[PATCH] D81359: [ELF] Add --[no-]relax for RISC-V

Fri Jun 12 08:38:11 PDT 2020

MaskRay added a comment.

I have a use case for x86 R_X86_64_[REX_]GOTPCRELX when debugging a glibc static-pie issue.

GNU ld --no-relax (loads an address from GOT):

  // elf_get_dynamic_info called by dl-reloc-static-pie.c
   316│ 176           if (__builtin_expect (GLRO(dl_debug_mask) & DL_DEBUG_FILES, 0)
   317│    0x00007fefd511896e <+462>:   mov    0x695b(%rip),%rcx        # 0x7fefd511f2d0
  ###### %rcx is 0 => null pointer dereference
   318│    0x00007fefd5118975 <+469>:   testb  $0x40,(%rcx)

--relax (PC-relative):

  316│ 176           if (__builtin_expect (GLRO(dl_debug_mask) & DL_DEBUG_FILES, 0)
  317│    0x00007f27dd56cdbe <+462>:   lea    0x6f87b(%rip),%rcx        # 0x7f27dd5dc640 <_dl_debug_mask>
  318│    0x00007f27dd56cdc5 <+469>:   testb  $0x40,(%rcx)

In a static pie program, it is a bug accessing GOT entry before the program relocates itself, as the GOT entry is likely 0 initially. lea is safe to use. If LLD has --relax --no-relax, it can be easier to verify that the behavior matches GNU ld. Honestly this is still a small use case and I managed to figure out the bug even if LLD does not support --no-relax.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D81359/new/

https://reviews.llvm.org/D81359