um: add a kmsg_dumper

Add a kmsg_dumper, that dumps the kmsg buffer to stdout, when no console
is available. This an enables the printing of early panic() calls
triggered in uml_postsetup().
When a panic() call happens so early in the UML kernel no
earlyprintk/console is available yet, but with a kmsg_dumper in place
the kernel message buffer will be outputted to the user, to give a
better hint, of what the failure was.

Signed-off-by: Thomas Meyer <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>

arch/um/kernel/Makefile

···
 obj-y = config.o exec.o exitcode.o irq.o ksyms.o mem.o \
 	physmem.o process.o ptrace.o reboot.o sigio.o \
 	signal.o syscall.o sysrq.o time.o tlb.o trap.o \
-	um_arch.o umid.o maccess.o skas/
+	um_arch.o umid.o maccess.o kmsg_dump.o skas/
 
 obj-$(CONFIG_BLK_DEV_INITRD) += initrd.o
 obj-$(CONFIG_GPROF)	+= gprof_syms.o

arch/um/kernel/kmsg_dump.c

···
+#include <linux/kmsg_dump.h>
+#include <linux/console.h>
+#include <shared/init.h>
+#include <shared/kern.h>
+#include <os.h>
+
+static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
+				enum kmsg_dump_reason reason)
+{
+	static char line[1024];
+
+	size_t len = 0;
+	bool con_available = false;
+
+	/* only dump kmsg when no console is available */
+	if (!console_trylock())
+		return;
+
+	if (console_drivers != NULL)
+		con_available = true;
+
+	console_unlock();
+
+	if (con_available == true)
+		return;
+
+	printf("kmsg_dump:\n");
+	while (kmsg_dump_get_line(dumper, true, line, sizeof(line), &len)) {
+		line[len] = '\0';
+		printf("%s", line);
+	}
+}
+
+static struct kmsg_dumper kmsg_dumper = {
+	.dump = kmsg_dumper_stdout
+};
+
+int __init kmsg_dumper_stdout_init(void)
+{
+	return kmsg_dump_register(&kmsg_dumper);
+}
+
+__uml_postsetup(kmsg_dumper_stdout_init);

arch/um/kernel/um_arch.c

···
 #include <linux/string.h>
 #include <linux/utsname.h>
 #include <linux/sched.h>
+#include <linux/kmsg_dump.h>
 #include <asm/pgtable.h>
 #include <asm/processor.h>
 #include <asm/sections.h>
···
 static int panic_exit(struct notifier_block *self, unsigned long unused1,
 		      void *unused2)
 {
+	kmsg_dump(KMSG_DUMP_PANIC);
 	bust_spinlocks(1);
 	bust_spinlocks(0);
 	uml_exitcode = 1;