bug fix pic:elf do not generate unwind table

[anjiahao1]

Summary

Nuttx's ELF dynamic loading cannot yet parse ARM exidx, so do not generate it yet.

modlib_read: Read 64 bytes from offset 192665
modlib_symvalue: ERROR: SHN_UNDEF: Exported symbol "__gnu_Unwind_Find_exidx" not found
modlib_relocate: ERROR: Section 2 reloc 12: Failed to get value of symbol[246]: -2
elf_loadbinary: Failed to bind symbols program binary: -2
builtin_loadbinary: Loading file: /pic/hello

Impact

ELF compilation on ARM

Testing

mps3-an546 build with pic, load hello elf.

[acassis]

@anjiahao1 nice to fix some ELF usage case, so without ELF .exidx we cannot have backtracing, crash dumps and C++ exception support on ELF binaries, right?

Please add this note to https://nuttx.apache.org/docs/latest/guides/partially_linked_elf.html

[cederom]

Thank you @anjiahao1 just some minor description update please :-)

• From the description it is unclear what behavior was before and after. Please update git commit and PR description to something like below. This will help users who encounter similar problem to find and apply you fix :-)

  arch/arm: Add arm toolchain unwinder configuration option.
  
  Nuttx's ELF dynamic loading cannot yet parse ARM exidx.
  Option is added not generate it yet to fix build/runtime(?).
  
  Before:
  modlib_read: Read 64 bytes from offset 192665
  modlib_symvalue: ERROR: SHN_UNDEF: Exported symbol "__gnu_Unwind_Find_exidx" not found
  modlib_relocate: ERROR: Section 2 reloc 12: Failed to get value of symbol[246]: -2
  elf_loadbinary: Failed to bind symbols program binary: -2
  
  After: 
  builtin_loadbinary: Loading file: /pic/hello
  

• I agree with @acassis Documentation needs an update too to stay in sync with the code.

[linguini1]

Can you please provide some logs for your test?

[anchao]

If unwind table support in ELF is not intended to be implemented, supplementary comments can be added. In addition, a statement indicating the lack of support should be included in the ELF chapter, preferably with details specified down to the ELF allocation type level.