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

From: Daniel Borkmann
Date: Mon Aug 08 2022 - 09:25:54 EST


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