kexec: add a kexec_crash_loaded() function

Provide a wrapper function to be used by kernel code to check whether a
crash kernel is loaded.  It returns the same value that can be seen in
/sys/kernel/kexec_crash_loaded by userspace programs.

I'm exporting the function, because it will be used by Xen, and it is
possible to compile Xen modules separately to enable the use of PV
drivers with unmodified bare-metal kernels.

Link: http://lkml.kernel.org/r/20160713121955.14969.69080.stgit@hananiah.suse.cz
Signed-off-by: Petr Tesarik <ptesarik@suse.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/kexec.h

@@ -230,6 +230,7 @@ extern void *kexec_purgatory_get_symbol_addr(struct kimage *image,
 extern void __crash_kexec(struct pt_regs *);
 extern void crash_kexec(struct pt_regs *);
 int kexec_should_crash(struct task_struct *);
+int kexec_crash_loaded(void);
 void crash_save_cpu(struct pt_regs *regs, int cpu);
 void crash_save_vmcoreinfo(void);
 void arch_crash_save_vmcoreinfo(void);
@@ -364,6 +365,7 @@ struct task_struct;
 static inline void __crash_kexec(struct pt_regs *regs) { }
 static inline void crash_kexec(struct pt_regs *regs) { }
 static inline int kexec_should_crash(struct task_struct *p) { return 0; }
+static inline int kexec_crash_loaded(void) { return 0; }
 #define kexec_in_progress false
 #endif /* CONFIG_KEXEC_CORE */
 

kernel/kexec_core.c

@@ -95,6 +95,12 @@ int kexec_should_crash(struct task_struct *p)
 	return 0;
 }
 
+int kexec_crash_loaded(void)
+{
+	return !!kexec_crash_image;
+}
+EXPORT_SYMBOL_GPL(kexec_crash_loaded);
+
 /*
  * When kexec transitions to the new kernel there is a one-to-one
  * mapping between physical and virtual addresses.  On processors

kernel/ksysfs.c

@@ -101,7 +101,7 @@ KERNEL_ATTR_RO(kexec_loaded);
 static ssize_t kexec_crash_loaded_show(struct kobject *kobj,
 				       struct kobj_attribute *attr, char *buf)
 {
-	return sprintf(buf, "%d\n", !!kexec_crash_image);
+	return sprintf(buf, "%d\n", kexec_crash_loaded());
 }
 KERNEL_ATTR_RO(kexec_crash_loaded);