Add vfs.Pathname{WithDeleted,ForGetcwd}.

The former is needed for vfs.FileDescription to implement
memmap.MappingIdentity, and the latter is needed to implement getcwd(2).

PiperOrigin-RevId: 285051855

pkg/sentry/fsimpl/ext/BUILD

@@ -38,6 +38,7 @@ go_library(
         "//pkg/abi/linux",
         "//pkg/binary",
         "//pkg/fd",
+        "//pkg/fspath",
         "//pkg/log",
         "//pkg/sentry/arch",
         "//pkg/sentry/context",

pkg/sentry/fsimpl/ext/filesystem.go

@@ -20,6 +20,7 @@ import (
 	"sync"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/fspath"
 	"gvisor.dev/gvisor/pkg/sentry/context"
 	"gvisor.dev/gvisor/pkg/sentry/fsimpl/ext/disklayout"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
@@ -441,3 +442,10 @@ func (fs *filesystem) UnlinkAt(ctx context.Context, rp *vfs.ResolvingPath) error
 
 	return syserror.EROFS
 }
+
+// PrependPath implements vfs.FilesystemImpl.PrependPath.
+func (fs *filesystem) PrependPath(ctx context.Context, vfsroot, vd vfs.VirtualDentry, b *fspath.Builder) error {
+	fs.mu.RLock()
+	defer fs.mu.RUnlock()
+	return vfs.GenericPrependPath(vfsroot, vd, b)
+}

pkg/sentry/fsimpl/memfs/BUILD

@@ -32,6 +32,7 @@ go_library(
     deps = [
         "//pkg/abi/linux",
         "//pkg/amutex",
+        "//pkg/fspath",
         "//pkg/sentry/arch",
         "//pkg/sentry/context",
         "//pkg/sentry/kernel/auth",

pkg/sentry/fsimpl/memfs/filesystem.go

@@ -19,6 +19,7 @@ import (
 	"sync/atomic"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/fspath"
 	"gvisor.dev/gvisor/pkg/sentry/context"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 	"gvisor.dev/gvisor/pkg/syserror"
@@ -582,3 +583,10 @@ func (fs *filesystem) UnlinkAt(ctx context.Context, rp *vfs.ResolvingPath) error
 	inode.decLinksLocked()
 	return nil
 }
+
+// PrependPath implements vfs.FilesystemImpl.PrependPath.
+func (fs *filesystem) PrependPath(ctx context.Context, vfsroot, vd vfs.VirtualDentry, b *fspath.Builder) error {
+	fs.mu.RLock()
+	defer fs.mu.RUnlock()
+	return vfs.GenericPrependPath(vfsroot, vd, b)
+}

pkg/sentry/vfs/BUILD

@@ -17,6 +17,7 @@ go_library(
         "mount.go",
         "mount_unsafe.go",
         "options.go",
+        "pathname.go",
         "permissions.go",
         "resolving_path.go",
         "testutil.go",

pkg/sentry/vfs/filesystem.go

@@ -18,6 +18,7 @@ import (
 	"sync/atomic"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/fspath"
 	"gvisor.dev/gvisor/pkg/sentry/context"
 )
 
@@ -185,5 +186,56 @@ type FilesystemImpl interface {
 	// UnlinkAt removes the non-directory file at rp.
 	UnlinkAt(ctx context.Context, rp *ResolvingPath) error
 
