netdev - Re: [bpf-next PATCH v2 05/18] bpf: create tcp_bpf_ulp allowing BPF to monitor socket TX/RX data

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Message-ID: <7791bf7d-5ca8-7e28-b740-bd6c40be402c@iogearbox.net>
Date:   Fri, 16 Mar 2018 01:37:47 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     John Fastabend <john.fastabend@...il.com>, davem@...emloft.net,
        ast@...nel.org, davejwatson@...com, netdev@...r.kernel.org
Subject: Re: [bpf-next PATCH v2 05/18] bpf: create tcp_bpf_ulp allowing BPF to
 monitor socket TX/RX data

On 03/16/2018 12:06 AM, Alexei Starovoitov wrote:
> On Thu, Mar 15, 2018 at 11:55:39PM +0100, Daniel Borkmann wrote:
>> On 03/15/2018 11:20 PM, Alexei Starovoitov wrote:
>>> On Thu, Mar 15, 2018 at 11:17:12PM +0100, Daniel Borkmann wrote:
>>>> On 03/15/2018 10:59 PM, Alexei Starovoitov wrote:
>>>>> On Mon, Mar 12, 2018 at 12:23:29PM -0700, John Fastabend wrote:
>>>>>>  
>>>>>> +/* User return codes for SK_MSG prog type. */
>>>>>> +enum sk_msg_action {
>>>>>> +	SK_MSG_DROP = 0,
>>>>>> +	SK_MSG_PASS,
>>>>>> +};
>>>>>
>>>>> do we really need new enum here?
>>>>> It's the same as 'enum sk_action' and SK_DROP == SK_MSG_DROP
>>>>> and there will be only drop/pass in both enums.
>>>>> Also I don't see where these two new SK_MSG_* are used...
>>>>>
>>>>>> +
>>>>>> +/* user accessible metadata for SK_MSG packet hook, new fields must
>>>>>> + * be added to the end of this structure
>>>>>> + */
>>>>>> +struct sk_msg_md {
>>>>>> +	__u32 data;
>>>>>> +	__u32 data_end;
>>>>>> +};
>>>>>
>>>>> I think it's time for me to ask for forgiveness :)
>>>>
>>>> :-)
>>>>
>>>>> I used __u32 for data and data_end only because all other fields
>>>>> in __sk_buff were __u32 at the time and I couldn't easily figure out
>>>>> how to teach verifier to recognize 8-byte rewrites.
>>>>> Unfortunately my mistake stuck and was copied over into xdp.
>>>>> Since this is new struct let's do it right and add
>>>>> 'void *data, *data_end' here,
>>>>> since bpf prog will use them as 'void *' pointers.
>>>>> There are no compat issues here, since bpf is always 64-bit.
>>>>
>>>> But at least offset-wise when you do the ctx rewrite this would then
>>>> be a bit more tricky when you have 64 bit kernel with 32 bit user
>>>> space since void * members are in each cases at different offset. So
>>>> unless I'm missing something, this still should either be __u32 or
>>>> __u64 instead of void *, no?
>>>
>>> there is no 32-bit user space. these structs are seen by bpf progs only
>>> and bpf is 64-bit only too.
>>> unless I'm missing your point.
>>
>> Ok, so lets say you have 32 bit LLVM binary and compile the prog where
>> you access md->data_end. Given the void * in the struct will that access
>> end up being BPF_W at ctx offset 4 or BPF_DW at ctx offset 8 from clang
>> perspective (iow, is the back end treating this special and always use
>> fixed BPF_DW in such case)? If not and it would be the first case with
>> offset 4, then we could have the case that underlying 64 bit kernel is
>> expecting ctx offset 8 for doing the md ctx conversion.
> 
> i'm still not quite following.
> Whether llvm itself is 32-bit binary or it's arm32 or sprac32 binary
> doesn't matter. It will produce the same 64-bit bpf code.
> It will see 'void *' deref from this struct and will emit DW.
> May be confusion is from newly added -mattr=+alu32 flag?
> That option doesn't change that sizeof(void*)==8.
> It only allows backend to emit 32-bit alu insns.

Ok, so conclusion we had is that while BPF target is unconditionally 64 bit,
it depends which clang front end you use for compilation wrt structs. E.g.
on 32 bit native (e.g. arm) clang front end it would compile the ctx void *
pointers as 4 byte while using clang -target bpf it would compile it as 8
byte. The native clang front end is needed in case of tracing when accessing
pt_regs for walking data structures, but not for networking use case, so
always using -target bpf there is proper way. Meaning there would be no
confusion on the void * since size will always be 8 regardless of underlying
arch being 32 or 64 bit or clang/llvm binary being 32 bit on 64 bit kernel.
Thus, sticking to void * would be fine, but definitely samples/sockmap/Makefile
must be fixed as well, such that people don't copy it wrongly.

Cheers,
Daniel