Support BTI in rtld

[andrew]

Summary

Read the elf note to decide when to set the guard page on arm64.

Sponsored by: Arm Ltd
Sponsored by: The FreeBSD Foundation

[andrew]

As with D39449 I've only added digest_note for arm64, but will add a no-op macro on other architectures if there are no issues.

[emaste]

libexec/rtld-elf/aarch64/reloc.c

57

we might pick this up from existing headers:

sys/sys/elf_common.h:#define    ELF_NOTE_GNU            "GNU"

[kib]

libexec/rtld-elf/rtld.c

1735

I would prefer if you changed the digest_note() name and contract. First, it should indicate that this is MD function, eg. arch_digest_note(). Second, it seems better to call arch_digest_note() as the first thing in the loop body, and expect a bool value as result. Then skip MI processing at all if result is true.

IMO one call is better than spreading it over the whole loop body.

[andrew]

• Update based on feedback
• Add arch_digest_note to all architectures

[andrew]

ping