-	// TODO: d_path(); extended attributes; inotify_add_watch(); bind()
+	// PrependPath prepends a path from vd to vd.Mount().Root() to b.
+	//
+	// If vfsroot.Ok(), it is the contextual VFS root; if it is encountered
+	// before vd.Mount().Root(), PrependPath should stop prepending path
+	// components and return a PrependPathAtVFSRootError.
+	//
+	// If traversal of vd.Dentry()'s ancestors encounters an independent
+	// ("root") Dentry that is not vd.Mount().Root() (i.e. vd.Dentry() is not a
+	// descendant of vd.Mount().Root()), PrependPath should stop prepending
+	// path components and return a PrependPathAtNonMountRootError.
+	//
+	// Filesystems for which Dentries do not have meaningful paths may prepend
+	// an arbitrary descriptive string to b and then return a
+	// PrependPathSyntheticError.
+	//
+	// Most implementations can acquire the appropriate locks to ensure that
+	// Dentry.Name() and Dentry.Parent() are fixed for vd.Dentry() and all of
+	// its ancestors, then call GenericPrependPath.
+	//
+	// Preconditions: vd.Mount().Filesystem().Impl() == this FilesystemImpl.
+	PrependPath(ctx context.Context, vfsroot, vd VirtualDentry, b *fspath.Builder) error
+
+	// TODO: extended attributes; inotify_add_watch(); bind()
+}
+
+// PrependPathAtVFSRootError is returned by implementations of
+// FilesystemImpl.PrependPath() when they encounter the contextual VFS root.
+type PrependPathAtVFSRootError struct{}
+
+// Error implements error.Error.
+func (PrependPathAtVFSRootError) Error() string {
+	return "vfs.FilesystemImpl.PrependPath() reached VFS root"
+}
+
+// PrependPathAtNonMountRootError is returned by implementations of
+// FilesystemImpl.PrependPath() when they encounter an independent ancestor
+// Dentry that is not the Mount root.
+type PrependPathAtNonMountRootError struct{}
+
+// Error implements error.Error.
+func (PrependPathAtNonMountRootError) Error() string {
+	return "vfs.FilesystemImpl.PrependPath() reached root other than Mount root"
+}
+
+// PrependPathSyntheticError is returned by implementations of
+// FilesystemImpl.PrependPath() for which prepended names do not represent real
+// paths.
+type PrependPathSyntheticError struct{}
+
+// Error implements error.Error.
+func (PrependPathSyntheticError) Error() string {
+	return "vfs.FilesystemImpl.PrependPath() prepended synthetic name"
 }

pkg/sentry/vfs/filesystem_impl_util.go

@@ -16,6 +16,8 @@ package vfs
 
 import (
 	"strings"
+
+	"gvisor.dev/gvisor/pkg/fspath"
 )
 
 // GenericParseMountOptions parses a comma-separated list of options of the
@@ -41,3 +43,27 @@ func GenericParseMountOptions(str string) map[string]string {
 	}
 	return m
 }
+
+// GenericPrependPath may be used by implementations of
+// FilesystemImpl.PrependPath() for which a single statically-determined lock
+// or set of locks is sufficient to ensure its preconditions (as opposed to
+// e.g. per-Dentry locks).
+//
+// Preconditions: Dentry.Name() and Dentry.Parent() must be held constant for
+// vd.Dentry() and all of its ancestors.
+func GenericPrependPath(vfsroot, vd VirtualDentry, b *fspath.Builder) error {
+	mnt, d := vd.mount, vd.dentry
+	for {
+		if mnt == vfsroot.mount && d == vfsroot.dentry {
+			return PrependPathAtVFSRootError{}
+		}
+		if d == mnt.root {
+			return nil
+		}
+		if d.parent == nil {
+			return PrependPathAtNonMountRootError{}
+		}
+		b.PrependComponent(d.name)
+		d = d.parent
+	}
+}

