[all-commits] [llvm/llvm-project] fb2944: [ELF][PPC32] Implement IPLT code sequence for non-...

[Fangrui Song via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: fb2944bd7f8ac6d7c4bccd3ac2033ba58c690038
      https://github.com/llvm/llvm-project/commit/fb2944bd7f8ac6d7c4bccd3ac2033ba58c690038
  Author: Fangrui Song <maskray at google.com>
  Date:   2019-12-29 (Sun, 29 Dec 2019)

  Changed paths:
    M lld/ELF/Arch/PPC.cpp
    M lld/ELF/Thunks.cpp
    M lld/ELF/Thunks.h
    R lld/test/ELF/ppc32-gnu-ifunc-nonpreemptable.s
    A lld/test/ELF/ppc32-ifunc-nonpreemptible-nopic.s
    A lld/test/ELF/ppc32-ifunc-nonpreemptible-pic.s

  Log Message:
  -----------
  [ELF][PPC32] Implement IPLT code sequence for non-preemptible IFUNC

Similar to D71509 (EM_PPC64), on EM_PPC, the IPLT code sequence should
be similar to a PLT call stub. Unlike EM_PPC64, EM_PPC -msecure-plt has
small/large PIC model differences.

* -fpic/-fpie: R_PPC_PLTREL24 r_addend=0.  The call stub loads an address relative to `_GLOBAL_OFFSET_TABLE_`.
* -fPIC/-fPIE: R_PPC_PLTREL24 r_addend=0x8000. (A partial linked object
  file may have an addend larger than 0x8000.) The call stub loads an address relative to .got2+0x8000.

Just assume large PIC model for now. This patch makes:

  // clang -fuse-ld=lld -msecure-plt -fno-pie -no-pie a.c
  // clang -fuse-ld=lld -msecure-plt -fPIE -pie a.c
  #include <stdio.h>
  static void impl(void) { puts("meow"); }
  void thefunc(void) __attribute__((ifunc("resolver")));
  void *resolver(void) { return &impl; }
  int main(void) {
    thefunc();
    void (*theptr)(void) = &thefunc;
    theptr();
  }

work on Linux glibc. -fpie will crash because the compiler and the
linker do not agree on the value which r30 stores (_GLOBAL_OFFSET_TABLE_
vs .got2+0x8000).

Differential Revision: https://reviews.llvm.org/D71621