[PPC32] Improve the 32-bit PowerPC port


[PPC32] Improve the 32-bit PowerPC port

Many -static/-no-pie/-shared/-pie applications linked against glibc or musl
should work with this patch. This also helps FreeBSD PowerPC64 to migrate
their lib32 (PR40888).

  • Fix default image base and max page size.
  • Support new-style Secure PLT (see below). Old-style BSS PLT is not implemented, so it is not suitable for FreeBSD rtld now because it doesn't support Secure PLT yet.
  • Support more initial relocation types: R_PPC_ADDR32, R_PPC_REL16*, R_PPC_LOCAL24PC, R_PPC_PLTREL24, and R_PPC_GOT16. The addend of R_PPC_PLTREL24 is special: it decides the call stub PLT type but it should be ignored for the computation of target symbol VA.
  • Support GNU ifunc
  • Support .glink used for lazy PLT resolution in glibc
  • Add a new thunk type: PPC32PltCallStub that is similar to PPC64PltCallStub. It is used by R_PPC_REL24 and R_PPC_PLTREL24.

A PLT stub used in -fPIE/-fPIC usually loads an address relative to
.got2+0x8000 (-fpie/-fpic code uses _GLOBAL_OFFSET_TABLE_ relative
Two .got2 sections in two object files have different addresses, thus a PLT stub
can't be shared by two object files. To handle this incompatibility,
change the parameters of Thunk::isCompatibleWith to
const InputSection &, const Relocation &.

PowerPC psABI specified an old-style .plt (BSS PLT) that is both
writable and executable. Linkers don't make separate RW- and RWE segments,
which causes all initially writable memory (think .data) executable.
This is a big security concern so a new PLT scheme (secure PLT) was developed to
address the security issue.

TLS will be implemented in D62940.

glibc older than ~2012 requires .rela.dyn to include .rela.plt, it can
not handle the DT_RELA+DT_RELASZ == DT_JMPREL case correctly. A hack
(not included in this patch) in LinkerScript.cpp addOrphanSections() to
work around the issue:

if (Config->EMachine == EM_PPC) {
  // Older glibc assumes .rela.dyn includes .rela.plt
  if (In.RelaPlt->isLive() && !In.RelaPlt->Parent)

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D62464


MaskRayJun 6 2019, 10:03 AM
Differential Revision
D62464: [PPC32] Improve the 32-bit PowerPC port