Add --file-access-mounts flag

--file-access-mounts flag is similar to --file-access, but controls
non-root mounts that were previously mounted in shared mode only.
This gives more flexibility to control how mounts are shared within
a container.

PiperOrigin-RevId: 364669882

runsc/boot/fs.go

@@ -792,7 +792,7 @@ func (c *containerMounter) getMountNameAndOptions(conf *config.Config, m specs.M
 	case bind:
 		fd := c.fds.remove()
 		fsName = gofervfs2.Name
-		opts = p9MountData(fd, c.getMountAccessType(m), conf.VFS2)
+		opts = p9MountData(fd, c.getMountAccessType(conf, m), conf.VFS2)
 		// If configured, add overlay to all writable mounts.
 		useOverlay = conf.Overlay && !mountFlags(m.Options).ReadOnly
 
@@ -802,12 +802,11 @@ func (c *containerMounter) getMountNameAndOptions(conf *config.Config, m specs.M
 	return fsName, opts, useOverlay, nil
 }
 
-func (c *containerMounter) getMountAccessType(mount specs.Mount) config.FileAccessType {
+func (c *containerMounter) getMountAccessType(conf *config.Config, mount specs.Mount) config.FileAccessType {
 	if hint := c.hints.findMount(mount); hint != nil {
 		return hint.fileAccessType()
 	}
-	// Non-root bind mounts are always shared if no hints were provided.
-	return config.FileAccessShared
+	return conf.FileAccessMounts
 }
 
 // mountSubmount mounts volumes inside the container's root. Because mounts may

runsc/boot/fs_test.go

@@ -243,7 +243,8 @@ func TestGetMountAccessType(t *testing.T) {
 				t.Fatalf("newPodMountHints failed: %v", err)
 			}
 			mounter := containerMounter{hints: podHints}
-			if got := mounter.getMountAccessType(specs.Mount{Source: source}); got != tst.want {
+			conf := &config.Config{FileAccessMounts: config.FileAccessShared}
+			if got := mounter.getMountAccessType(conf, specs.Mount{Source: source}); got != tst.want {
 				t.Errorf("getMountAccessType(), want: %v, got: %v", tst.want, got)
 			}
 		})

runsc/boot/vfs.go

@@ -494,7 +494,7 @@ func (c *containerMounter) getMountNameAndOptionsVFS2(conf *config.Config, m *mo
 			// but unlikely to be correct in this context.
 			return "", nil, false, fmt.Errorf("9P mount requires a connection FD")
 		}
-		data = p9MountData(m.fd, c.getMountAccessType(m.Mount), true /* vfs2 */)
+		data = p9MountData(m.fd, c.getMountAccessType(conf, m.Mount), true /* vfs2 */)
 		iopts = gofer.InternalFilesystemOptions{
 			UniqueID: m.Destination,
 		}

runsc/config/config.go

@@ -58,9 +58,12 @@ type Config struct {
 	// DebugLogFormat is the log format for debug.
 	DebugLogFormat string `flag:"debug-log-format"`
 
-	// FileAccess indicates how the filesystem is accessed.
+	// FileAccess indicates how the root filesystem is accessed.
 	FileAccess FileAccessType `flag:"file-access"`
 
+	// FileAccessMounts indicates how non-root volumes are accessed.
+	FileAccessMounts FileAccessType `flag:"file-access-mounts"`
+
 	// Overlay is whether to wrap the root filesystem in an overlay.
 	Overlay bool `flag:"overlay"`
 
@@ -197,13 +200,19 @@ func (c *Config) validate() error {
 type FileAccessType int
 
 const (
-	// FileAccessExclusive is the same as FileAccessShared, but enables
-	// extra caching for improved performance. It should only be used if
-	// the sandbox has exclusive access to the filesystem.
+	// FileAccessExclusive gives the sandbox exclusive access over files and
+	// directories in the filesystem. No external modifications are permitted and
+	// can lead to undefined behavior.
+	//
+	// Exclusive filesystem access enables more aggressive caching and offers
+	// significantly better performance. This is the default mode for the root
+	// volume.
 	FileAccessExclusive FileAccessType = iota
 
-	// FileAccessShared sends IO requests to a Gofer process that validates the
-	// requests and forwards them to the host.
+	// FileAccessShared is used for volumes that can have external changes. It
+	// requires revalidation on every filesystem access to detect external
+	// changes, and reduces the amount of caching that can be done. This is the
+	// default mode for non-root volumes.
 	FileAccessShared
 )
 

runsc/config/flags.go

@@ -67,7 +67,8 @@ func RegisterFlags() {
 		flag.Bool("oci-seccomp", false, "Enables loading OCI seccomp filters inside the sandbox.")
 
 		// Flags that control sandbox runtime behavior: FS related.
-		flag.Var(fileAccessTypePtr(FileAccessExclusive), "file-access", "specifies which filesystem to use for the root mount: exclusive (default), shared. Volume mounts are always shared.")
+		flag.Var(fileAccessTypePtr(FileAccessExclusive), "file-access", "specifies which filesystem validation to use for the root mount: exclusive (default), shared.")
+		flag.Var(fileAccessTypePtr(FileAccessShared), "file-access-mounts", "specifies which filesystem validation to use for volumes other than the root mount: shared (default), exclusive.")
 		flag.Bool("overlay", false, "wrap filesystem mounts with writable overlay. All modifications are stored in memory inside the sandbox.")
 		flag.Bool("verity", false, "specifies whether a verity file system will be mounted.")
 		flag.Bool("overlayfs-stale-read", true, "assume root mount is an overlay filesystem")