pkg/sentry/vfs/pathname.go

@@ -0,0 +1,153 @@
+// Copyright 2019 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 vfs
+
+import (
+	"sync"
+
+	"gvisor.dev/gvisor/pkg/fspath"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+var fspathBuilderPool = sync.Pool{
+	New: func() interface{} {
+		return &fspath.Builder{}
+	},
+}
+
+func getFSPathBuilder() *fspath.Builder {
+	return fspathBuilderPool.Get().(*fspath.Builder)
+}
+
+func putFSPathBuilder(b *fspath.Builder) {
+	// No methods can be called on b after b.String(), so reset it to its zero
+	// value (as returned by fspathBuilderPool.New) instead.
+	*b = fspath.Builder{}
+	fspathBuilderPool.Put(b)
+}
+
+// PathnameWithDeleted returns an absolute pathname to vd, consistent with
+// Linux's d_path(). In particular, if vd.Dentry() has been disowned,
+// PathnameWithDeleted appends " (deleted)" to the returned pathname.
+func (vfs *VirtualFilesystem) PathnameWithDeleted(ctx context.Context, vfsroot, vd VirtualDentry) (string, error) {
+	b := getFSPathBuilder()
+	defer putFSPathBuilder(b)
+	haveRef := false
+	defer func() {
+		if haveRef {
+			vd.DecRef()
+		}
+	}()
+
+	origD := vd.dentry
+loop:
+	for {
+		err := vd.mount.fs.impl.PrependPath(ctx, vfsroot, vd, b)
+		switch err.(type) {
+		case nil:
+			if vd.mount == vfsroot.mount && vd.mount.root == vfsroot.dentry {
+				// GenericPrependPath() will have returned
+				// PrependPathAtVFSRootError in this case since it checks
+				// against vfsroot before mnt.root, but other implementations
+				// of FilesystemImpl.PrependPath() may return nil instead.
+				break loop
+			}
+			nextVD := vfs.getMountpointAt(vd.mount, vfsroot)
+			if !nextVD.Ok() {
+				break loop
+			}
+			if haveRef {
+				vd.DecRef()
+			}
+			vd = nextVD
+			haveRef = true
+			// continue loop
+		case PrependPathSyntheticError:
+			// Skip prepending "/" and appending " (deleted)".
+			return b.String(), nil
+		case PrependPathAtVFSRootError, PrependPathAtNonMountRootError:
+			break loop
+		default:
+			return "", err
+		}
+	}
+	b.PrependByte('/')
+	if origD.IsDisowned() {
+		b.AppendString(" (deleted)")
+	}
+	return b.String(), nil
+}
+
+// PathnameForGetcwd returns an absolute pathname to vd, consistent with
+// Linux's sys_getcwd().
+func (vfs *VirtualFilesystem) PathnameForGetcwd(ctx context.Context, vfsroot, vd VirtualDentry) (string, error) {
+	if vd.dentry.IsDisowned() {
+		return "", syserror.ENOENT
+	}
+
+	b := getFSPathBuilder()
+	defer putFSPathBuilder(b)
+	haveRef := false
+	defer func() {
+		if haveRef {
+			vd.DecRef()
+		}
+	}()
+	unreachable := false
+loop:
+	for {
+		err := vd.mount.fs.impl.PrependPath(ctx, vfsroot, vd, b)
+		switch err.(type) {
+		case nil:
+			if vd.mount == vfsroot.mount && vd.mount.root == vfsroot.dentry {
+				break loop
+			}
+			nextVD := vfs.getMountpointAt(vd.mount, vfsroot)
+			if !nextVD.Ok() {
+				unreachable = true
+				break loop
+			}
+			if haveRef {
+				vd.DecRef()
+			}
+			vd = nextVD
+			haveRef = true
+		case PrependPathAtVFSRootError:
+			break loop
+		case PrependPathAtNonMountRootError, PrependPathSyntheticError:
+			unreachable = true
+			break loop
+		default:
+			return "", err
+		}
+	}
+	b.PrependByte('/')
+	if unreachable {
+		b.PrependString("(unreachable)")
+	}
+	return b.String(), nil
+}
+
+// As of this writing, we do not have equivalents to:
+//
+// - d_absolute_path(), which returns EINVAL if (effectively) any call to
+// FilesystemImpl.PrependPath() would return PrependPathAtNonMountRootError.
+//
+// - dentry_path(), which does not walk up mounts (and only returns the path
+// relative to Filesystem root), but also appends "//deleted" for disowned
+// Dentries.
+//
+// These should be added as necessary.

pkg/sentry/vfs/testutil.go

@@ -15,7 +15,10 @@
 package vfs
 
 import (
+	"fmt"
+
 	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/fspath"
 	"gvisor.dev/gvisor/pkg/sentry/context"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
 	"gvisor.dev/gvisor/pkg/syserror"
@@ -114,6 +117,12 @@ func (fs *FDTestFilesystem) UnlinkAt(ctx context.Context, rp *ResolvingPath) err
 	return syserror.EPERM
 }
 
+// PrependPath implements FilesystemImpl.PrependPath.
+func (fs *FDTestFilesystem) PrependPath(ctx context.Context, vfsroot, vd VirtualDentry, b *fspath.Builder) error {
+	b.PrependComponent(fmt.Sprintf("vfs.fdTestDentry:%p", vd.dentry.impl.(*fdTestDentry)))
+	return PrependPathSyntheticError{}
+}
+
 type fdTestDentry struct {
 	vfsd Dentry
 }