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

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Message-ID: <CA+khW7iknv0hcn-D2tRt8HFseUnyTV7BwpohQHtEyctbA1k27w@mail.gmail.com>
Date:   Fri, 29 Jul 2022 10:11:54 -0700
From:   Hao Luo <haoluo@...gle.com>
To:     Youlin Li <liulin063@...il.com>
Cc:     ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com,
        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: Re: [PATCH bpf] bpf: Do more tight ALU bounds tracking

Hi Youlin,

On Thu, Jul 28, 2022 at 9:44 PM Youlin Li <liulin063@...il.com> wrote:
>
> 32bit bounds and 64bit bounds are updated separately in
> adjust_scalar_min_max_vals() currently, let them learn from each other to
> get more tight bounds tracking. Similar operation can be found in
> reg_set_min_max().
>
> 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
>
> Fixes: 3f50f132d840 ("bpf: Verifier, do explicit ALU32 bounds tracking")

This change looks to me like an improvement, rather than a bug fix. We
probably don't need this tag.

> Signed-off-by: Youlin Li <liulin063@...il.com>
> ---
>  kernel/bpf/verifier.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 0efbac0fd126..888aa50fbdc0 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -8934,10 +8934,13 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
>                 break;
>         }
>
> -       /* ALU32 ops are zero extended into 64bit register */
> -       if (alu32)
> +       if (alu32) {
> +               /* ALU32 ops are zero extended into 64bit register */
>                 zext_32_to_64(dst_reg);
> -       reg_bounds_sync(dst_reg);
> +               __reg_combine_32_into_64(dst_reg);

This __reg_combine_32_into_64 can be replaced with simply
reg_bounds_sync, because the above zext_32_to_64 has already
propagated 32 to 64. Using reg_bounds_sync would be more efficient.

It turns out we can now fold reg_bounds_sync into zext_32_to_64. Can
you do that and resend? IMO that will make the code slightly cleaner.

> +       } else {
> +               __reg_combine_64_into_32(dst_reg);
> +       }
>         return 0;
>  }
>
> --
> 2.25.1
>