linux-kernel - Re: [PATCH bpf RESEND 2/4] bpf: Remove size check for sk in bpf_skb_is_valid

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Date:   Mon, 7 Nov 2022 17:12:10 +0800
From:   Yang Jihong <yangjihong1@...wei.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
CC:     <ast@...nel.org>, <daniel@...earbox.net>, <andrii@...nel.org>,
        <martin.lau@...ux.dev>, <song@...nel.org>, <yhs@...com>,
        <john.fastabend@...il.com>, <kpsingh@...nel.org>, <sdf@...gle.com>,
        <haoluo@...gle.com>, <jolsa@...nel.org>,
        <illusionist.neo@...il.com>, <davem@...emloft.net>,
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        <mykolal@...com>, <shuah@...nel.org>,
        <benjamin.tissoires@...hat.com>, <memxor@...il.com>,
        <delyank@...com>, <asavkov@...hat.com>, <bpf@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>,
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH bpf RESEND 2/4] bpf: Remove size check for sk in
 bpf_skb_is_valid_access for 32-bit architecture

Hello,

On 2022/11/3 19:23, Russell King (Oracle) wrote:
> On Thu, Nov 03, 2022 at 05:21:16PM +0800, Yang Jihong wrote:
>> The error code -EACCES is returned when bpf prog is tested in 32-bit environment,
>> This is because bpf_object__relocate modifies the instruction to change memory
>> size to 4 bytes, as shown in the following messages:
>>
>> libbpf: prog 'kfunc_call_test1': relo #2: matching candidate #0 <byte_off> [18342] struct __sk_buff.sk (0:30:0 @ offset 168)
>> libbpf: prog 'kfunc_call_test1': relo #2: patched insn #1 (LDX/ST/STX) off 168 -> 168
>> libbpf: prog 'kfunc_call_test1': relo #2: patched insn #1 (LDX/ST/STX) mem_sz 8 -> 4
>>
>> As a result, the bpf_skb_is_valid_access check fails. For 32-bit architecture,
>> unnecessary checks need to be deleted.
> 
> Isn't the purpose of this check to ensure that the entire pointer is
> written, and BPF can't write half of it?
> 
> 
>>   	case offsetof(struct __sk_buff, sk):
>> -		if (type == BPF_WRITE || size != sizeof(__u64))
>> -			return false;
> 
> Wouldn't "(size != sizeof(struct bpf_sock *) && size != sizeof(__u64))"
> be more appropriate here, so 32-bit can only write the 32-bit pointer
> or the full 64-bit value, and 64-bit can only write the 64-bit pointer?
> Or is there a reason not to? bpf folk?
> 
Thanks for the detailed proposals, will fix it in next version.

Thanks,
Yang