[PATCH bpf-next 0/7] bpf: Improve verifier for cond_op and spilled loop index variables

[Yonghong Song <yhs@fb.com>]

LLVM commit [1] introduced hoistMinMax optimization like
  (i < VIRTIO_MAX_SGS) && (i < out_sgs)
to
  upper = MIN(VIRTIO_MAX_SGS, out_sgs)
  ... i < upper ...
and caused the verification failure. Commit [2] workarounded the issue by
adding some bpf assembly code to prohibit the above optimization.
This patch improved verifier such that verification can succeed without
the above workaround.

Without [2], the current verifier will hit the following failures:
  ...
  119: (15) if r1 == 0x0 goto pc+1      
  The sequence of 8193 jumps is too complex.
  verification time 525829 usec
  stack depth 64
  processed 156616 insns (limit 1000000) max_states_per_insn 8 total_states 1754 peak_states 1712 mark_read 12
  -- END PROG LOAD LOG --
  libbpf: prog 'trace_virtqueue_add_sgs': failed to load: -14
  libbpf: failed to load object 'loop6.bpf.o'
  ...
The failure is due to verifier inadequately handling '<const> <cond_op> <non_const>' which will
go through both pathes and generate the following verificaiton states:
  ...
  89: (07) r2 += 1                      ; R2_w=5
  90: (79) r8 = *(u64 *)(r10 -48)       ; R8_w=scalar() R10=fp0
  91: (79) r1 = *(u64 *)(r10 -56)       ; R1_w=scalar(umax=5,var_off=(0x0; 0x7)) R10=fp0
  92: (ad) if r2 < r1 goto pc+41        ; R0_w=scalar() R1_w=scalar(umin=6,umax=5,var_off=(0x4; 0x3))
      R2_w=5 R6_w=scalar(id=385) R7_w=0 R8_w=scalar() R9_w=scalar(umax=21474836475,var_off=(0x0; 0x7ffffffff))
      R10=fp0 fp-8=mmmmmmmm fp-16=mmmmmmmm fp-24=mmmm???? fp-32= fp-40_w=4 fp-48=mmmmmmmm fp-56= fp-64=mmmmmmmm    
  ...
  89: (07) r2 += 1                      ; R2_w=6
  90: (79) r8 = *(u64 *)(r10 -48)       ; R8_w=scalar() R10=fp0
  91: (79) r1 = *(u64 *)(r10 -56)       ; R1_w=scalar(umax=5,var_off=(0x0; 0x7)) R10=fp0
  92: (ad) if r2 < r1 goto pc+41        ; R0_w=scalar() R1_w=scalar(umin=7,umax=5,var_off=(0x4; 0x3))
      R2_w=6 R6=scalar(id=388) R7=0 R8_w=scalar() R9_w=scalar(umax=25769803770,var_off=(0x0; 0x7ffffffff))
      R10=fp0 fp-8=mmmmmmmm fp-16=mmmmmmmm fp-24=mmmm???? fp-32= fp-40=5 fp-48=mmmmmmmm fp-56= fp-64=mmmmmmmm
    ...
  89: (07) r2 += 1                      ; R2_w=4088
  90: (79) r8 = *(u64 *)(r10 -48)       ; R8_w=scalar() R10=fp0
  91: (79) r1 = *(u64 *)(r10 -56)       ; R1_w=scalar(umax=5,var_off=(0x0; 0x7)) R10=fp0
  92: (ad) if r2 < r1 goto pc+41        ; R0=scalar() R1=scalar(umin=4089,umax=5,var_off=(0x0; 0x7))
      R2=4088 R6=scalar(id=12634) R7=0 R8=scalar() R9=scalar(umax=17557826301960,var_off=(0x0; 0xfffffffffff))
      R10=fp0 fp-8=mmmmmmmm fp-16=mmmmmmmm fp-24=mmmm???? fp-32= fp-40=4087 fp-48=mmmmmmmm fp-56= fp-64=mmmmmmmm

Patch 3 fixed the above issue by handling '<const> <cond_op> <non_const>' properly.
During developing selftests for Patch 3, I found some issues with bound deduction with
BPF_EQ/BPF_NE and fixed the issue in Patch 1.

