powerpc/fadump: fix PT_LOAD segment for boot memory area

Boot memory area is setup as separate PT_LOAD segment in the vmcore
as it is moved by f/w, on crash, to a destination address provided by
the kernel. Having separate PT_LOAD segment helps in handling the
different physical address and offset for boot memory area in the
vmcore.

Commit ced1bf52 ("powerpc/fadump: merge adjacent memory ranges to
reduce PT_LOAD segements") inadvertly broke this pre-condition for
cases where some of the first kernel memory is available adjacent to
boot memory area. This scenario is rare but possible when memory for
fadump could not be reserved adjacent to boot memory area owing to
memory hole or such. Reading memory from a vmcore exported in such
scenario provides incorrect data.  Fix it by ensuring no other region
is folded into boot memory area.

Fixes: ced1bf52 ("powerpc/fadump: merge adjacent memory ranges to reduce PT_LOAD segements")
Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220406093839.206608-2-hbathini@linux.ibm.com

arch/powerpc/kernel/fadump.c

@@ -867,7 +867,6 @@ static int fadump_alloc_mem_ranges(struct fadump_mrange_info *mrange_info)
 				       sizeof(struct fadump_memory_range));
 	return 0;
 }
-
 static inline int fadump_add_mem_range(struct fadump_mrange_info *mrange_info,
 				       u64 base, u64 end)
 {
@@ -886,7 +885,12 @@ static inline int fadump_add_mem_range(struct fadump_mrange_info *mrange_info,
 		start = mem_ranges[mrange_info->mem_range_cnt - 1].base;
 		size  = mem_ranges[mrange_info->mem_range_cnt - 1].size;
 
-		if ((start + size) == base)
+		/*
+		 * Boot memory area needs separate PT_LOAD segment(s) as it
+		 * is moved to a different location at the time of crash.
+		 * So, fold only if the region is not boot memory area.
+		 */
+		if ((start + size) == base && start >= fw_dump.boot_mem_top)
 			is_adjacent = true;
 	}
 	if (!is_adjacent) {