Re: [PATCH bpf] bpf: Do more tight ALU bounds tracking

[Daniel Borkmann]

On 7/30/22 12:48 AM, Hao Luo wrote:
> On Fri, Jul 29, 2022 at 3:43 PM Youlin Li <liulin063@xxxxxxxxx> wrote:
> > In adjust_scalar_min_max_vals(), let 32bit bounds learn from 64bit bounds
> > to get more tight bounds tracking. Similar operation can be found in
> > reg_set_min_max().
> >
> > Also, we can now fold reg_bounds_sync() into zext_32_to_64().
> >
> > Before:
> >
> >      func#0 @0
> >      0: R1=ctx(off=0,imm=0) R10=fp0
> >      0: (b7) r0 = 0                        ; R0_w=0
> >      1: (b7) r1 = 0                        ; R1_w=0
> >      2: (87) r1 = -r1                      ; R1_w=scalar()
> >      3: (87) r1 = -r1                      ; R1_w=scalar()
> >      4: (c7) r1 s>>= 63                    ; R1_w=scalar(smin=-1,smax=0)
> >      5: (07) r1 += 2                       ; R1_w=scalar(umin=1,umax=2,var_off=(0x0; 0xffffffff))  <--- [*]
> >      6: (95) exit
> >
> > It can be seen that even if the 64bit bounds is clear here, the 32bit
> > bounds is still in the state of 'UNKNOWN'.
> >
> > After:
> >
> >      func#0 @0
> >      0: R1=ctx(off=0,imm=0) R10=fp0
> >      0: (b7) r0 = 0                        ; R0_w=0
> >      1: (b7) r1 = 0                        ; R1_w=0
> >      2: (87) r1 = -r1                      ; R1_w=scalar()
> >      3: (87) r1 = -r1                      ; R1_w=scalar()
> >      4: (c7) r1 s>>= 63                    ; R1_w=scalar(smin=-1,smax=0)
> >      5: (07) r1 += 2                       ; R1_w=scalar(umin=1,umax=2,var_off=(0x0; 0x3))  <--- [*]
> >      6: (95) exit
> >
> > Signed-off-by: Youlin Li <liulin063@xxxxxxxxx>
>
> Looks good to me. Thanks Youlin.
>
> Acked-by: Hao Luo <haoluo@xxxxxxxxxx>

Thanks Youlin! Looks like the patch breaks CI [0] e.g.:

  #142/p bounds check after truncation of non-boundary-crossing range FAIL
  Failed to load prog 'Permission denied'!
  invalid access to map value, value_size=8 off=16777215 size=1
  R0 max value is outside of the allowed memory range
  verification time 296 usec
  stack depth 8
  processed 15 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0

Please take a look. Also it would be great to add a test_verifier selftest to
assert above case from commit log against future changes.

Thanks,
Daniel

  [0] https://github.com/kernel-patches/bpf/runs/7696324041?check_suite_focus=true