Re: ebpf: issue with clang

[Eric Leblond <eric@regit.org>]

Hello,

On Sun, 2016-06-12 at 20:35 +0200, Daniel Borkmann wrote:
> On 06/12/2016 07:37 PM, Eric Leblond wrote:
> > On Thu, 2016-06-09 at 17:34 -0700, Alexei Starovoitov wrote:
> > > On Thu, Jun 09, 2016 at 11:10:05PM +0200, Eric Leblond wrote:
> > > > Hello,
> > > > 
> > > > I'm working on integrating ebpf cluster load balancing for
> > > > AF_PACKET
> > > > and I've got some problem to get real code inside the EBPF
> > > > filter.
> > > > 
> > > > I've tried different command lines in the build process. One of
> > > > them
> > > > is:
> > > > clang-3.9 -Wall -O2 -emit-llvm -c hash_ports.c -o - | llc-3.9
> > > > -march=bpf -filetype=obj -o hash_ports.bpf
> > > > 
> > > > If I use that one, then the generated code is almost void. If I
> > > > remove
> > > > the -O2 then I've got a generated code that fails during load.
> > > > When
> > > > not
> > > > using -O2, I manage to load a trivial filter (return of static
> > > > value).
> > > > 
> > > > The C code is the following (a derivative of http-simple-
> > > > filter.c
> > > > used
> > > > for testing):
> > > > 
> > > > int filter(struct __sk_buff *skb) {
> > > > 	uint8_t *cursor = 0;
> > > > 	struct ethernet_t *ethernet = cursor_advance(cursor,
> > > > sizeof(*ethernet));
> > > 
> > > this is bcc C syntax that is hiding the explicit
> > > load_byte/half/word
> > > operations
> > > we have to do when using plain C.
> > > If you want to compile C code with clang -O2 -target bpf file.c
> > > -c -o
> > > file.o
> > > and copy .o around to be used in tc like:
> > > tc filter add dev eth0 ingress bpf da obj file.o
> > > then plain C should be used like in all samples/bpf/*_kern.c
> > > examples.
> > > Other folks like the convenience of bcc that hides clang/llvm
> > > invocation.
> > > It mostly applicable to tracing tools where both bcc-C and
> > > corresponding python or lua bits are in the same file
> > > like in iovisor/bcc/tools/* scripts.
> > > The iovisor/bcc/examples/networking/* (where this http-simple-
> > > filter.c came from)
> > > are also suitable for networking and relying on pyroute2 to talk
> > > to
> > > kernel to create netns, veth and to attach bpf to qdisc.
> > > 
> > > In summary there are several ways to write bpf C code:
> > > 1. plain C syntax as in samples/bpf/*_kern.c
> > > Pro: compiles with clang into .o
> > > Con: .o requires elf loader (integrated into tc already for
> > > networking),
> > 
> > Yes, that's not an easy part. I've devel one loader for suricata
> > but I
> > will check the one in tc to see if I can take advantage of it.
> 
> Sure, feel free to rip it out and adapt it.
> 
> With AF_PACKET load balancing you mean a packet fanout eBPF demuxing
> or
> something else controlled via tc ingress?

I'm using fanout eBPF demuxing to implement load balancing in Suricata.
Current alpha level code is here:

https://github.com/regit/suricata/commit/f299abe90bfed3590a9f3de1179091
b7afc2d90c

I'm currently working on the demuxing to implement something more
realistic than what current demuxing function.

> 
> If packet fanout, then you also need to adapt the program type into
> BPF_PROG_TYPE_SOCKET_FILTER.

Yes, already done that (or at least it seems to work).

BR,
-- 

Eric Leblond <eric@regit.org>