On Mon, Aug 21, 2017 at 02:09:31AM +0200, Mickaël Salaün wrote:

Add a basic sandbox tool to create a process isolated from some part of
the system. This sandbox create a read-only environment. It is only
allowed to write to a character device such as a TTY:

  # :> X
  # echo $?
  0
  # ./samples/bpf/landlock1 /bin/sh -i
  Launching a new sandboxed process.
  # :> Y
  cannot create Y: Operation not permitted

Signed-off-by: Mickaël Salaün <redacted>

...

+SEC("landlock1")
+static int landlock_fs_prog1(struct landlock_context *ctx)
+{
+	char fmt_error_mode[] = "landlock1: error: get_mode:%lld\n";
+	char fmt_error_access[] = "landlock1: error: access denied\n";
+	long long ret;
+
+	/*
+	 * The argument ctx->arg2 contains bitflags of actions for which the
+	 * rule is run.  The flag LANDLOCK_ACTION_FS_WRITE means that a write
+	 * is requested by one of the userspace processes restricted by this
+	 * rule. The following test allows any actions which does not include a
+	 * write.
+	 */
+	if (!(ctx->arg2 & LANDLOCK_ACTION_FS_WRITE))
+		return 0;
+
+	/*
+	 * The argument ctx->arg1 is a file handle for which the process want
+	 * to access. The function bpf_handle_fs_get_mode() return the mode of
+	 * a file (e.g. S_IFBLK, S_IFDIR, S_IFREG...). If there is an error,
+	 * for example if the argument is not a file handle, then an
+	 * -errno value is returned. Otherwise the caller get the file mode as
+	 *  with stat(2).
+	 */
+	ret = bpf_handle_fs_get_mode((void *)ctx->arg1);
+	if (ret < 0) {
+
+		/*
+		 * The bpf_trace_printk() function enable to write in the
+		 * kernel eBPF debug log, accessible through
+		 * /sys/kernel/debug/tracing/trace_pipe . To be allowed to call
+		 * this function, a Landlock rule must have the
+		 * LANDLOCK_SUBTYPE_ABILITY_DEBUG ability, which is only
+		 * allowed for CAP_SYS_ADMIN.
+		 */
+		bpf_trace_printk(fmt_error_mode, sizeof(fmt_error_mode), ret);
+		return 1;
+	}
+
+	/*
+	 * This check allows the action on the file if it is a directory or a
+	 * pipe. Otherwise, a message is printed to the eBPF log.
+	 */
+	if (S_ISCHR(ret) || S_ISFIFO(ret))
+		return 0;
+	bpf_trace_printk(fmt_error_access, sizeof(fmt_error_access));
+	return 1;
+}
+
+/*
+ * This subtype enable to set the ABI, which ensure that the eBPF context and
+ * program behavior will be compatible with this Landlock rule.
+ */
+SEC("subtype")
+static const union bpf_prog_subtype _subtype = {
+	.landlock_rule = {
+		.abi = 1,
+		.event = LANDLOCK_SUBTYPE_EVENT_FS,
+		.ability = LANDLOCK_SUBTYPE_ABILITY_DEBUG,
+	}
+};

from rule writer perspective can you somehow merge subtype definition
with the program? It seems they go hand in hand.
Like section name of the program can be:
SEC("landlock_rule1/event=fs/ability=debug")
static int landlock_fs_prog1(struct landlock_context *ctx)...
and the loader can parse this string and prepare appropriate
data structures for the kernel.