Subject: Re: [bpf-next PATCH 2/3] samples/bpf: make xdp_fwd more practically usable via devmap lookup

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 8 Aug 2019 08:51:15 +0200

On Wed, 7 Aug 2019 11:04:17 -0700
Y Song <[email-masked]> wrote:

> On Wed, Aug 7, 2019 at 5:37 AM Jesper Dangaard Brouer <[email-masked]> wrote:
> >
> > This address the TODO in samples/bpf/xdp_fwd_kern.c, which points out
> > that the chosen egress index should be checked for existence in the
> > devmap. This can now be done via taking advantage of Toke's work in
> > commit 0cdbb4b09a06 ("devmap: Allow map lookups from eBPF").
> >
> > This change makes xdp_fwd more practically usable, as this allows for
> > a mixed environment, where IP-forwarding fallback to network stack, if
> > the egress device isn't configured to use XDP.
> >
> > Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> > ---
> >  samples/bpf/xdp_fwd_kern.c |   20 ++++++++++++++------
> >  samples/bpf/xdp_fwd_user.c |   36 +++++++++++++++++++++++++-----------
> >  2 files changed, 39 insertions(+), 17 deletions(-)
> >
> > diff --git a/samples/bpf/xdp_fwd_kern.c b/samples/bpf/xdp_fwd_kern.c
> > index e6ffc4ea06f4..4a5ad381ed2a 100644
> > --- a/samples/bpf/xdp_fwd_kern.c
> > +++ b/samples/bpf/xdp_fwd_kern.c
> > @@ -104,13 +104,21 @@ static __always_inline int xdp_fwd_flags(struct xdp_md *ctx, u32 flags)
> >
> >         rc = bpf_fib_lookup(ctx, &fib_params, sizeof(fib_params), flags);
> >
> > -       /* verify egress index has xdp support
> > -        * TO-DO bpf_map_lookup_elem(&tx_port, &key) fails with
> > -        *       cannot pass map_type 14 into func bpf_map_lookup_elem#1:
> > -        * NOTE: without verification that egress index supports XDP
> > -        *       forwarding packets are dropped.
> > -        */
> >         if (rc == 0) {
> > +               int *val;
> > +
> > +               /* Verify egress index has been configured as TX-port.
> > +                * (Note: User can still have inserted an egress ifindex that
> > +                * doesn't support XDP xmit, which will result in packet drops).
> > +                *
> > +                * Note: lookup in devmap supported since 0cdbb4b09a0.
> > +                * If not supported will fail with:
> > +                *  cannot pass map_type 14 into func bpf_map_lookup_elem#1:
> > +                */
> > +               val = bpf_map_lookup_elem(&tx_port, &fib_params.ifindex);  
> 
> It should be "xdp_tx_ports". Otherwise, you will have compilation errors.

Ups. This happened in my rebase, where I moved the rename patch [1/3]
before this one.  Thanks for catching this!

> > +               if (!val)
> > +                       return XDP_PASS;  
> 
> Also, maybe we can do
>          if (!bpf_map_lookup_elem(&tx_port, &fib_params.ifindex))
>             return XDP_PASS;
> so we do not need to define val at all.

I had it this way, because I also checked the contents (of the pointer
*val) to check if this was the correct ifindex, but I removed that
check again (as user side always insert correctly).  So, I guess I
could take your suggestion now.

> > +
> >                 if (h_proto == htons(ETH_P_IP))
> >                         ip_decrease_ttl(iph);
> >                 else if (h_proto == htons(ETH_P_IPV6))
> > diff --git a/samples/bpf/xdp_fwd_user.c b/samples/bpf/xdp_fwd_user.c
> > index ba012d9f93dd..20951bc27477 100644
> > --- a/samples/bpf/xdp_fwd_user.c
> > +++ b/samples/bpf/xdp_fwd_user.c
> > @@ -27,14 +27,20 @@
> >  #include "libbpf.h"
> >  #include <bpf/bpf.h>
> >
> > -
> > -static int do_attach(int idx, int fd, const char *name)
> > +static int do_attach(int idx, int prog_fd, int map_fd, const char
> > *name) {
> >         int err;
> >
> > -       err = bpf_set_link_xdp_fd(idx, fd, 0);
> > -       if (err < 0)
> > +       err = bpf_set_link_xdp_fd(idx, prog_fd, 0);
> > +       if (err < 0) {
> >                 printf("ERROR: failed to attach program to %s\n",
> > name);
> > +               return err;
> > +       }
> > +
> > +       /* Adding ifindex as a possible egress TX port */
> > +       err = bpf_map_update_elem(map_fd, &idx, &idx, 0);
> > +       if (err)
> > +               printf("ERROR: failed using device %s as
> > TX-port\n", name);
> >
> >         return err;
> >  }
> > @@ -47,6 +53,9 @@ static int do_detach(int idx, const char *name)
> >         if (err < 0)
> >                 printf("ERROR: failed to detach program from %s\n",
> > name);
> >
> > +       /* TODO: Remember to cleanup map, when adding use of shared
> > map
> > +        *  bpf_map_delete_elem((map_fd, &idx);
> > +        */
> >         return err;
> >  }
> >
> > @@ -67,10 +76,10 @@ int main(int argc, char **argv)
> >         };
> >         const char *prog_name = "xdp_fwd";
> >         struct bpf_program *prog;
> > +       int prog_fd, map_fd = -1;
> >         char filename[PATH_MAX];
> >         struct bpf_object *obj;
> >         int opt, i, idx, err;
> > -       int prog_fd, map_fd;
> >         int attach = 1;
> >         int ret = 0;
> >
> > @@ -103,8 +112,17 @@ int main(int argc, char **argv)
> >                         return 1;
> >                 }
> >
> > -               if (bpf_prog_load_xattr(&prog_load_attr, &obj,
> > &prog_fd))
> > +               err = bpf_prog_load_xattr(&prog_load_attr, &obj,
> > &prog_fd);
> > +               if (err) {
> > +                       if (err == -22) {  
> 
> -EINVAL?

Yes.

> For -EINVAL, many things could go wrong. But maybe the blow error
> is the most common one so I am fine with that.

Yes, it is rather sad, that we don't have better/more return codes,
such that we can react better to these.

E.g. if this was part of a open source project (external to the
kernel), I could have two XDP-BPF programs, one that use this feature
and one that don't.  If I had a more specific return code, then I could
load the other if the first failed.

> > +                               printf("Does kernel support devmap lookup?\n");
> > +                               /* If not, the error message will be:
> > +                                * "cannot pass map_type 14 into func
> > +                                * bpf_map_lookup_elem#1"
> > +                                */
> > +                       }
> >                         return 1;
> > +               }
> >
> >                 prog = bpf_object__find_program_by_title(obj, prog_name);
> >                 prog_fd = bpf_program__fd(prog);
> > @@ -119,10 +137,6 @@ int main(int argc, char **argv)
> >                         return 1;
> >                 }
> >         }
> > -       if (attach) {
> > -               for (i = 1; i < 64; ++i)
> > -                       bpf_map_update_elem(map_fd, &i, &i, 0);
> > -       }
> >
> >         for (i = optind; i < argc; ++i) {
> >                 idx = if_nametoindex(argv[i]);
> > @@ -138,7 +152,7 @@ int main(int argc, char **argv)
> >                         if (err)
> >                                 ret = err;
> >                 } else {
> > -                       err = do_attach(idx, prog_fd, argv[i]);
> > +                       err = do_attach(idx, prog_fd, map_fd, argv[i]);
> >                         if (err)
> >                                 ret = err;
> >                 }
> >  

I'll send a V2

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer