kernel/ksysfs.c: expose file_caps_enabled in sysfs

A kernel booted with no_file_caps allows to install fscaps on a binary
but doesn't actually honor the fscaps when running the binary. Userspace
currently has no sane way to determine whether installing fscaps
actually has any effect. Since parsing /proc/cmdline is fragile this
patch exposes the current setting (1 or 0) via /sys/kernel/fscaps

Signed-off-by: Ludwig Nussel <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/ABI/testing/sysfs-kernel-fscaps

···
+What:		/sys/kernel/fscaps
+Date:		February 2011
+KernelVersion:	2.6.38
+Contact:	Ludwig Nussel <[e-mail]>
+Description
+		Shows whether file system capabilities are honored
+		when executing a binary
+

kernel/ksysfs.c

···
 #include <linux/kexec.h>
 #include <linux/profile.h>
 #include <linux/sched.h>
+#include <linux/capability.h>
 
 #define KERNEL_ATTR_RO(_name) \
 static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
···
 
 #endif /* CONFIG_KEXEC */
 
+/* whether file capabilities are enabled */
+static ssize_t fscaps_show(struct kobject *kobj,
+				  struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%d\n", file_caps_enabled);
+}
+KERNEL_ATTR_RO(fscaps);
+
 /*
  * Make /sys/kernel/notes give the raw contents of our kernel .notes section.
  */
···
 EXPORT_SYMBOL_GPL(kernel_kobj);
 
 static struct attribute * kernel_attrs[] = {
+	&fscaps_attr.attr,
 #if defined(CONFIG_HOTPLUG)
 	&uevent_seqnum_attr.attr,
 	&uevent_helper_attr.attr,