bpf: enable callchain sensitive stack liveness tracking

bpf: enable callchain sensitive stack liveness tracking

Allocate analysis instance:
- Add bpf_stack_liveness_{init,free}() calls to bpf_check().

Notify the instance about any stack reads and writes:
- Add bpf_mark_stack_write() call at every location where
  REG_LIVE_WRITTEN is recorded for a stack slot.
- Add bpf_mark_stack_read() call at every location mark_reg_read() is
  called.
- Both bpf_mark_stack_{read,write}() rely on
  env->liveness->cur_instance callchain being in sync with
  env->cur_state. It is possible to update env->liveness->cur_instance
  every time a mark read/write is called, but that costs a hash table
  lookup and is noticeable in the performance profile. Hence, manually
  reset env->liveness->cur_instance whenever the verifier changes
  env->cur_state call stack:
  - call bpf_reset_live_stack_callchain() when the verifier enters a
    subprogram;
  - call bpf_update_live_stack() when the verifier exits a subprogram
    (it implies the reset).

Make sure bpf_update_live_stack() is called for a callchain before
issuing liveness queries. And make sure that bpf_update_live_stack()
is called for any callee callchain first:
- Add bpf_update_live_stack() call at every location that processes
  BPF_EXIT:
  - exit from a subprogram;
  - before pop_stack() call.
  This makes sure that bpf_update_live_stack() is called for callee
  callchains before caller callchains.

Make sure must_write marks are set to zero for instructions that
do not always access the stack:
- Wrap do_check_insn() with bpf_reset_stack_write_marks() /
  bpf_commit_stack_write_marks() calls.
  Any calls to bpf_mark_stack_write() are accumulated between this
  pair of calls. If no bpf_mark_stack_write() calls were made
  it means that the instruction does not access stack (at-least
  on the current verification path) and it is important to record
  this fact.

Finally, use bpf_live_stack_query_init() / bpf_stack_slot_alive()
to query stack liveness info.

The manual tracking of the correct order for callee/caller
bpf_update_live_stack() calls is a bit convoluted and may warrant some
automation in future revisions.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250918-callchain-sensitive-liveness-v3-7-c3cd27bacc60@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>