powerpc/kexec: Check crash_base for relocatable kernel

Enforce that the crash kernel region never overlaps the current kernel,
as it will be written directly on kexec load.

Also, default to the previous KDUMP_KERNELBASE if the start is 0.

Other architectures (x86, ia64) state that specifying the start address
0 (or omitting it) will result in the kernel allocating it.  Before the
relocatable patch in 2.6.28, powerpc would adjust any other start value
to the hardcoded KDUMP_KERNELBASE of 32M.
Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/machine_kexec.c

@@ -93,10 +93,35 @@ void __init reserve_crashkernel(void)
 				KDUMP_KERNELBASE);
 
 	crashk_res.start = KDUMP_KERNELBASE;
+#else
+	if (!crashk_res.start) {
+		/*
+		 * unspecified address, choose a region of specified size
+		 * can overlap with initrd (ignoring corruption when retained)
+		 * ppc64 requires kernel and some stacks to be in first segemnt
+		 */
+		crashk_res.start = KDUMP_KERNELBASE;
+	}
+
+	crash_base = PAGE_ALIGN(crashk_res.start);
+	if (crash_base != crashk_res.start) {
+		printk("Crash kernel base must be aligned to 0x%lx\n",
+				PAGE_SIZE);
+		crashk_res.start = crash_base;
+	}
+
 #endif
 	crash_size = PAGE_ALIGN(crash_size);
 	crashk_res.end = crashk_res.start + crash_size - 1;
 
+	/* The crash region must not overlap the current kernel */
+	if (overlaps_crashkernel(__pa(_stext), _end - _stext)) {
+		printk(KERN_WARNING
+			"Crash kernel can not overlap current kernel\n");
+		crashk_res.start = crashk_res.end = 0;
+		return;
+	}
+
 	/* Crash kernel trumps memory limit */
 	if (memory_limit && memory_limit <= crashk_res.end) {
 		memory_limit = crashk_res.end + 1;