Update manpage and usage for mount.composefs

Signed-off-by: Alexander Larsson <[email protected]>

man/mount.composefs.md

@@ -5,27 +5,61 @@
 mount.composefs - mount a composefs filesystem image
 
 # SYNOPSIS
-**mount.composefs** *IMAGE* *TARGETDIR*
+**mount.composefs** [-o OPTIONS] *IMAGE* *TARGETDIR*
 
 # DESCRIPTION
 
-The composefs project uses EROFS to store metadata, and a distinct
-underlying backing store for regular files.  At runtime, composefs
-uses `overlayfs` on top of a loopback mount.
+The composefs project uses EROFS image file to store metadata, and one
+or more separate directories containing content-addressed backing data
+for regular files.
 
-**mount.composefs** 
+**mount.composefs** mounts such an EROFS file in combination with a given
+set of basedir at the specified location. It can be called directly, or
+as a mount helper by running `mount -t composefst ...`.
 
 # OPTIONS
 
 The provided *IMAGE* argument must be a valid composefs (EROFS)
 metadata image.  The *TARGETDIR* will be used as a mount target.
 
-**mount.composefs** accepts the following options:
+**mount.composefs** accepts the following colon-separated mount
+options when passed via the `-o OPTIONS` argument.
 
-**\-\-basedir**=*PATH*
+**basedir**=*PATH*
 :   This path will be used to resolve non-empty file references
     stored in the composefs metadata image.  A primary use case is to have
-    this be the same path provided to `mkcomposefs --digest-store`.
+    this be the same path provided to `mkcomposefs --digest-store=PATH`.
+
+    Multiple paths can be specified, separated by `:`.
+
+**digest**=*DIGEST*
+:   The image file is validated to have the specified fs-verity digest
+    before being used. This allows a chain of trust the ensures only
+    the expected data is ever visible in the mount.
+
+    This option also implies **verity**.
+
+**verity**
+:   If this is specified, all files in the *IMAGE* must specify an fs-verity
+    digest, and all the files in the base dirs must have a matching fs-verity
+    digest.
+
+    Note: This needs support for the overlayfs "verity" option in the
+    kernel, which was added in 6.6rc1.
+
+**ro**
+:  Mounts the filesystem read-only. This is mainly useful when using
+   **upperdir** as unlayered composefs images are naturally readonly.
+
+**rw**
+:  Overrides a previous **ro** option
+
+**upperdir**
+:  Specify an upper dir in the overlayfs mount that composefs uses. This allows
+   a writable layer on top of the composefs image. See overlayfs docs for details.
+
+**workdir**
+:  Specifies an overlayfs workdir to go with **upperdir**.
 
 # SEE ALSO
 

tools/mountcomposefs.c

@@ -51,11 +51,24 @@ static void printexit(const char *format, ...)
 
 static void usage(const char *argv0)
 {
+	const char *bin = basename(argv0);
 	fprintf(stderr,
 		"usage: %s [-t type] [-o opt[,opts..]] IMAGE MOUNTPOINT\n"
 		"Example:\n"
-		"  %s -o basedir=/composefs/objects exampleimage.cfs /mnt/exampleimage\n",
-		argv0, argv0);
+		"  %s -o basedir=/composefs/objects exampleimage.cfs /mnt/exampleimage\n"
+		"or, as a mount helper:\n"
+		"  mount -t composefs -o basedir=/composefs/objects exampleimage.cfs /mnt/exampleimage\n"
+		"\n"
+		"Supported options:\n"
+		"  basedir=PATH[:PATH]    Specify location of basedir(s)\n"
+		"  digest=DIGEST          Specify required image digest\n"
+		"  verity                 Require all files to have specified and valid fs-verity digests\n"
+		"  ro                     Read only\n"
+		"  rw                     Read/write\n"
+		"  upperdir               Overlayfs upperdir\n"
+		"  workdir                Overlayfs workdir\n"
+		,
+		bin, bin);
 }
 
 static void unescape_option(char *s)