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.
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).
Andrii Nakryiko (10):
bpf: support non-r10 register spill/fill to/from stack in precision
tracking
selftests/bpf: add stack access precision test
bpf: fix check for attempt to corrupt spilled pointer
bpf: preserve STACK_ZERO slots on partial reg spills
selftests/bpf: validate STACK_ZERO is preserved on subreg spill
bpf: preserve constant zero when doing partial register restore
selftests/bpf: validate zero preservation for sub-slot loads
bpf: track aligned STACK_ZERO cases as imprecise spilled registers
selftests/bpf: validate precision logic in
partial_stack_load_preserves_zeros
bpf: use common instruction history across all states
include/linux/bpf_verifier.h | 42 ++-
kernel/bpf/verifier.c | 297 +++++++++++-------
.../selftests/bpf/progs/verifier_spill_fill.c | 124 ++++++++
.../bpf/progs/verifier_subprog_precision.c | 87 ++++-
.../testing/selftests/bpf/verifier/precise.c | 38 ++-
5 files changed, 435 insertions(+), 153 deletions(-)
--
2.34.1
