[PATCH] D68094: ELF: Don't merge SHF_LINK_ORDER sections in relocatable links.

[Peter Smith via Phabricator via llvm-commits]

peter.smith added a comment.

I've got a test case that certainly doesn't work well when doing a -r, and then relinking the corresponding object.

exidx.s

  	.section .text.f1,"ax",%progbits
  	.globl f1
  	.type f1, %function
  f1:     .fnstart
  	.cantunwind
  	bx lr
  	.fnend
  
  	.section .text.f2,"ax",%progbits
  	.globl f2
  	.type f2, %function
  f2:     .fnstart
  	bx lr
          .save {r7, lr}
          .setfp r7, sp, #0
  	.fnend

exidx2.s

  	.section .text.f1,"ax",%progbits
  	.globl f3
  	.type f3, %function
  f3:     .fnstart
          .save {r7, lr}
          .setfp r7, sp, #0
  	bx lr
  	.fnend
  
  	.section .text.f2,"ax",%progbits
  	.globl f4
  	.type f2, %function
  f4:     .fnstart
  	.cantunwind
  	bx lr
  	.fnend
  
  	.text
  	.globl __aeabi_unwind_cpp_pr0
  __aeabi_unwind_cpp_pr0:	bx lr

exidx.lds

  SECTIONS {
    .foo : { *(.text.f2) *(.text.f1) }
  }

Commands

  clang --target=armv7a-linux-gnu exidx.s exidx2.s -c
  ld.lld exidx.o exidx2.o -r -o exidxr.o -T exidx.lds 
  ld.lld exidxr.o -o exidx.exe

The creation of the relocatable object produces a single output section with 4 .ARM.exidx sections with a SHF_LINK_ORDER dependency on it. The linker script scrambles the order of the .text.f* functions so that it doesn't match the order of the functions in the output executable section. This will break a linker using SHF_LINK_ORDER to construct the table.

In lld it is worse as there is custom code that is expecting a 1:1 mapping between .ARM.exidx and executable section. Looking at the ARMExidxSyntheticSection::finalizeContents() the code frequently goes from the executable section back to its SHF_LINK_ORDER section (first .ARM.exidx in the dependent sections). This doesn't work well when there are multiple .ARM.exidx sections on the dependent sections list.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D68094/new/

https://reviews.llvm.org/D68094