Add vfs.MkdirOptions.ForSyntheticMountpoint.

PiperOrigin-RevId: 308143529

1 files changed, 114 insertions, 63 deletions

diff --git a/pkg/sentry/fsimpl/gofer/gofer.go b/pkg/sentry/fsimpl/gofer/gofer.go
index 293df2545..8b4e91d17 100644
--- a/pkg/sentry/fsimpl/gofer/gofer.go
+++ b/pkg/sentry/fsimpl/gofer/gofer.go
@@ -27,8 +27,9 @@
 //             dentry.handleMu
 //               dentry.dataMu
 //
-// Locking dentry.dirMu in multiple dentries requires holding
-// filesystem.renameMu for writing.
+// Locking dentry.dirMu in multiple dentries requires that either ancestor
+// dentries are locked before descendant dentries, or that filesystem.renameMu
+// is locked for writing.
 package gofer
 
 import (
@@ -102,11 +103,12 @@ type filesystem struct {
 	cachedDentries    dentryList
 	cachedDentriesLen uint64
 
-	// dentries contains all dentries in this filesystem. specialFileFDs
-	// contains all open specialFileFDs. These fields are protected by syncMu.
-	syncMu         sync.Mutex
-	dentries       map[*dentry]struct{}
-	specialFileFDs map[*specialFileFD]struct{}
+	// syncableDentries contains all dentries in this filesystem for which
+	// !dentry.file.isNil(). specialFileFDs contains all open specialFileFDs.
+	// These fields are protected by syncMu.
+	syncMu           sync.Mutex
+	syncableDentries map[*dentry]struct{}
+	specialFileFDs   map[*specialFileFD]struct{}
 }
 
 type filesystemOptions struct {
@@ -187,7 +189,8 @@ const (
 	// InteropModeShared is appropriate when there are users of the remote
 	// filesystem that may mutate its state other than the client.
 	//
-	// - The client must verify cached filesystem state before using it.
+	// - The client must verify ("revalidate") cached filesystem state before
+	// using it.
 	//
 	// - Client changes to filesystem state must be sent to the remote
 	// filesystem synchronously.
@@ -376,14 +379,14 @@ func (fstype FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 
 	// Construct the filesystem object.
 	fs := &filesystem{
-		mfp:            mfp,
-		opts:           fsopts,
-		uid:            creds.EffectiveKUID,
-		gid:            creds.EffectiveKGID,
-		client:         client,
-		clock:          ktime.RealtimeClockFromContext(ctx),
-		dentries:       make(map[*dentry]struct{}),
-		specialFileFDs: make(map[*specialFileFD]struct{}),
+		mfp:              mfp,
+		opts:             fsopts,
+		uid:              creds.EffectiveKUID,
+		gid:              creds.EffectiveKGID,
+		client:           client,
+		clock:            ktime.RealtimeClockFromContext(ctx),
+		syncableDentries: make(map[*dentry]struct{}),
+		specialFileFDs:   make(map[*specialFileFD]struct{}),
 	}
 	fs.vfsfs.Init(vfsObj, &fstype, fs)
 
@@ -409,7 +412,7 @@ func (fs *filesystem) Release() {
 	mf := fs.mfp.MemoryFile()
 
 	fs.syncMu.Lock()
-	for d := range fs.dentries {
+	for d := range fs.syncableDentries {
 		d.handleMu.Lock()
 		d.dataMu.Lock()
 		if d.handleWritable {
@@ -444,9 +447,11 @@ type dentry struct {
 	vfsd vfs.Dentry
 
 	// refs is the reference count. Each dentry holds a reference on its
-	// parent, even if disowned. refs is accessed using atomic memory
-	// operations. When refs reaches 0, the dentry may be added to the cache or
-	// destroyed. If refs==-1 the dentry has already been destroyed.
+	// parent, even if disowned. An additional reference is held on all
+	// synthetic dentries until they are unlinked or invalidated. When refs
+	// reaches 0, the dentry may be added to the cache or destroyed. If refs ==
+	// -1, the dentry has already been destroyed. refs is accessed using atomic
+	// memory operations.
 	refs int64
 
 	// fs is the owning filesystem. fs is immutable.
@@ -465,6 +470,12 @@ type dentry struct {
 	// We don't support hard links, so each dentry maps 1:1 to an inode.
 
