s390/kexec_file: fix error handling when applying relocations

arch_kexec_apply_relocations_add currently ignores all errors returned
by arch_kexec_do_relocs. This means that every unknown relocation is
silently skipped causing unpredictable behavior while the relocated code
runs. Fix this by checking for errors and fail kexec_file_load if an
unknown relocation type is encountered.

The problem was found after gcc changed its behavior and used
R_390_PLT32DBL relocations for brasl instruction and relied on ld to
resolve the relocations in the final link in case direct calls are
possible. As the purgatory code is only linked partially (option -r)
ld didn't resolve the relocations leaving them for arch_kexec_do_relocs.
But arch_kexec_do_relocs doesn't know how to handle R_390_PLT32DBL
relocations so they were silently skipped. This ultimately caused an
endless loop in the purgatory as the brasl instructions kept branching
to itself.

Fixes: 71406883 ("s390/kexec_file: Add kexec_file_load system call")
Reported-by: Tao Liu <ltao@redhat.com>
Signed-off-by: Philipp Rudo <prudo@redhat.com>
Link: https://lore.kernel.org/r/20211208130741.5821-3-prudo@redhat.comSigned-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/kernel/machine_kexec_file.c

@@ -296,6 +296,7 @@ int arch_kexec_apply_relocations_add(struct purgatory_info *pi,
 	const Elf_Shdr *sechdrs;
 	Elf_Rela *relas;
 	int i, r_type;
+	int ret;
 
 	/* String & section header string table */
 	sechdrs = (void *)pi->ehdr + pi->ehdr->e_shoff;
@@ -347,7 +348,11 @@ int arch_kexec_apply_relocations_add(struct purgatory_info *pi,
 		addr = section->sh_addr + relas[i].r_offset;
 
 		r_type = ELF64_R_TYPE(relas[i].r_info);
-		arch_kexec_do_relocs(r_type, loc, val, addr);
+		ret = arch_kexec_do_relocs(r_type, loc, val, addr);
+		if (ret) {
+			pr_err("Unknown rela relocation: %d\n", r_type);
+			return -ENOEXEC;
+		}
 	}
 	return 0;
 }