[PATCH] D42748: [ELF] Don't create a .dynamic section when linking with -Bstatic

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added a comment.

>> And if you decide to support static pie, you also need _DYNAMIC.
> 
> Are you sure about this ? For static PIE, as I understand, we miss some kind of relocator in csu.

On the linker side, static pie is `-static -pie --no-dynamic-linker`. The `-pie` causes lld to create dynamic sections.

  void
  _start(char **ap, void (*cleanup)(void))
  {
  	int argc;
  	char **argv;
  	char **env;
  
  	argc = *(long *)(void *)ap;
  	argv = ap + 1;
  	env = ap + 2 + argc;
  	handle_argv(argc, argv, env);
  
  	if (&_DYNAMIC != NULL) {
  		atexit(cleanup);
  	} else {
  		process_irelocs();
  		_init_tls();
  	}

If FreeBSD used to differentiate dynamically/statically linked programs with `&_DYNAMIC != NULL`, I think it is probably time to revisit.
We have two ways to have dynamic sections in a statically linked program, `--export-dynamic` (https://reviews.llvm.org/rL295240) or `-pie` (static pie). If the distinction of dynamic/static is whether the program is interpreted by PT_INTERP, isn't testing this property directly more straightforward?

>   However, I'm pretty sure I saw some application level code use the check to determine if they are dynamically linked.

I think that is extremely rare, but they should also be updated to make static pie work.

> Also I just had a look in llvm-project.git and it seems that compiler-rt/lib/sanitizer_common/sanitizer_linux.cc uses the following: level = &_DYNAMIC == nullptr ? AndroidDetectApiLevelStatic(). I think this may be a common way of checking for the presence of a dynamic linker so emitting it for static binaries could be problematic.

It is used by `AndroidDetectApiLevelStatic()`. If Android ever supports static pie, that piece of code has to change.


Repository:
  rLLD LLVM Linker

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  https://reviews.llvm.org/D42748/new/

https://reviews.llvm.org/D42748