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

Thu May 23 23:38:20 PDT 2019

MaskRay added a comment.

> Regarding this patch: I don't think _DYNAMIC would be useful for -static-pie since we would need to relocate it first so _DYNAMIC could still be undefined and used to check for dynamic linker presence.

You need `_DYNAMIC` for -static-pie. You need `_DYNAMIC` to find `DT_REL*` tags, then perform relocations.

> I had a quick look at glibc's static-pie code and it seems like glibc loads the first GOT entry for most architectures to get the address of the dynamic section.

It is one thing I don't like (I don't know any other program that needs this) about glibc (this is wrriten in x86 psABI, but probably not in other psABIs): the first entry of .got.plt or .got holds the link-time address of `_DYNAMIC`. At runtime a pcrel load of `_GLOBAL_OFFSET_TABLE_` gets the link-time address, subtracts it from the runtime address of `_DYNAMIC`, then you get the load base. glibc could parse the program header to get `p_vaddr` of `PT_DYNAMIC` instead.

> Also, as I noted above, PT_INTERP absence does not really mean that the binary is static.

@kib I'm happy to know about FreeBSD crt internals but I'm afraid we've digressed from the topic, or I fail to understand how you justify the this patch.
I tried not using the term dynamically/statically linked program (the file utility may say a `-static-pie` is "dynamic" while glibc ldd says it is "static"). I'm sorry if I failed to do that.

I'm not happy with some decisions made in the GNU toolchain: `-static` does not compose with `-pie` (it overrides `-pie`) so they added `-static-pie`, ld.bfd has default PT_INTERP path for various platforms so it needs `--no-dynamic-linker`, `-static` seems to override `-export-dynamic`, etc.

@ed's explanation of `-export-dynamic` makes sense to me. I don't understand why this patch wants to override part of the semantics of `-export-dynamic` with `-static`, making the overall logic harder to reason about.

A -static-pie needs `_DYNAMIC`. When FreeBSD gets to -static-pie, if the expectation is that it resembles more a -static than a dynamic (finalizers called in the exe, not in ld.so/libc.so), the simplistic check `&_DYNAMIC != NULL` will not work.
Or when you decide to support dlopen() from a static program, it also needs `_DYNAMIC`. If the current (`&_DYNAMIC!=` is true) status makes applications crash, you need to figure out a more reliable approach.

Repository:
  rLLD LLVM Linker

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D42748/new/

https://reviews.llvm.org/D42748