| Message ID | 20240718202357.1746514-1-eddyz87@gmail.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | bpf: track find_equal_scalars history on per-instruction level | expand |
Hello: This series was applied to bpf/bpf-next.git (master) by Andrii Nakryiko <andrii@kernel.org>: On Thu, 18 Jul 2024 13:23:52 -0700 you wrote: > This is a fix for precision tracking bug reported in [0]. > It supersedes my previous attempt to fix similar issue in commit [1]. > Here is a minimized test case from [0]: > > 0: call bpf_get_prandom_u32; > 1: r7 = r0; > 2: r8 = r0; > 3: call bpf_get_prandom_u32; > 4: if r0 > 1 goto +0; > /* --- checkpoint #1: r7.id=1, r8.id=1 --- */ > 5: if r8 >= r0 goto 9f; > 6: r8 += r8; > /* --- checkpoint #2: r7.id=1, r8.id=0 --- */ > 7: if r7 == 0 goto 9f; > 8: r0 /= 0; > /* --- checkpoint #3 --- */ > 9: r0 = 42; > 10: exit; > > [...] Here is the summary with links: - [bpf-next,v3,1/4] bpf: track equal scalars history on per-instruction level https://git.kernel.org/bpf/bpf-next/c/2b7350d7ca65 - [bpf-next,v3,2/4] bpf: remove mark_precise_scalar_ids() https://git.kernel.org/bpf/bpf-next/c/cb0f94d85874 - [bpf-next,v3,3/4] selftests/bpf: tests for per-insn sync_linked_regs() precision tracking https://git.kernel.org/bpf/bpf-next/c/c0087d59e504 - [bpf-next,v3,4/4] selftests/bpf: update comments find_equal_scalars->sync_linked_regs https://git.kernel.org/bpf/bpf-next/c/f081cd17c0bc You are awesome, thank you!
This is a fix for precision tracking bug reported in [0]. It supersedes my previous attempt to fix similar issue in commit [1]. Here is a minimized test case from [0]: 0: call bpf_get_prandom_u32; 1: r7 = r0; 2: r8 = r0; 3: call bpf_get_prandom_u32; 4: if r0 > 1 goto +0; /* --- checkpoint #1: r7.id=1, r8.id=1 --- */ 5: if r8 >= r0 goto 9f; 6: r8 += r8; /* --- checkpoint #2: r7.id=1, r8.id=0 --- */ 7: if r7 == 0 goto 9f; 8: r0 /= 0; /* --- checkpoint #3 --- */ 9: r0 = 42; 10: exit; W/o this fix verifier incorrectly assumes that instruction at label (8) is unreachable. The issue is caused by failure to infer precision mark for r0 at checkpoint #1: - first verification path is: - (0-4): r0 range [0,1]; - (5): r8 range [0,0], propagated to r7; - (6): r8.id is reset; - (7): jump is predicted to happen; - (9-10): safe exit. - when jump at (7) is predicted mark_chain_precision() for r7 is called and backtrack_insn() proceeds as follows: - at (7) r7 is marked as precise; - at (5) r8 is not currently tracked and thus r0 is not marked; - at (4-5) boundary logic from [1] is triggered and r7,r8 are marked as precise; - => r0 precision mark is missed. - when second branch of (4) is considered, verifier prunes the state because r0 is not marked as precise in the visited state. Basically, backtracking logic fails to notice that at (5) range information is gained for both r7 and r8, and thus both r8 and r0 have to be marked as precise. This happens because [1] can only account for such range transfers at parent/child state boundaries. The solution suggested by Andrii Nakryiko in [0] is to use jump history to remember which registers gained range as a result of find_equal_scalars() [renamed to sync_linked_regs()] and use this information in backtrack_insn(). Which is what this patch-set does. The patch-set uses u64 value as a vector of 10-bit values that identify registers gaining range in find_equal_scalars(). This amounts to maximum of 6 possible values. To check if such capacity is sufficient I've instrumented kernel to track a histogram for maximal amount of registers that gain range in find_equal_scalars per program verification [2]. Measurements done for verifier selftests and Cilium bpf object files from [3] show that number of such registers is *always* <= 4 and in 98% of cases it is <= 2. When tested on a subset of selftests identified by selftests/bpf/veristat.cfg and Cilium bpf object files from [3] this patch-set has minimal verification performance impact: File Program Insns (DIFF) States (DIFF) ------------------------ ------------------------ -------------- ------------- bpf_host.o tail_handle_nat_fwd_ipv4 -75 (-0.61%) -3 (-0.39%) pyperf600_nounroll.bpf.o on_event +1673 (+0.33%) +3 (+0.01%) [0] https://lore.kernel.org/bpf/CAEf4BzZ0xidVCqB47XnkXcNhkPWF6_nTV7yt+_Lf0kcFEut2Mg@mail.gmail.com/ [1] commit 904e6ddf4133 ("bpf: Use scalar ids in mark_chain_precision()") [2] https://github.com/eddyz87/bpf/tree/find-equal-scalars-in-jump-history-with-stats [3] https://github.com/anakryiko/cilium Changes: - v2 -> v3: A number of stylistic changes suggested by Andrii: - renamings: - struct reg_or_spill -> linked_reg; - find_equal_scalars() -> collect_linked_regs; - copy_known_reg() -> sync_linked_regs; - collect_linked_regs() now returns linked regs set of size 2 or larger; - dropped usage of bit fields in struct linked_reg; - added a patch changing references to find_equal_scalars() in selftests comments. - v1 -> v2: - patch "bpf: replace env->cur_hist_ent with a getter function" is dropped (Andrii); - added structure linked_regs and helper functions to [de]serialize u64 value as such structure (Andrii); - bt_set_equal_scalars() renamed to bt_sync_linked_regs(), moved to start and end of backtrack_insn() in order to untie linked register logic from conditional jumps backtracking. Andrii requested a more radical change of moving linked registers processing to bt_set_xxx() functions, I did an experiment in this direction: https://github.com/eddyz87/bpf/tree/find-equal-scalars-in-jump-history--linked-regs-in-bt-set-reg the end result of the experiment seems much uglier than version presented in v2. Revisions: - v1: https://lore.kernel.org/bpf/20240222005005.31784-1-eddyz87@gmail.com/ - v2: https://lore.kernel.org/bpf/20240705205851.2635794-1-eddyz87@gmail.com/ Eduard Zingerman (4): bpf: track equal scalars history on per-instruction level bpf: remove mark_precise_scalar_ids() selftests/bpf: tests for per-insn sync_linked_regs() precision tracking selftests/bpf: update comments find_equal_scalars->sync_linked_regs include/linux/bpf_verifier.h | 4 + kernel/bpf/verifier.c | 363 +++++++++++------- .../selftests/bpf/progs/verifier_scalar_ids.c | 256 ++++++++---- .../selftests/bpf/progs/verifier_spill_fill.c | 16 +- .../bpf/progs/verifier_subprog_precision.c | 2 +- .../testing/selftests/bpf/verifier/precise.c | 28 +- 6 files changed, 434 insertions(+), 235 deletions(-)