ARM: 8650/1: module: handle negative R_ARM_PREL31 addends correctly

According to the spec 'ELF for the ARM Architecture' (IHI 0044E),
addends for R_ARM_PREL31 relocations are 31-bit signed quantities,
so we need to sign extend the value to 32 bits before it can be used
as an offset in the calculation of the relocated value.

We have not been bitten by this because these relocations are usually
emitted against the start of a section, which means the addends never
assume negative values in practice. But it is a bug nonetheless, so fix
it.
Signed-off-by: NArd Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: NRussell King <rmk+kernel@armlinux.org.uk>

arch/arm/kernel/module.c

@@ -155,8 +155,17 @@ apply_relocate(Elf32_Shdr *sechdrs, const char *strtab, unsigned int symindex,
 		       break;
 
 		case R_ARM_PREL31:
-			offset = *(u32 *)loc + sym->st_value - loc;
-			*(u32 *)loc = offset & 0x7fffffff;
+			offset = (*(s32 *)loc << 1) >> 1; /* sign extend */
+			offset += sym->st_value - loc;
+			if (offset >= 0x40000000 || offset < -0x40000000) {
+				pr_err("%s: section %u reloc %u sym '%s': relocation %u out of range (%#lx -> %#x)\n",
+				       module->name, relindex, i, symname,
+				       ELF32_R_TYPE(rel->r_info), loc,
+				       sym->st_value);
+				return -ENOEXEC;
+			}
+			*(u32 *)loc &= 0x80000000;
+			*(u32 *)loc |= offset & 0x7fffffff;
 			break;
 
 		case R_ARM_MOVW_ABS_NC: