exit: Expose "oops_count" to sysfs

Since Oops count is now tracked and is a fairly interesting signal, add
the entry /sys/kernel/oops_count to expose it to userspace.

Cc: "Eric W. Biederman" <[e-mail]>
Cc: Jann Horn <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Reviewed-by: Luis Chamberlain <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/ABI/testing/sysfs-kernel-oops_count

···
+What:		/sys/kernel/oops_count
+Date:		November 2022
+KernelVersion:	6.2.0
+Contact:	Linux Kernel Hardening List <[e-mail]>
+Description:
+		Shows how many times the system has Oopsed since last boot.

MAINTAINERS

···
 L:	[e-mail]
 S:	Supported
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git for-next/hardening
+F:	Documentation/ABI/testing/sysfs-kernel-oops_count
 F:	include/linux/overflow.h
 F:	include/linux/randomize_kstack.h
 F:	mm/usercopy.c

kernel/exit.c

···
 #include <linux/io_uring.h>
 #include <linux/kprobes.h>
 #include <linux/rethook.h>
+#include <linux/sysfs.h>
 
 #include <linux/uaccess.h>
 #include <asm/unistd.h>
···
 	return 0;
 }
 late_initcall(kernel_exit_sysctls_init);
+#endif
+
+static atomic_t oops_count = ATOMIC_INIT(0);
+
+#ifdef CONFIG_SYSFS
+static ssize_t oops_count_show(struct kobject *kobj, struct kobj_attribute *attr,
+			       char *page)
+{
+	return sysfs_emit(page, "%d\n", atomic_read(&oops_count));
+}
+
+static struct kobj_attribute oops_count_attr = __ATTR_RO(oops_count);
+
+static __init int kernel_exit_sysfs_init(void)
+{
+	sysfs_add_file_to_group(kernel_kobj, &oops_count_attr.attr, NULL);
+	return 0;
+}
+late_initcall(kernel_exit_sysfs_init);
 #endif
 
 static void __unhash_process(struct task_struct *p, bool group_dead)
···
 
 void __noreturn make_task_dead(int signr)
 {
-	static atomic_t oops_count = ATOMIC_INIT(0);
-
 	/*
 	 * Take the task off the cpu after something catastrophic has
 	 * happened.