linux-kernel - Re: [PATCH bpf v2 1/2] bpf: Do more tight ALU bounds tracking

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Date:   Mon, 29 Aug 2022 17:19:10 -0700
From:   Hao Luo <haoluo@...gle.com>
To:     Youlin Li <liulin063@...il.com>
Cc:     daniel@...earbox.net, ast@...nel.org, 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 v2 1/2] bpf: Do more tight ALU bounds tracking

Hi Youlin,

On Sat, Aug 27, 2022 at 6:57 AM 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().
>
> Note that we cannot simply add a call to __reg_combine_64_into_32(). 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 in adjust_scalar_min_max_vals(), 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.
>
> 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>
> ---

It might be better to put the code that performs the actual bounds
deduction into a helper function. It avoids code duplication. But the
current version looks fine to me. Thanks for the patch!

Acked-by: Hao Luo <haoluo@...gle.com>