 	// file is the unopened p9.File that backs this dentry. file is immutable.
+	//
+	// If file.isNil(), this dentry represents a synthetic file, i.e. a file
+	// that does not exist on the remote filesystem. As of this writing, this
+	// is only possible for a directory created with
+	// MkdirOptions.ForSyntheticMountpoint == true.
+	// TODO(gvisor.dev/issue/1476): Support synthetic sockets (and pipes).
 	file p9file
 
 	// If deleted is non-zero, the file represented by this dentry has been
@@ -484,15 +495,21 @@ type dentry struct {
 	// - Mappings of child filenames to dentries representing those children.
 	//
 	// - Mappings of child filenames that are known not to exist to nil
-	// dentries (only if InteropModeShared is not in effect).
+	// dentries (only if InteropModeShared is not in effect and the directory
+	// is not synthetic).
 	//
 	// children is protected by dirMu.
 	children map[string]*dentry
 
-	// If this dentry represents a directory, InteropModeShared is not in
-	// effect, and dirents is not nil, it is a cache of all entries in the
-	// directory, in the order they were returned by the server. dirents is
-	// protected by dirMu.
+	// If this dentry represents a directory, syntheticChildren is the number
+	// of child dentries for which dentry.isSynthetic() == true.
+	// syntheticChildren is protected by dirMu.
+	syntheticChildren int
+
+	// If this dentry represents a directory,
+	// dentry.cachedMetadataAuthoritative() == true, and dirents is not nil, it
+	// is a cache of all entries in the directory, in the order they were
+	// returned by the server. dirents is protected by dirMu.
 	dirents []vfs.Dirent
 
 	// Cached metadata; protected by metadataMu and accessed using atomic
@@ -589,6 +606,8 @@ func dentryAttrMask() p9.AttrMask {
 // initially has no references, but is not cached; it is the caller's
 // responsibility to set the dentry's reference count and/or call
 // dentry.checkCachingLocked() as appropriate.
+//
+// Preconditions: !file.isNil().
 func (fs *filesystem) newDentry(ctx context.Context, file p9file, qid p9.QID, mask p9.AttrMask, attr *p9.Attr) (*dentry, error) {
 	if !mask.Mode {
 		ctx.Warningf("can't create gofer.dentry without file type")
@@ -612,10 +631,10 @@ func (fs *filesystem) newDentry(ctx context.Context, file p9file, qid p9.QID, ma
 		},
 	}
 	d.pf.dentry = d
-	if mask.UID {
+	if mask.UID && attr.UID != auth.NoID {
 		d.uid = uint32(attr.UID)
 	}
-	if mask.GID {
+	if mask.GID && attr.GID != auth.NoID {
 		d.gid = uint32(attr.GID)
 	}
 	if mask.Size {
@@ -642,11 +661,19 @@ func (fs *filesystem) newDentry(ctx context.Context, file p9file, qid p9.QID, ma
 	d.vfsd.Init(d)
 
 	fs.syncMu.Lock()
-	fs.dentries[d] = struct{}{}
+	fs.syncableDentries[d] = struct{}{}
 	fs.syncMu.Unlock()
 	return d, nil
 }
 
+func (d *dentry) isSynthetic() bool {
+	return d.file.isNil()
+}
+
+func (d *dentry) cachedMetadataAuthoritative() bool {
+	return d.fs.opts.interop != InteropModeShared || d.isSynthetic()
+}
+
 // updateFromP9Attrs is called to update d's metadata after an update from the
 // remote filesystem.
 func (d *dentry) updateFromP9Attrs(mask p9.AttrMask, attr *p9.Attr) {
@@ -691,6 +718,7 @@ func (d *dentry) updateFromP9Attrs(mask p9.AttrMask, attr *p9.Attr) {
 	d.metadataMu.Unlock()
 }
 
