rtld: implement _dl_iterate_phdr_locked

[kevans]

Summary

Some sanitizers need to be able to use dl_iterate_phdr() after stopping
the rest of the process, but it's very hard to do so reliably as a
non-participant in the main logic of the program.

Introduce _dl_iterate_phdr_locked to bypass the locking that's normally
required for dl_iterate_phdr() and slap some scary warning on it. It
will remain undocumented and probably shouldn't be used for anything
else.

[kib]

Now after a fresh look, I suggest to change the new function to be read-only. What I mean, is to remove hold/unhold_object and marker linkage. I believe this is 'better' in the sense that it would induce the UB memory accesses on the caller, but other threads would not see unlocked updates.

[kevans]

Re-implement r/o

Revert the previous implementation and just write a new short and sweet
implementation that iterates over the globallist and invokes the callback.

[kib]

libexec/rtld-elf/rtld.c

4216

You still want to do rtld_fill_dl_phdr_info(&obj_rtld, &phdr_info); and then execute callback on rtld data.

[kevans]

Don't skip the rtld object itself, and use consistent sizeof() while here.

[kevans]

libexec/rtld-elf/rtld.c

4216

D'oh! Thanks

[kib]

libexec/rtld-elf/rtld.c

4207

I think the initialization is not needed.