Merge branch 'complete-bpf-verifier-precision-tracking-support-for-register-spills'

Merge branch 'complete-bpf-verifier-precision-tracking-support-for-register-spills'

Andrii Nakryiko says:

====================
Complete BPF verifier precision tracking support for register spills

Add support to BPF verifier to track and support register spill/fill to/from
stack regardless if it was done through read-only R10 register (which is the
only form supported today), or through a general register after copying R10
into it, while also potentially modifying offset.

Once we add register this generic spill/fill support to precision
backtracking, we can take advantage of it to stop doing eager STACK_ZERO
conversion on register spill. Instead we can rely on (im)precision of spilled
const zero register to improve verifier state pruning efficiency. This
situation of using const zero register to initialize stack slots is very
common with __builtin_memset() usage or just zero-initializing variables on
the stack, and it causes unnecessary state duplication, as that STACK_ZERO
knowledge is often not necessary for correctness, as those zero values are
never used in precise context. Thus, relying on register imprecision helps
tremendously, especially in real-world BPF programs.

To make spilled const zero register behave completely equivalently to
STACK_ZERO, we need to improve few other small pieces, which is done in the
second part of the patch set. See individual patches for details. There are
also two small bug fixes spotted during STACK_ZERO debugging.

The patch set consists of logically three changes:
  - patch #1 (and corresponding tests in patch #2) is fixing/impoving precision
    propagation for stack spills/fills. This can be landed as a stand-alone
    improvement;
  - patches #3 through #9 is improving verification scalability by utilizing
    register (im)precision instead of eager STACK_ZERO. These changes depend
    on patch #1.
  - patch #10 is a memory efficiency improvement to how instruction/jump
    history is tracked and maintained. It depends on patch #1, but is not
    strictly speaking required, even though I believe it's a good long-term
    solution to have a path-dependent per-instruction information. Kind
    of like a path-dependent counterpart to path-agnostic insn_aux array.

v3->v3:
  - fixed up Fixes tag (Alexei);
  - fixed few more selftests to not use BPF_ST instruction in inline asm
    directly, checked with CI, it was happy (CI);
v2->v3:
  - BPF_ST instruction workaround (Eduard);
  - force dereference in added tests to catch problems (Eduard);
  - some commit message massaging (Alexei);
v1->v2:
  - clean ups, WARN_ONCE(), insn_flags helpers added (Eduard);
  - added more selftests for STACK_ZERO/STACK_MISC cases (Eduard);
  - a bit more detailed explanation of effect of avoiding STACK_ZERO in favor
    of register spill in patch #8 commit (Alexei);
  - global shared instruction history refactoring moved to be the last patch
    in the series to make it easier to revert it, if applied (Alexei).
====================

Link: https://lore.kernel.org/r/20231205184248.1502704-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>