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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Date:   Thu, 15 Mar 2018 16:06:07 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
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 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.