+// Preconditions: !d.isSynthetic()
 func (d *dentry) updateFromGetattr(ctx context.Context) error {
 	// Use d.handle.file, which represents a 9P fid that has been opened, in
 	// preference to d.file, which represents a 9P fid that has not. This may
@@ -758,7 +786,7 @@ func (d *dentry) setStat(ctx context.Context, creds *auth.Credentials, stat *lin
 	defer mnt.EndWrite()
 	setLocalAtime := false
 	setLocalMtime := false
-	if d.fs.opts.interop != InteropModeShared {
+	if d.cachedMetadataAuthoritative() {
 		// Timestamp updates will be handled locally.
 		setLocalAtime = stat.Mask&linux.STATX_ATIME != 0
 		setLocalMtime = stat.Mask&linux.STATX_MTIME != 0
@@ -771,35 +799,37 @@ func (d *dentry) setStat(ctx context.Context, creds *auth.Credentials, stat *lin
 	}
 	d.metadataMu.Lock()
 	defer d.metadataMu.Unlock()
-	if stat.Mask != 0 {
-		if err := d.file.setAttr(ctx, p9.SetAttrMask{
-			Permissions:        stat.Mask&linux.STATX_MODE != 0,
-			UID:                stat.Mask&linux.STATX_UID != 0,
-			GID:                stat.Mask&linux.STATX_GID != 0,
-			Size:               stat.Mask&linux.STATX_SIZE != 0,
-			ATime:              stat.Mask&linux.STATX_ATIME != 0,
-			MTime:              stat.Mask&linux.STATX_MTIME != 0,
-			ATimeNotSystemTime: stat.Atime.Nsec != linux.UTIME_NOW,
-			MTimeNotSystemTime: stat.Mtime.Nsec != linux.UTIME_NOW,
-		}, p9.SetAttr{
-			Permissions:      p9.FileMode(stat.Mode),
-			UID:              p9.UID(stat.UID),
-			GID:              p9.GID(stat.GID),
-			Size:             stat.Size,
-			ATimeSeconds:     uint64(stat.Atime.Sec),
-			ATimeNanoSeconds: uint64(stat.Atime.Nsec),
-			MTimeSeconds:     uint64(stat.Mtime.Sec),
-			MTimeNanoSeconds: uint64(stat.Mtime.Nsec),
-		}); err != nil {
-			return err
+	if !d.isSynthetic() {
+		if stat.Mask != 0 {
+			if err := d.file.setAttr(ctx, p9.SetAttrMask{
+				Permissions:        stat.Mask&linux.STATX_MODE != 0,
+				UID:                stat.Mask&linux.STATX_UID != 0,
+				GID:                stat.Mask&linux.STATX_GID != 0,
+				Size:               stat.Mask&linux.STATX_SIZE != 0,
+				ATime:              stat.Mask&linux.STATX_ATIME != 0,
+				MTime:              stat.Mask&linux.STATX_MTIME != 0,
+				ATimeNotSystemTime: stat.Atime.Nsec != linux.UTIME_NOW,
+				MTimeNotSystemTime: stat.Mtime.Nsec != linux.UTIME_NOW,
+			}, p9.SetAttr{
+				Permissions:      p9.FileMode(stat.Mode),
+				UID:              p9.UID(stat.UID),
+				GID:              p9.GID(stat.GID),
+				Size:             stat.Size,
+				ATimeSeconds:     uint64(stat.Atime.Sec),
+				ATimeNanoSeconds: uint64(stat.Atime.Nsec),
+				MTimeSeconds:     uint64(stat.Mtime.Sec),
+				MTimeNanoSeconds: uint64(stat.Mtime.Nsec),
+			}); err != nil {
+				return err
+			}
+		}
+		if d.fs.opts.interop == InteropModeShared {
+			// There's no point to updating d's metadata in this case since
+			// it'll be overwritten by revalidation before the next time it's
+			// used anyway. (InteropModeShared inhibits client caching of
+			// regular file data, so there's no cache to truncate either.)
+			return nil
 		}
-	}
-	if d.fs.opts.interop == InteropModeShared {
-		// There's no point to updating d's metadata in this case since it'll
-		// be overwritten by revalidation before the next time it's used
-		// anyway. (InteropModeShared inhibits client caching of regular file
-		// data, so there's no cache to truncate either.)
-		return nil
 	}
 	now := d.fs.clock.Now().Nanoseconds()
 	if stat.Mask&linux.STATX_MODE != 0 {
@@ -897,6 +927,15 @@ func (d *dentry) DecRef() {
 	}
 }
 
