Merge branch 'BPF verifier precision tracking improvements'

Andrii Nakryiko says:

====================

This patch set fixes and improves BPF verifier's precision tracking logic for
SCALAR registers.

Patches #1 and #2 are bug fixes discovered while working on these changes.

Patch #3 enables precision tracking for BPF programs that contain subprograms.
This was disabled before and prevent any modern BPF programs that use
subprograms from enjoying the benefits of SCALAR (im)precise logic.

Patch #4 is few lines of code changes and many lines of explaining why those
changes are correct. We establish why ignoring precise markings in current
state is OK.

Patch #5 build on explanation in patch #4 and pushes it to the limit by
forcefully forgetting inherited precise markins. Patch #4 by itself doesn't
prevent current state from having precise=true SCALARs, so patch #5 is
necessary to prevent such stray precise=true registers from creeping in.

Patch #6 adjusts test_align selftests to work around BPF verifier log's
limitations when it comes to interactions between state output and precision
backtracking output.

Overall, the goal of this patch set is to make BPF verifier's state tracking
a bit more efficient by trying to preserve as much generality in checkpointed
states as possible.

v1->v2:
- adjusted patch #1 commit message to make it clear we are fixing forward
  step, not precision backtracking (Alexei);
- moved last_idx/first_idx verbose logging up to make it clear when global
  func reaches the first empty state (Alexei).
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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