linux-kernel - Re: [PATCH] bpf/verifier: fix control flow issues in __reg64_bound

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Message-ID: <e186cbd5-4c22-8069-717d-35bb8f8e4fff@fb.com>
Date:   Fri, 29 Jul 2022 10:15:21 -0700
From:   Yonghong Song <yhs@...com>
To:     Zeng Jingxiang <zengjx95@...il.com>, ast@...nel.org,
        daniel@...earbox.net, john.fastabend@...il.com, andrii@...nel.org,
        martin.lau@...ux.dev, song@...nel.org, kpsingh@...nel.org,
        sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        Zeng Jingxiang <linuszeng@...cent.com>
Subject: Re: [PATCH] bpf/verifier: fix control flow issues in
 __reg64_bound_u32()



On 7/28/22 10:49 PM, Zeng Jingxiang wrote:
> From: Zeng Jingxiang <linuszeng@...cent.com>
> 
> This greater-than-or-equal-to-zero comparison of an unsigned value
> is always true. "a >= U32_MIN".
> 1632	return a >= U32_MIN && a <= U32_MAX;
> 
> Fixes: b9979db83401 ("bpf: Fix propagation of bounds from 64-bit min/max into 32-bit and var_off.")
> Signed-off-by: Zeng Jingxiang <linuszeng@...cent.com>
> ---
>   kernel/bpf/verifier.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 0efbac0fd126..dd67108fb1d7 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -1629,7 +1629,7 @@ static bool __reg64_bound_s32(s64 a)
>   
>   static bool __reg64_bound_u32(u64 a)
>   {
> -	return a >= U32_MIN && a <= U32_MAX;
> +	return a <= U32_MAX;
>   }

I cannot find the related link. But IIRC, Alexei commented that
the code is written this way to express the intention (within
32bit bounds) so this patch is unnecessary...

>   
>   static void __reg_combine_64_into_32(struct bpf_reg_state *reg)