netdev - Re: [PATCH] bpf: Fix off-by-one in tail call count limiting

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Date:   Wed, 28 Jul 2021 12:13:17 -0700
From:   Yonghong Song <yhs@...com>
To:     Johan Almbladh <johan.almbladh@...finetworks.com>,
        <ast@...nel.org>, <daniel@...earbox.net>, <andrii@...nel.org>
CC:     <kafai@...com>, <songliubraving@...com>,
        <john.fastabend@...il.com>, <kpsingh@...nel.org>,
        <Tony.Ambardar@...il.com>, <netdev@...r.kernel.org>,
        <bpf@...r.kernel.org>
Subject: Re: [PATCH] bpf: Fix off-by-one in tail call count limiting



On 7/28/21 9:47 AM, Johan Almbladh wrote:
> Before, the interpreter allowed up to MAX_TAIL_CALL_CNT + 1 tail calls.
> Now precisely MAX_TAIL_CALL_CNT is allowed, which is in line with the
> behavior of the x86 JITs.
> 
> Signed-off-by: Johan Almbladh <johan.almbladh@...finetworks.com>

LGTM.

Acked-by: Yonghong Song <yhs@...com>

I also checked arm/arm64 jit. I saw the following comments:

         /* if (tail_call_cnt > MAX_TAIL_CALL_CNT)
          *      goto out;
          * tail_call_cnt++;
          */

Maybe we have this MAX_TAIL_CALL_CNT + 1 issue
for arm/arm64 jit?

> ---
>   kernel/bpf/core.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 9b1577498373..67682b3afc84 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -1559,7 +1559,7 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
>   
>   		if (unlikely(index >= array->map.max_entries))
>   			goto out;
> -		if (unlikely(tail_call_cnt > MAX_TAIL_CALL_CNT))
> +		if (unlikely(tail_call_cnt >= MAX_TAIL_CALL_CNT))
>   			goto out;
>   
>   		tail_call_cnt++;
>