[vfs] Allow mountpoint to be an existing non-directory.

Unlike linux mount(2), OCI spec allows mounting on top of an existing
non-directory file.

PiperOrigin-RevId: 327914342

2 files changed, 28 insertions, 2 deletions

diff --git a/pkg/sentry/vfs/mount.go b/pkg/sentry/vfs/mount.go
index 09fea3628..cd5456eef 100644
--- a/pkg/sentry/vfs/mount.go
+++ b/pkg/sentry/vfs/mount.go
@@ -18,12 +18,14 @@ import (
 	"bytes"
 	"fmt"
 	"math"
+	"path"
 	"sort"
 	"strings"
 	"sync/atomic"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/fspath"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
 	"gvisor.dev/gvisor/pkg/syserror"
 )
@@ -888,6 +890,30 @@ func (vfs *VirtualFilesystem) GenerateProcMountInfo(ctx context.Context, taskRoo
 	}
 }
 
+// MakeSyntheticMountpoint creates parent directories of target if they do not
+// exist and attempts to create a directory for the mountpoint. If a
+// non-directory file already exists there then we allow it.
+func (vfs *VirtualFilesystem) MakeSyntheticMountpoint(ctx context.Context, target string, root VirtualDentry, creds *auth.Credentials) error {
+	mkdirOpts := &MkdirOptions{Mode: 0777, ForSyntheticMountpoint: true}
+
+	// Make sure the parent directory of target exists.
+	if err := vfs.MkdirAllAt(ctx, path.Dir(target), root, creds, mkdirOpts); err != nil {
+		return fmt.Errorf("failed to create parent directory of mountpoint %q: %w", target, err)
+	}
+
+	// Attempt to mkdir the final component. If a file (of any type) exists
+	// then we let allow mounting on top of that because we do not require the
+	// target to be an existing directory, unlike Linux mount(2).
+	if err := vfs.MkdirAt(ctx, creds, &PathOperation{
+		Root:  root,
+		Start: root,
+		Path:  fspath.Parse(target),
+	}, mkdirOpts); err != nil && err != syserror.EEXIST {
+		return fmt.Errorf("failed to create mountpoint %q: %w", target, err)
+	}
+	return nil
+}
+
 // manglePath replaces ' ', '\t', '\n', and '\\' with their octal equivalents.
 // See Linux fs/seq_file.c:mangle_path.
 func manglePath(p string) string {
diff --git a/runsc/boot/vfs.go b/runsc/boot/vfs.go
index fb200e988..66b6cf19b 100644
--- a/runsc/boot/vfs.go
+++ b/runsc/boot/vfs.go
@@ -292,7 +292,7 @@ func (c *containerMounter) mountSubmountVFS2(ctx context.Context, conf *config.C
 		return nil, nil
 	}
 
-	if err := c.k.VFS().MkdirAllAt(ctx, submount.Destination, root, creds, &vfs.MkdirOptions{Mode: 0777, ForSyntheticMountpoint: true}); err != nil {
+	if err := c.k.VFS().MakeSyntheticMountpoint(ctx, submount.Destination, root, creds); err != nil {
 		return nil, err
 	}
 	mnt, err := c.k.VFS().MountAt(ctx, creds, "", target, fsName, opts)
@@ -496,7 +496,7 @@ func (c *containerMounter) mountSharedSubmountVFS2(ctx context.Context, conf *co
 
 	root := mns.Root()
 	defer root.DecRef(ctx)
-	if err := c.k.VFS().MkdirAllAt(ctx, mount.Destination, root, creds, &vfs.MkdirOptions{Mode: 0777, ForSyntheticMountpoint: true}); err != nil {
+	if err := c.k.VFS().MakeSyntheticMountpoint(ctx, mount.Destination, root, creds); err != nil {
 		return nil, err
 	}