linux-kernel - Fwd: [PATCH bpf] bpf: Do more tight ALU bounds tracking

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Date:   Mon, 8 Aug 2022 23:14:31 +0800
From:   Kuee k1r0a <liulin063@...il.com>
To:     haoluo@...gle.com
Cc:     Alexei Starovoitov <ast@...nel.org>, john.fastabend@...il.com,
        Andrii Nakryiko <andrii@...nel.org>, martin.lau@...ux.dev,
        song@...nel.org, yhs@...com, kpsingh@...nel.org, sdf@...gle.com,
        jolsa@...nel.org, bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Fwd: [PATCH bpf] bpf: Do more tight ALU bounds tracking

---------- Forwarded message ---------
From: Kuee k1r0a <liulin063@...il.com>
Date: Mon, Aug 8, 2022 at 11:11 PM
Subject: Re: [PATCH bpf] bpf: Do more tight ALU bounds tracking
To: Daniel Borkmann <daniel@...earbox.net>


On Mon, Aug 8, 2022 at 9:25 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 7/30/22 12:48 AM, Hao Luo wrote:
> > On Fri, Jul 29, 2022 at 3:43 PM Youlin Li <liulin063@...il.com> 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@...il.com>
> >
> > Looks good to me. Thanks Youlin.
> >
> > Acked-by: Hao Luo <haoluo@...gle.com>
>
> 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

This test case fails because the 32bit boundary information is lost
after the 11th instruction is executed:
Before:
    11: (07) r1 += 2147483647             ;
R1_w=scalar(umin=70866960383,umax=70866960638,var_off=(0x1000000000;
0xffffffff),u32_min=2147483647,u32_max=-2147483394)
After:
    11: (07) r1 += 2147483647             ;
R1_w=scalar(umin=70866960383,umax=70866960638,var_off=(0x1000000000;
0xffffffff))

This may be because, in previous versions of the code, when
__reg_combine_64_into_32() was called, the 32bit boundary was
completely deduced from the 64bit boundary, so there was a call to
__mark_reg32_unbounded() in __reg_combine_64_into_32().

But now, before adjust_scalar_min_max_vals() calls
__reg_combine_64_into_32() , the 32bit bounds are already calculated
to some extent, and __mark_reg32_unbounded() will eliminate these
information.

Simply copying a code without __mark_reg32_unbounded() should work,
perhaps it would be more elegant to introduce a flag into
__reg_combine_64_into_32()?

Sorry for not completing the tests because I did not 'make selftests'
successfully, and uploaded the code that caused the error.