After the above issue is fixed, the second issue shows up.
  ...
  67: (07) r1 += -16                    ; R1_w=fp-16
  ; bpf_probe_read_kernel(&sgp, sizeof(sgp), sgs + i);
  68: (b4) w2 = 8                       ; R2_w=8
  69: (85) call bpf_probe_read_kernel#113       ; R0_w=scalar() fp-16=mmmmmmmm
  ; return sgp;
  70: (79) r6 = *(u64 *)(r10 -16)       ; R6=scalar() R10=fp0
  ; for (n = 0, sgp = get_sgp(sgs, i); sgp && (n < SG_MAX);
  71: (15) if r6 == 0x0 goto pc-49      ; R6=scalar()
  72: (b4) w1 = 0                       ; R1_w=0
  73: (05) goto pc-46
  ; for (i = 0; (i < VIRTIO_MAX_SGS) && (i < out_sgs); i++) {
  28: (bc) w7 = w1                      ; R1_w=0 R7_w=0
  ; bpf_probe_read_kernel(&len, sizeof(len), &sgp->length);
  ...
  23: (79) r3 = *(u64 *)(r10 -40)       ; R3_w=2 R10=fp0
  ; for (i = 0; (i < VIRTIO_MAX_SGS) && (i < out_sgs); i++) {
  24: (07) r3 += 1                      ; R3_w=3
  ; for (i = 0; (i < VIRTIO_MAX_SGS) && (i < out_sgs); i++) {
  25: (79) r1 = *(u64 *)(r10 -56)       ; R1_w=scalar(umax=5,var_off=(0x0; 0x7)) R10=fp0
  26: (ad) if r3 < r1 goto pc+34 61: R0=scalar() R1_w=scalar(umin=4,umax=5,var_off=(0x4; 0x1)) R3_w=3 R6=scalar(id=1658)
     R7=0 R8=scalar(id=1653) R9=scalar(umax=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0 fp-8=mmmmmmmm fp-16=mmmmmmmm
     fp-24=mmmm???? fp-32= fp-40=2 fp-56= fp-64=mmmmmmmm
  ; if (sg_is_chain(&sg))               
  61: (7b) *(u64 *)(r10 -40) = r3       ; R3_w=3 R10=fp0 fp-40_w=3
    ...
  67: (07) r1 += -16                    ; R1_w=fp-16
  ; bpf_probe_read_kernel(&sgp, sizeof(sgp), sgs + i);
  68: (b4) w2 = 8                       ; R2_w=8
  69: (85) call bpf_probe_read_kernel#113       ; R0_w=scalar() fp-16=mmmmmmmm
  ; return sgp;
  70: (79) r6 = *(u64 *)(r10 -16)       
  ; for (n = 0, sgp = get_sgp(sgs, i); sgp && (n < SG_MAX);
  infinite loop detected at insn 71     
  verification time 90800 usec
  stack depth 64
  processed 25017 insns (limit 1000000) max_states_per_insn 20 total_states 491 peak_states 169 mark_read 12
  -- END PROG LOAD LOG --
  libbpf: prog 'trace_virtqueue_add_sgs': failed to load: -22

Further analysis found the index variable 'i' is spilled but since it is not marked as precise, regsafe will ignore
comparison since they are scalar values.

Since it is hard for verifier to determine whether a particular scalar is index variable or not, Patch 5 implemented
a heuristic such that if both old and new reg states are constant, mark the old one as precise to force scalar value
comparison and this fixed the problem.

The rest of patches are selftests related.

  [1] https://reviews.llvm.org/D143726
  [2] Commit 3c2611bac08a ("selftests/bpf: Fix trace_virtqueue_add_sgs test issue with LLVM 17.")

Yonghong Song (7):
  bpf: Improve verifier JEQ/JNE insn branch taken checking
  selftests/bpf: Add tests for non-constant cond_op NE/EQ bound
    deduction
  bpf: Improve handling of pattern '<const> <cond_op> <non_const>' in
    verifier
  selftests/bpf: Add verifier tests for code pattern '<const> <cond_op>
    <non_const>'
  bpf: Mark potential spilled loop index variable as precise
  selftests/bpf: Remove previous workaround for test verif_scale_loop6
  selftests/bpf: Add a new test based on loop6.c

 kernel/bpf/verifier.c                         |  40 +-
 .../bpf/prog_tests/bpf_verif_scale.c          |   5 +
 .../selftests/bpf/prog_tests/verifier.c       |   2 +
 tools/testing/selftests/bpf/progs/loop6.c     |   2 -
 tools/testing/selftests/bpf/progs/loop7.c     | 102 ++++
 .../verifier_bounds_deduction_non_const.c     | 553 ++++++++++++++++++
 .../progs/verifier_bounds_mix_sign_unsign.c   |   2 +-
 7 files changed, 701 insertions(+), 5 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/loop7.c
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_bounds_deduction_non_const.c

-- 
2.34.1