[PATCH v2] kbuild: modversions: Fix relative CRC byte order interpretation

[Fredrik Noring <noring@nocrew.org>]

Fix commit 56067812d5b0 ("kbuild: modversions: add infrastructure for
emitting relative CRCs") where CRCs are interpreted in host byte order
rather than proper kernel byte order. The bug is conditional on
CONFIG_MODULE_REL_CRCS.

For example, when loading a BE module into a BE kernel compiled with a LE
system, the error "disagrees about version of symbol module_layout" is
produced. A message such as "Found checksum D7FA6856 vs module 5668FAD7"
will be given with debug enabled, which indicates an obvious endian
problem within __kcrctab within the kernel image.

The general solution is to use the macro TO_NATIVE, as is done in
similar cases throughout modpost.c. With this correction it has been
verified that a BE kernel compiled with a LE system accepts BE modules.

This change has also been verified with a LE kernel compiled with a LE
system, in which case TO_NATIVE returns its value unmodified since the
byte orders match. This is by far the common case.

Fixes: 56067812d5b0 ("kbuild: modversions: add infrastructure for emitting relative CRCs")
Signed-off-by: Fredrik Noring <noring@nocrew.org>
Cc: stable@vger.kernel.org
---
Thank you for your review, Masahiro,

Changes in v2:
- Fix subject spelling mistake.
- Note that the bug is conditional on CONFIG_MODULE_REL_CRCS.
- Clarify kernel image and __kcrctab compilation details.
- Mention verification of matching LE byte order common case.

Fredrik
---
diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 0b0d1080b1c5..f277e116e0eb 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -639,7 +639,7 @@ static void handle_modversions(struct module *mod, struct elf_info *info,
 			       info->sechdrs[sym->st_shndx].sh_offset -
 			       (info->hdr->e_type != ET_REL ?
 				info->sechdrs[sym->st_shndx].sh_addr : 0);
-			crc = *crcp;
+			crc = TO_NATIVE(*crcp);
 		}
 		sym_update_crc(symname + strlen("__crc_"), mod, crc,
 				export);