modpost: detect section mismatch for R_ARM_{MOVW_ABS_NC,MOVT_ABS}

For ARM defconfig (i.e. multi_v7_defconfig), modpost fails to detect
some types of section mismatches.

  [test code]

    #include <linux/init.h>

    int __initdata foo;
    int get_foo(void) { return foo; }

It is apparently a bad reference, but modpost does not report anything.

The test code above produces the following relocations.

  Relocation section '.rel.text' at offset 0x200 contains 2 entries:
   Offset     Info    Type            Sym.Value  Sym. Name
  00000000  0000062b R_ARM_MOVW_ABS_NC 00000000   .LANCHOR0
  00000004  0000062c R_ARM_MOVT_ABS    00000000   .LANCHOR0

Currently, R_ARM_MOVW_ABS_NC and R_ARM_MOVT_ABS are just skipped.

Add code to handle them. I checked arch/arm/kernel/module.c to learn
how the offset is encoded in the instruction.

The referenced symbol in relocation might be a local anchor.
If is_valid_name() returns false, let's search for a better symbol name.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/mod/modpost.c

@@ -1078,7 +1078,7 @@ static inline int is_valid_name(struct elf_info *elf, Elf_Sym *sym)
 /**
  * Find symbol based on relocation record info.
  * In some cases the symbol supplied is a valid symbol so
- * return refsym. If st_name != 0 we assume this is a valid symbol.
+ * return refsym. If is_valid_name() == true, we assume this is a valid symbol.
  * In other cases the symbol needs to be looked up in the symbol table
  * based on section and address.
  *  **/
@@ -1091,7 +1091,7 @@ static Elf_Sym *find_tosym(struct elf_info *elf, Elf64_Sword addr,
 	Elf64_Sword d;
 	unsigned int relsym_secindex;
 
-	if (relsym->st_name != 0)
+	if (is_valid_name(elf, relsym))
 		return relsym;
 
 	/*
@@ -1297,6 +1297,13 @@ static int addend_arm_rel(struct elf_info *elf, Elf_Shdr *sechdr, Elf_Rela *r)
 		inst = TO_NATIVE(*(uint32_t *)loc);
 		r->r_addend = inst + sym->st_value;
 		break;
+	case R_ARM_MOVW_ABS_NC:
+	case R_ARM_MOVT_ABS:
+		inst = TO_NATIVE(*(uint32_t *)loc);
+		offset = sign_extend32(((inst & 0xf0000) >> 4) | (inst & 0xfff),
+				       15);
+		r->r_addend = offset + sym->st_value;
+		break;
 	case R_ARM_PC24:
 	case R_ARM_CALL:
 	case R_ARM_JUMP24: