s390/kaslr: add support for R_390_GLOB_DAT relocation type

Commit "bpf: Process in-kernel BTF" in linux-next introduced an undefined
__weak symbol, which results in an R_390_GLOB_DAT relocation type. That
is not yet handled by the KASLR relocation code, and the kernel stops with
the message "Unknown relocation type".

Add code to detect and handle R_390_GLOB_DAT relocation types and undefined
symbols.

Fixes: 805bc0bc ("s390/kernel: build a relocatable kernel")
Cc: <stable@vger.kernel.org> # v5.2+
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/boot/startup.c

@@ -101,10 +101,18 @@ static void handle_relocs(unsigned long offset)
 	dynsym = (Elf64_Sym *) vmlinux.dynsym_start;
 	for (rela = rela_start; rela < rela_end; rela++) {
 		loc = rela->r_offset + offset;
-		val = rela->r_addend + offset;
+		val = rela->r_addend;
 		r_sym = ELF64_R_SYM(rela->r_info);
-		if (r_sym)
-			val += dynsym[r_sym].st_value;
+		if (r_sym) {
+			if (dynsym[r_sym].st_shndx != SHN_UNDEF)
+				val += dynsym[r_sym].st_value + offset;
+		} else {
+			/*
+			 * 0 == undefined symbol table index (STN_UNDEF),
+			 * used for R_390_RELATIVE, only add KASLR offset
+			 */
+			val += offset;
+		}
 		r_type = ELF64_R_TYPE(rela->r_info);
 		rc = arch_kexec_do_relocs(r_type, (void *) loc, val, 0);
 		if (rc)

arch/s390/kernel/machine_kexec_reloc.c

@@ -27,6 +27,7 @@ int arch_kexec_do_relocs(int r_type, void *loc, unsigned long val,
 		*(u32 *)loc = val;
 		break;
 	case R_390_64:		/* Direct 64 bit.  */
+	case R_390_GLOB_DAT:
 		*(u64 *)loc = val;
 		break;
 	case R_390_PC16:	/* PC relative 16 bit.	*/