kernel/kexec_file.c: remove mis-use of sh_offset field during purgatory load

The current code uses the sh_offset field in purgatory_info->sechdrs to
store a pointer to the current load address of the section.  Depending
whether the section will be loaded or not this is either a pointer into
purgatory_info->purgatory_buf or kexec_purgatory.  This is not only a
violation of the ELF standard but also makes the code very hard to
understand as you cannot tell if the memory you are using is read-only
or not.

Remove this misuse and store the offset of the section in
pugaroty_info->purgatory_buf in sh_offset.

Link: http://lkml.kernel.org/r/20180321112751.22196-10-prudo@linux.vnet.ibm.com
Signed-off-by: Philipp Rudo <prudo@linux.vnet.ibm.com>
Acked-by: Dave Young <dyoung@redhat.com>
Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/kernel/machine_kexec_64.c

@@ -417,13 +417,15 @@ int arch_kexec_apply_relocations_add(struct purgatory_info *pi,
 		 * rel[i].r_offset contains byte offset from beginning
 		 * of section to the storage unit affected.
 		 *
-		 * This is location to update (->sh_offset). This is temporary
-		 * buffer where section is currently loaded. This will finally
-		 * be loaded to a different address later, pointed to by
+		 * This is location to update. This is temporary buffer
+		 * where section is currently loaded. This will finally be
+		 * loaded to a different address later, pointed to by
 		 * ->sh_addr. kexec takes care of moving it
 		 *  (kexec_load_segment()).
 		 */
-		location = (void *)(section->sh_offset + rel[i].r_offset);
+		location = pi->purgatory_buf;
+		location += section->sh_offset;
+		location += rel[i].r_offset;
 
 		/* Final address of the location */
 		address = section->sh_addr + rel[i].r_offset;