Re: [RFC bpf-next 0/5] Support for BPF_ST instruction in LLVM C compiler

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Sat, Dec 31, 2022 at 8:31 AM Eduard Zingerman <eddyz87@gmail.com> wrote:
>
> BPF has two documented (non-atomic) memory store instructions:
>
> BPF_STX: *(size *) (dst_reg + off) = src_reg
> BPF_ST : *(size *) (dst_reg + off) = imm32
>
> Currently LLVM BPF back-end does not emit BPF_ST instruction and does
> not allow one to be specified as inline assembly.
>
> Recently I've been exploring ways to port some of the verifier test
> cases from tools/testing/selftests/bpf/verifier/*.c to use inline assembly
> and machinery provided in tools/testing/selftests/bpf/test_loader.c
> (which should hopefully simplify tests maintenance).
> The BPF_ST instruction is popular in these tests: used in 52 of 94 files.
>
> While it is possible to adjust LLVM to only support BPF_ST for inline
> assembly blocks it seems a bit wasteful. This patch-set contains a set
> of changes to verifier necessary in case when LLVM is allowed to
> freely emit BPF_ST instructions (source code is available here [1]).

Would we gate LLVM's emitting of BPF_ST for C code behind some new
cpu=v4? What is the benefit for compiler to start automatically emit
such instructions? Such thinking about logistics, if there isn't much
benefit, as BPF application owner I wouldn't bother enabling this
behavior risking regressions on old kernels that don't have these
changes.

So I feel like the biggest benefit is to be able to use this
instruction in embedded assembly, to make writing and maintaining
tests easier.

> The changes include:
>  - update to verifier.c:check_stack_write_*() functions to track
>    constant values spilled to stack via BPF_ST instruction in a same
>    way stack spills of known registers by BPF_STX are tracked;
>  - updates to verifier.c:convert_ctx_access() and it's callbacks to
>    handle BPF_ST instruction in a way similar to BPF_STX;
>  - some test adjustments and a few new tests.
>
> With the above changes applied and LLVM from [1] all test_verifier,
> test_maps, test_progs and test_progs-no_alu32 test cases are passing.
>
> When built using the LLVM version from [1] BPF programs generated for
> selftests and Cilium programs (taken from [2]) see certain reduction
> in size, e.g. below are total numbers of instructions for files with
> over 5K instructions:
>
> File                                    Insns   Insns   Insns   Diff
>                                         w/o     with    diff    pct
>                                         BPF_ST  BPF_ST
> cilium/bpf_host.o                       44620   43774   -846    -1.90%
> cilium/bpf_lxc.o                        36842   36060   -782    -2.12%
> cilium/bpf_overlay.o                    23557   23186   -371    -1.57%
> cilium/bpf_xdp.o                        26397   25931   -466    -1.77%
> selftests/core_kern.bpf.o               12359   12359    0       0.00%
> selftests/linked_list_fail.bpf.o        5501    5302    -199    -3.62%
> selftests/profiler1.bpf.o               17828   17709   -119    -0.67%
> selftests/pyperf100.bpf.o               49793   49268   -525    -1.05%
> selftests/pyperf180.bpf.o               88738   87813   -925    -1.04%
> selftests/pyperf50.bpf.o                25388   25113   -275    -1.08%
> selftests/pyperf600.bpf.o               78330   78300   -30     -0.04%
> selftests/pyperf_global.bpf.o           5244    5188    -56     -1.07%
> selftests/pyperf_subprogs.bpf.o         5262    5192    -70     -1.33%
> selftests/strobemeta.bpf.o              17154   16065   -1089   -6.35%
> selftests/test_verif_scale2.bpf.o       11337   11337    0       0.00%
>
> (Instructions are counted by counting the number of instruction lines
>  in disassembly).
>
> Is community interested in this work?
> Are there any omissions in my changes to the verifier?
>
> Known issue:
>
> There are two programs (one Cilium, one selftest) that exhibit
> anomalous increase in verifier processing time with this patch-set:
>
>  File                 Program                        Insns (A)  Insns (B)  Insns   (DIFF)
>  -------------------  -----------------------------  ---------  ---------  --------------
>  bpf_host.o           tail_ipv6_host_policy_ingress       1576       2403  +827 (+52.47%)
>  map_kptr.bpf.o       test_map_kptr                        400        475   +75 (+18.75%)
>  -------------------  -----------------------------  ---------  ---------  --------------
>
> These are under investigation.
>
> Thanks,
> Eduard
>
> [1] https://reviews.llvm.org/D140804
> [2] git@github.com:anakryiko/cilium.git
>
> Eduard Zingerman (5):
>   bpf: more precise stack write reasoning for BPF_ST instruction
>   selftests/bpf: check if verifier tracks constants spilled by
>     BPF_ST_MEM
>   bpf: allow ctx writes using BPF_ST_MEM instruction
>   selftests/bpf: test if pointer type is tracked for BPF_ST_MEM
>   selftests/bpf: don't match exact insn index in expected error message
>
>  kernel/bpf/cgroup.c                           |  49 +++++---
>  kernel/bpf/verifier.c                         | 102 +++++++++-------
>  net/core/filter.c                             |  72 ++++++------
>  .../selftests/bpf/prog_tests/log_fixup.c      |   2 +-
>  .../selftests/bpf/prog_tests/spin_lock.c      |   8 +-
>  .../bpf/verifier/bounds_mix_sign_unsign.c     | 110 ++++++++++--------
>  .../selftests/bpf/verifier/bpf_st_mem.c       |  29 +++++
>  tools/testing/selftests/bpf/verifier/ctx.c    |  11 --
>  tools/testing/selftests/bpf/verifier/unpriv.c |  23 ++++
>  9 files changed, 249 insertions(+), 157 deletions(-)
>  create mode 100644 tools/testing/selftests/bpf/verifier/bpf_st_mem.c
>
> --
> 2.39.0
>