uint32, ino uint64
 }
 
 // Init initializes StaticDirectory.
-func (s *StaticDirectory) Init(creds *auth.Credentials, devMajor, devMinor uint32, ino uint64, perm linux.FileMode) {
+func (s *StaticDirectory) Init(creds *auth.Credentials, devMajor, devMinor uint32, ino uint64, perm linux.FileMode, fdOpts GenericDirectoryFDOptions) {
 	if perm&^linux.PermissionsMask != 0 {
 		panic(fmt.Sprintf("Only permission mask must be set: %x", perm&linux.PermissionsMask))
 	}
+	s.fdOpts = fdOpts
 	s.InodeAttrs.Init(creds, devMajor, devMinor, ino, linux.ModeDirectory|perm)
 }
 
 // Open implements kernfs.Inode.
 func (s *StaticDirectory) Open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd, err := NewGenericDirectoryFD(rp.Mount(), vfsd, &s.OrderedChildren, &s.locks, &opts)
+	fd, err := NewGenericDirectoryFD(rp.Mount(), vfsd, &s.OrderedChildren, &s.locks, &opts, s.fdOpts)
 	if err != nil {
 		return nil, err
 	}
diff --git a/pkg/sentry/fsimpl/kernfs/kernfs_test.go b/pkg/sentry/fsimpl/kernfs/kernfs_test.go
index c5d5afedf..e5c28c0e4 100644
--- a/pkg/sentry/fsimpl/kernfs/kernfs_test.go
+++ b/pkg/sentry/fsimpl/kernfs/kernfs_test.go
@@ -119,7 +119,9 @@ func (fs *filesystem) newReadonlyDir(creds *auth.Credentials, mode linux.FileMod
 }
 
 func (d *readonlyDir) Open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &d.OrderedChildren, &d.locks, &opts)
+	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &d.OrderedChildren, &d.locks, &opts, kernfs.GenericDirectoryFDOptions{
+		SeekEnd: kernfs.SeekEndStaticEntries,
+	})
 	if err != nil {
 		return nil, err
 	}
@@ -151,7 +153,9 @@ func (fs *filesystem) newDir(creds *auth.Credentials, mode linux.FileMode, conte
 }
 
 func (d *dir) Open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &d.OrderedChildren, &d.locks, &opts)
+	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &d.OrderedChildren, &d.locks, &opts, kernfs.GenericDirectoryFDOptions{
+		SeekEnd: kernfs.SeekEndStaticEntries,
+	})
 	if err != nil {
 		return nil, err
 	}