Reduce startup delays caused by libsystrap

[stephenrkell]

Currently, starting a process under liballocs causes a noticeable delay while each text segment's instructions are scanned for system calls.

We could avoid this by scanning offline and caching the syscall locations in yet another part of the meta-DSO. I've very sketchily started some work on this.

Possibly a neat way to store the records is as R_X86_64_16 relocs-with-addends. Do all architectures/ABIs define a 16-bit absolute fixup? It seems very common indeed, from a scan of elf.h.

There is obviously a security issue, if we can trick liballocs into loading a mismatched meta-DSO for a given binary. I wonder how to mitigate this.

[stephenrkell]

On the security thing, if build IDs are a cryptographically secure hash, we could verify the build ID (not cheap!) to be sure that the loaded binary is the one that the metadata applies to. However, how can we be sure that the metadata itself is trustworthy? We might want to verify its build ID, but we don't have a reference for that.

[stephenrkell]

I worry that any such verification will be more costly than the original scanning of instructions. Perhaps we are overthinking it... the host system generally assumes that installed binaries are not malicious, and all manner of things can go wrong if they are.