Validate ELF headers, fail on heap setup errors, add tracked launches

[ItsJokerZz]

What changed

elfldr.c

• elfldr_read rejects headers that are not 64-bit ELF, have a bad section
  header size or count, or point past a 64 MB cap, and checks each section's
  offset and size before using them. These values come straight off the
  network and were used as allocation and receive sizes unchecked.
• Fixed sizeof(Elf64_Ehdr) where sizeof(Elf64_Shdr) was meant (same
  size, no behaviour change).
• A failed heap setup now kills the child and fails the spawn. The result
  of elfldr_set_heap_size was ignored before, so the payload would run
  with a default-sized heap.

socksrv.c

• New optional request prefix: TRACKED, a 32-bit big-endian input
  length, up to 191 bytes of input, then the payload as usual.
• The payload is spawned with stdio detached from the client socket
  (/dev/null, or a socket pair carrying the input), and the client gets
  back TPID plus the pid, or -1 on failure.
• When input is given, the loader waits up to 15 s for the payload to
  print PAYLOAD_READY and kills it otherwise.
• A tracked request ignores the pipe URI option, since its output never
  goes to the client.
• Requests without the prefix take exactly the same path as before.

Why

A client that launches payloads has no way to learn which process it
started, so it cannot stop or monitor it later. The tracked request gives
it the pid without mixing payload output into the reply. The rest is
hardening found along the way.

This brings the PS5 loader in line with the same changes proposed for the
PS4 loader, so one client can speak the same request to both.

[john-tornblom]

Please separate this into two: one with bug fixes which I will likely merge, and a second one for the new feature. I don’t think I will merge the feature as it is designed right now, better to use http get request for this where you can pass parameters.

[ItsJokerZz]

Sounds good- will do.

[ItsJokerZz]

Bug fixes here on commit 8530ff5

[john-tornblom]

What does the max_size constant do? Limit elf file size?

[ItsJokerZz]

It was an upper bound to stop the header's offsets and sizes from overflowing when added together. Checking for overflow directly does the same job without limiting file size, so I'll just remove it.