add support for powerpc/powerpc64 unaligned relocations

R_PPC_UADDR32 (R_PPC64_UADDR64) has the same meaning as R_PPC_ADDR32
(R_PPC64_ADDR64), except that its address need not be aligned. For
powerpc64, BFD ld(1) will automatically convert between ADDR<->UADDR
relocations when the address is/isn't at its native alignment. This
will happen if, for example, there is a pointer in a packed struct.

gold and lld do not currently generate R_PPC64_UADDR64, but pass
through misaligned R_PPC64_ADDR64 relocations from object files,
possibly relaxing them to misaligned R_PPC64_RELATIVE. In both cases
(relaxed or not) this violates the PSABI, which defines the relevant
field type as "a 64-bit field occupying 8 bytes, the alignment of
which is 8 bytes unless otherwise specified."

All three linkers violate the PSABI on 32-bit powerpc, where the only
difference is that the field is 32 bits wide, aligned to 4 bytes.

Currently musl fails to load executables linked by BFD ld containing
R_PPC64_UADDR64, with the error "unsupported relocation type 43".
This change provides compatibility with BFD ld on powerpc64, and any
static linker on either architecture that starts following the PSABI
more closely.

1 files changed, 3 insertions, 0 deletions

diff --git a/ldso/dynlink.c b/ldso/dynlink.c
index 531811cc..edd91acd 100644
--- a/ldso/dynlink.c
+++ b/ldso/dynlink.c
@@ -407,6 +407,9 @@ static void do_relocs(struct dso *dso, size_t *rel, size_t rel_size, size_t stri
 		case REL_PLT:
 			*reloc_addr = sym_val + addend;
 			break;
+		case REL_USYMBOLIC:
+			memcpy(reloc_addr, &(size_t){sym_val + addend}, sizeof(size_t));
+			break;
 		case REL_RELATIVE:
 			*reloc_addr = (size_t)base + addend;
 			break;