module: handle ppc64 relocating kcrctabs when CONFIG_RELOCATABLE=y

powerpc applies relocations to the kcrctab.  They're absolute symbols,
but it's not completely unreasonable: other archs may too, but the
relocation is often 0.

http://lists.ozlabs.org/pipermail/linuxppc-dev/2009-November/077972.html

Inspired-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Tested-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/include/asm/module.h

@@ -87,5 +87,10 @@ struct exception_table_entry;
 void sort_ex_table(struct exception_table_entry *start,
 		   struct exception_table_entry *finish);
 
+#ifdef CONFIG_MODVERSIONS
+#define ARCH_RELOCATES_KCRCTAB
+
+extern const unsigned long reloc_start[];
+#endif
 #endif /* __KERNEL__ */
 #endif	/* _ASM_POWERPC_MODULE_H */

arch/powerpc/kernel/vmlinux.lds.S

@@ -38,6 +38,9 @@ jiffies = jiffies_64 + 4;
 #endif
 SECTIONS
 {
+	. = 0;
+	reloc_start = .;
+
 	. = KERNELBASE;
 
 /*

kernel/module.c

@@ -880,11 +880,23 @@ static int try_to_force_load(struct module *mod, const char *reason)
 }
 
 #ifdef CONFIG_MODVERSIONS
+/* If the arch applies (non-zero) relocations to kernel kcrctab, unapply it. */
+static unsigned long maybe_relocated(unsigned long crc,
+				     const struct module *crc_owner)
+{
+#ifdef ARCH_RELOCATES_KCRCTAB
+	if (crc_owner == NULL)
+		return crc - (unsigned long)reloc_start;
+#endif
+	return crc;
+}
+
 static int check_version(Elf_Shdr *sechdrs,
 			 unsigned int versindex,
 			 const char *symname,
 			 struct module *mod, 
-			 const unsigned long *crc)
+			 const unsigned long *crc,
+			 const struct module *crc_owner)
 {
 	unsigned int i, num_versions;
 	struct modversion_info *versions;
@@ -905,10 +917,10 @@ static int check_version(Elf_Shdr *sechdrs,
 		if (strcmp(versions[i].name, symname) != 0)
 			continue;
 
-		if (versions[i].crc == *crc)
+		if (versions[i].crc == maybe_relocated(*crc, crc_owner))
 			return 1;
 		DEBUGP("Found checksum %lX vs module %lX\n",
-		       *crc, versions[i].crc);
+		       maybe_relocated(*crc, crc_owner), versions[i].crc);
 		goto bad_version;
 	}
 
@@ -931,7 +943,8 @@ static inline int check_modstruct_version(Elf_Shdr *sechdrs,
 	if (!find_symbol(MODULE_SYMBOL_PREFIX "module_layout", NULL,
 			 &crc, true, false))
 		BUG();
-	return check_version(sechdrs, versindex, "module_layout", mod, crc);
+	return check_version(sechdrs, versindex, "module_layout", mod, crc,
+			     NULL);
 }
 
 /* First part is kernel version, which we ignore if module has crcs. */
@@ -949,7 +962,8 @@ static inline int check_version(Elf_Shdr *sechdrs,
 				unsigned int versindex,
 				const char *symname,
 				struct module *mod, 
-				const unsigned long *crc)
+				const unsigned long *crc,
+				const struct module *crc_owner)
 {
 	return 1;
 }
@@ -984,8 +998,8 @@ static const struct kernel_symbol *resolve_symbol(Elf_Shdr *sechdrs,
 	/* use_module can fail due to OOM,
 	   or module initialization or unloading */
 	if (sym) {
-		if (!check_version(sechdrs, versindex, name, mod, crc) ||
-		    !use_module(mod, owner))
+		if (!check_version(sechdrs, versindex, name, mod, crc, owner)
+		    || !use_module(mod, owner))
 			sym = NULL;
 	}
 	return sym;