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SubjectRe: [PATCH bpf RESEND 2/4] bpf: Remove size check for sk in bpf_skb_is_valid_access for 32-bit architecture
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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

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