On Thu, 2017-04-20 at 16:10 +0200, Daniel Borkmann wrote:
>
> I think this would be a rather more complex operation on the BPF
> side, it would need changes from LLVM (which assumes initial ctx sits
> in r1), verifier for tracking this ctx2, all the way down to JITs
> plus some way to handle 1 and 2 argument program calls generically.
> Much easier to pass additional meta data for the program via cb[],
> for example.
Yeah, it did seem very complex :)
> > Alternatively I can clear another pointer (u64) in the CB, store a
> > pointer there, and always emit code following that pointer - should
> > be possible right?
>
> What kind of pointer? If it's something like data_end as read-only,
> then this needs to be tracked in the verifier in addition, of course.
> Other option you could do (depending on what you want to achieve) is
> to have a bpf_probe_read() version as a helper for your prog type
> that would further walk that pointer/struct (similar to tracing)
> where this comes w/o any backward compat guarantees, though.
I meant something like this
struct wifi_cb {
struct wifi_data *wifi_data;
...
void *data_end; // with BUILD_BUG_ON to the right offset
};
Then struct wifi_data can contain extra data that doesn't fit into
wifi_cb, like the stuff I evicted for *data_end and *wifi_data. Let's
say one of those fields is "u64 boottime_ns;" (as I did in my patch
now), so we have
struct wifi_data {
u64 boottime_ns;
};
then I can still have
struct __wifi_sk_buff {
u32 len;
u32 data;
u32 data_end;
u32 boottime_ns; // this is strange but
// seems to be done this way?
};
And then when boottime_ns is accessed, I can have:
case offsetof(struct __wifi_sk_buff, boottime_ns):
off = si->off;
off -= offsetof(struct __wifi_sk_buff, boottime_ns);
off += offsetof(struct sk_buff, cb);
off += offsetof(struct wifi_cb, wifi_data);
*insn++ = BPF_LDX_MEM(BPF_SIZEOF(void *), si->dst_reg,
si->src_reg, off);
off = offsetof(struct wifi_data, boottime_ns);
*isns++ = BPF_LDX_MEM(BPF_SIZEOF(u64), si->dst_reg,
si->src_reg, off);
break;
no?
It seems to me this should work, and essentially emit code to follow
the pointer to inside struct wifi_data. Assuming
johannes
