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 <thomas@m3y3r.de>
Signed-off-by: Richard Weinberger <richard@nod.at>

3 files changed, 46 insertions, 1 deletions

diff --git a/arch/um/kernel/Makefile b/arch/um/kernel/Makefile
index b7e31e5a8c0c..a6a5e42caaef 100644
--- a/arch/um/kernel/Makefile
+++ b/arch/um/kernel/Makefile
@@ -13,7 +13,7 @@ clean-files :=
 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
diff --git a/arch/um/kernel/kmsg_dump.c b/arch/um/kernel/kmsg_dump.c
new file mode 100644
index 000000000000..407d49251d6f
--- /dev/null
+++ b/arch/um/kernel/kmsg_dump.c
@@ -0,0 +1,43 @@
+#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);
diff --git a/arch/um/kernel/um_arch.c b/arch/um/kernel/um_arch.c
index 9b06957e98ce..926ecdceba86 100644
--- a/arch/um/kernel/um_arch.c
+++ b/arch/um/kernel/um_arch.c
@@ -11,6 +11,7 @@
 #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>
@@ -211,6 +212,7 @@ static void __init uml_postsetup(void)
 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;