+// decRefLocked decrements d's reference count without calling
+// d.checkCachingLocked, even if d's reference count reaches 0; callers are
+// responsible for ensuring that d.checkCachingLocked will be called later.
+func (d *dentry) decRefLocked() {
+	if refs := atomic.AddInt64(&d.refs, -1); refs < 0 {
+		panic("gofer.dentry.decRefLocked() called without holding a reference")
+	}
+}
+
 // checkCachingLocked should be called after d's reference count becomes 0 or it
 // becomes disowned.
 //
@@ -1013,11 +1052,11 @@ func (d *dentry) destroyLocked() {
 	if !d.file.isNil() {
 		d.file.close(ctx)
 		d.file = p9file{}
+		// Remove d from the set of syncable dentries.
+		d.fs.syncMu.Lock()
+		delete(d.fs.syncableDentries, d)
+		d.fs.syncMu.Unlock()
 	}
-	// Remove d from the set of all dentries.
-	d.fs.syncMu.Lock()
-	delete(d.fs.dentries, d)
-	d.fs.syncMu.Unlock()
 	// Drop the reference held by d on its parent without recursively locking
 	// d.fs.renameMu.
 	if d.parent != nil {
@@ -1040,6 +1079,9 @@ func (d *dentry) setDeleted() {
 // We only support xattrs prefixed with "user." (see b/148380782). Currently,
 // there is no need to expose any other xattrs through a gofer.
 func (d *dentry) listxattr(ctx context.Context, creds *auth.Credentials, size uint64) ([]string, error) {
+	if d.file.isNil() {
+		return nil, nil
+	}
 	xattrMap, err := d.file.listXattr(ctx, size)
 	if err != nil {
 		return nil, err
@@ -1054,6 +1096,9 @@ func (d *dentry) listxattr(ctx context.Context, creds *auth.Credentials, size ui
 }
 
 func (d *dentry) getxattr(ctx context.Context, creds *auth.Credentials, opts *vfs.GetxattrOptions) (string, error) {
+	if d.file.isNil() {
+		return "", syserror.ENODATA
+	}
 	if err := d.checkPermissions(creds, vfs.MayRead); err != nil {
 		return "", err
 	}
@@ -1064,6 +1109,9 @@ func (d *dentry) getxattr(ctx context.Context, creds *auth.Credentials, opts *vf
 }
 
 func (d *dentry) setxattr(ctx context.Context, creds *auth.Credentials, opts *vfs.SetxattrOptions) error {
+	if d.file.isNil() {
+		return syserror.EPERM
+	}
 	if err := d.checkPermissions(creds, vfs.MayWrite); err != nil {
 		return err
 	}
@@ -1074,6 +1122,9 @@ func (d *dentry) setxattr(ctx context.Context, creds *auth.Credentials, opts *vf
 }
 
 func (d *dentry) removexattr(ctx context.Context, creds *auth.Credentials, name string) error {
+	if d.file.isNil() {
+		return syserror.EPERM
+	}
 	if err := d.checkPermissions(creds, vfs.MayWrite); err != nil {
 		return err
 	}
@@ -1083,7 +1134,7 @@ func (d *dentry) removexattr(ctx context.Context, creds *auth.Credentials, name
 	return d.file.removeXattr(ctx, name)
 }
 
-// Preconditions: d.isRegularFile() || d.isDirectory().
+// Preconditions: !d.file.isNil(). d.isRegularFile() || d.isDirectory().
 func (d *dentry) ensureSharedHandle(ctx context.Context, read, write, trunc bool) error {
 	// O_TRUNC unconditionally requires us to obtain a new handle (opened with
 	// O_TRUNC).
@@ -1213,7 +1264,7 @@ func (fd *fileDescription) dentry() *dentry {
 func (fd *fileDescription) Stat(ctx context.Context, opts vfs.StatOptions) (linux.Statx, error) {
 	d := fd.dentry()
 	const validMask = uint32(linux.STATX_MODE | linux.STATX_UID | linux.STATX_GID | linux.STATX_ATIME | linux.STATX_MTIME | linux.STATX_CTIME | linux.STATX_SIZE | linux.STATX_BLOCKS | linux.STATX_BTIME)
-	if d.fs.opts.interop == InteropModeShared && opts.Mask&(validMask) != 0 && opts.Sync != linux.AT_STATX_DONT_SYNC {
+	if !d.cachedMetadataAuthoritative() && opts.Mask&validMask != 0 && opts.Sync != linux.AT_STATX_DONT_SYNC {
 		// TODO(jamieliu): Use specialFileFD.handle.file for the getattr if
 		// available?
 		if err := d.updateFromGetattr(ctx); err != nil {