Date:   Thu, 19 Sep 2019 10:52:39 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     "Daniel T. Lee" <danieltimlee@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [bpf-next,v3] samples: bpf: add max_pckt_size option at xdp_adjust_tail

On Thu, Sep 19, 2019 at 2:16 AM Daniel T. Lee <danieltimlee@...il.com> wrote:
>
> On Thu, Sep 19, 2019 at 3:00 AM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Wed, Sep 18, 2019 at 10:37 AM Daniel T. Lee <danieltimlee@...il.com> wrote:
> > >
> > > On Tue, Sep 17, 2019 at 1:04 PM Andrii Nakryiko
> > > <andrii.nakryiko@...il.com> wrote:
> > > >
> > > > On Wed, Sep 11, 2019 at 2:33 PM Daniel T. Lee <danieltimlee@...il.com> wrote:
> > > > >
> > > > > Currently, at xdp_adjust_tail_kern.c, MAX_PCKT_SIZE is limited
> > > > > to 600. To make this size flexible, a new map 'pcktsz' is added.
> > > > >
> > > > > By updating new packet size to this map from the userland,
> > > > > xdp_adjust_tail_kern.o will use this value as a new max_pckt_size.
> > > > >
> > > > > If no '-P <MAX_PCKT_SIZE>' option is used, the size of maximum packet
> > > > > will be 600 as a default.
> > > > >
> > > > > Signed-off-by: Daniel T. Lee <danieltimlee@...il.com>
> > > > >
> > > > > ---
> > > > > Changes in v2:
> > > > >     - Change the helper to fetch map from 'bpf_map__next' to
> > > > >     'bpf_object__find_map_fd_by_name'.
> > > > >
> > > > >  samples/bpf/xdp_adjust_tail_kern.c | 23 +++++++++++++++++++----
> > > > >  samples/bpf/xdp_adjust_tail_user.c | 28 ++++++++++++++++++++++------
> > > > >  2 files changed, 41 insertions(+), 10 deletions(-)
> > > > >
> > > > > diff --git a/samples/bpf/xdp_adjust_tail_kern.c b/samples/bpf/xdp_adjust_tail_kern.c
> > > > > index 411fdb21f8bc..d6d84ffe6a7a 100644
> > > > > --- a/samples/bpf/xdp_adjust_tail_kern.c
> > > > > +++ b/samples/bpf/xdp_adjust_tail_kern.c
> > > > > @@ -25,6 +25,13 @@
> > > > >  #define ICMP_TOOBIG_SIZE 98
> > > > >  #define ICMP_TOOBIG_PAYLOAD_SIZE 92
> > > > >
> > > > > +struct bpf_map_def SEC("maps") pcktsz = {
> > > > > +       .type = BPF_MAP_TYPE_ARRAY,
> > > > > +       .key_size = sizeof(__u32),
> > > > > +       .value_size = sizeof(__u32),
> > > > > +       .max_entries = 1,
> > > > > +};
> > > > > +
> > > >
> > > > Hey Daniel,
> > > >
> > > > This looks like an ideal use case for global variables on BPF side. I
> > > > think it's much cleaner and will make BPF side of things simpler.
> > > > Would you mind giving global data a spin instead of adding this map?
> > > >
> > >
> > > Sure thing!
> > > But, I'm not sure there is global variables for BPF?
> > > AFAIK, there aren't any support for global variables yet in BPF
> > > program (_kern.c).
> > >
> > >     # when defining global variable at _kern.c
> > >     libbpf: bpf: relocation: not yet supported relo for non-static
> > > global '<var>' variable found in insns[39].code 0x18
> >
> > just what it says: use static global variable (also volatile to
> > prevent compiler optimizations) :)
> >
> > static volatile __u32 pcktsz; /* this should work */
> >
>
> My apologies, but I'm not sure I'm following.
> What you are saying is, should I define global variable to _kern,c
> and access and modify this variable from _user.c?

Exactly.

>
> For example,
>
> <_kern.c>
> static volatile __u32 pcktsz = 300;

So this part is right.

>
> <_user.c>
> extern __u32 pcktsz;
> // Later in code
> pcktsz = 400;

This one is not as simple, unfortunately. From user side you need to
find an internal map (it will most probably have .bss suffix) and
update it. See selftests/bpf/prog_tests/global_data.c for how it can
be done. Eventually working with BPF global data will be much more
natural, but we don't yet have that implemented.


>
> Is this code means similar to what you've said?
> AFAIK, 'static' keyword for global variable restricts scope to file itself,
> so the 'accessing' and 'modifying' this variable from the <_user.c>
> isn't available.
>
> The reason why I've used bpf map for this 'pcktsz' option is,
> I've wanted to run this kernel xdp program (xdp_adjust_tail_kern.o)
> as it itself, not heavily controlled by user program (./xdp_adjust_tail).
>
> When this 'pcktsz' option is implemented in bpf map, user can simply
> modify 'map' to change this size. (such as bpftool prog map)
>
> But when this variable comes to global data, it can't be changed
> after the program gets loaded.
>
> I really appreciate your time and effort for the review.
> But I'm sorry that I seem to get it wrong.

I understand your confusion, but BPF global data behaves exactly like
what you explain. From BPF side it looks like a normal variable.
Performance-wise it's also faster than doing explicit map lookup. From
user space side it's still a map, though, so you can read/modify it
and generally do the same communication between BPF kernel and user
space as you are used to with maps. Check selftests, it should make it
clearer.

>
> Thanks,
> Daniel
>
> > >
> > > By the way, thanks for the review.
> > >
> > > Thanks,
> > > Daniel
> > >
> > >
> > > > >  struct bpf_map_def SEC("maps") icmpcnt = {
> > > > >         .type = BPF_MAP_TYPE_ARRAY,
> > > > >         .key_size = sizeof(__u32),
> > > > > @@ -64,7 +71,8 @@ static __always_inline void ipv4_csum(void *data_start, int data_size,
> > > > >         *csum = csum_fold_helper(*csum);
> > > > >  }
> > > > >
> > > >
> > > > [...]

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