[PATCH] kdump: crashk_res init check for /sys/kernel/kexec_crash_size

[Michael Holzheu <holzheu@linux.vnet.ibm.com>]

From: Michael Holzheu <holzheu@linux.vnet.ibm.com>

Currently it is possible to set the crash_size via the sysfs
/sys/kernel/kexec_crash_size even if no crash kernel memory has
been defined with the "crashkernel" parameter. In this case
"crashk_res" is not initialized and crashk_res.start = crashk_res.end = 0.
Unfortunately resource_size(&crashk_res) returns 1 in this case.
This breaks the s390 implementation of crash_(un)map_reserved_pages().

To fix the problem the correct "old_size" is now calculated in
crash_shrink_memory(). "old_size is set to "0" if crashk_res is
not initialized. With this change crash_shrink_memory() will do nothing,
when "crashk_res" is not initialized. It will return "0" for
"echo 0 > /sys/kernel/kexec_crash_size" and -EINVAL for
"echo [not zero] > /sys/kernel/kexec_crash_size".

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
---
 kernel/kexec.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/kernel/kexec.c
+++ b/kernel/kexec.c
@@ -1131,7 +1131,7 @@ void __weak crash_free_reserved_phys_ran
 int crash_shrink_memory(unsigned long new_size)
 {
 	int ret = 0;
-	unsigned long start, end;
+	unsigned long start, end, old_size;
 
 	mutex_lock(&kexec_mutex);
 
@@ -1141,10 +1141,10 @@ int crash_shrink_memory(unsigned long ne
 	}
 	start = crashk_res.start;
 	end = crashk_res.end;
-
-	if (new_size >= end - start + 1) {
+	old_size = (end == 0) ? 0 : end - start + 1;
+	if (new_size >= old_size) {
 		ret = -EINVAL;
-		if (new_size == end - start + 1)
+		if (new_size == old_size)
 			ret = 0;
 		goto unlock;
 	}