Re: [bpf-next v5] samples: bpf: add max_pckt_size option at xdp_adjust_tail

["Daniel T. Lee" <danieltimlee@gmail.com>]

On Mon, Oct 7, 2019 at 12:37 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Sun, Oct 6, 2019 at 4:58 AM Daniel T. Lee <danieltimlee@gmail.com> wrote:
> >
> > Currently, at xdp_adjust_tail_kern.c, MAX_PCKT_SIZE is limited
> > to 600. To make this size flexible, static global variable
> > 'max_pcktsz' is added.
> >
> > By updating new packet size from the user space, xdp_adjust_tail_kern.o
> > will use this value as a new max packet size.
> >
> > This static global variable can be accesible from .data section with
> > bpf_object__find_map* from user space, since it is considered as
> > internal map (accessible with .bss/.data/.rodata suffix).
> >
> > If no '-P <MAX_PCKT_SIZE>' option is used, the size of maximum packet
> > will be 600 as a default.
> >
> > Changed the way to test prog_fd, map_fd from '!= 0' to '< 0',
> > since fd could be 0 when stdin is closed.
> >
> > Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
> >
> > ---
>
> See nit below, but otherwise looks good.
>
> Acked-by: Andrii Nakryiko <andriin@fb.com>
>
> > Changes in v5:
> >     - Change pcktsz map to static global variable
> > Changes in v4:
> >     - make pckt_size no less than ICMP_TOOBIG_SIZE
> >     - Fix code style
> > 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 |  7 +++++--
> >  samples/bpf/xdp_adjust_tail_user.c | 32 ++++++++++++++++++++++--------
> >  2 files changed, 29 insertions(+), 10 deletions(-)
> >
> > diff --git a/samples/bpf/xdp_adjust_tail_kern.c b/samples/bpf/xdp_adjust_tail_kern.c
> > index 411fdb21f8bc..c616508befb9 100644
> > --- a/samples/bpf/xdp_adjust_tail_kern.c
> > +++ b/samples/bpf/xdp_adjust_tail_kern.c
> > @@ -25,6 +25,9 @@
> >  #define ICMP_TOOBIG_SIZE 98
> >  #define ICMP_TOOBIG_PAYLOAD_SIZE 92
> >
> > +/* volatile to prevent compiler optimizations */
> > +static volatile __u32 max_pcktsz = MAX_PCKT_SIZE;
> > +
> >  struct bpf_map_def SEC("maps") icmpcnt = {
> >         .type = BPF_MAP_TYPE_ARRAY,
> >         .key_size = sizeof(__u32),
> > @@ -92,7 +95,7 @@ static __always_inline int send_icmp4_too_big(struct xdp_md *xdp)
> >         orig_iph = data + off;
> >         icmp_hdr->type = ICMP_DEST_UNREACH;
> >         icmp_hdr->code = ICMP_FRAG_NEEDED;
> > -       icmp_hdr->un.frag.mtu = htons(MAX_PCKT_SIZE-sizeof(struct ethhdr));
> > +       icmp_hdr->un.frag.mtu = htons(max_pcktsz - sizeof(struct ethhdr));
> >         icmp_hdr->checksum = 0;
> >         ipv4_csum(icmp_hdr, ICMP_TOOBIG_PAYLOAD_SIZE, &csum);
> >         icmp_hdr->checksum = csum;
> > @@ -121,7 +124,7 @@ static __always_inline int handle_ipv4(struct xdp_md *xdp)
> >         int pckt_size = data_end - data;
> >         int offset;
> >
> > -       if (pckt_size > MAX_PCKT_SIZE) {
> > +       if (pckt_size > max(max_pcktsz, ICMP_TOOBIG_SIZE)) {
> >                 offset = pckt_size - ICMP_TOOBIG_SIZE;
> >                 if (bpf_xdp_adjust_tail(xdp, 0 - offset))
> >                         return XDP_PASS;
> > diff --git a/samples/bpf/xdp_adjust_tail_user.c b/samples/bpf/xdp_adjust_tail_user.c
> > index a3596b617c4c..bcdebd3be83e 100644
> > --- a/samples/bpf/xdp_adjust_tail_user.c
> > +++ b/samples/bpf/xdp_adjust_tail_user.c
> > @@ -23,6 +23,7 @@
> >  #include "libbpf.h"
> >
> >  #define STATS_INTERVAL_S 2U
> > +#define MAX_PCKT_SIZE 600
> >
> >  static int ifindex = -1;
> >  static __u32 xdp_flags = XDP_FLAGS_UPDATE_IF_NOEXIST;
> > @@ -72,6 +73,7 @@ static void usage(const char *cmd)
> >         printf("Usage: %s [...]\n", cmd);
> >         printf("    -i <ifname|ifindex> Interface\n");
> >         printf("    -T <stop-after-X-seconds> Default: 0 (forever)\n");
> > +       printf("    -P <MAX_PCKT_SIZE> Default: %u\n", MAX_PCKT_SIZE);
> >         printf("    -S use skb-mode\n");
> >         printf("    -N enforce native mode\n");
> >         printf("    -F force loading prog\n");
> > @@ -85,13 +87,14 @@ int main(int argc, char **argv)
> >                 .prog_type      = BPF_PROG_TYPE_XDP,
> >         };
> >         unsigned char opt_flags[256] = {};
> > -       const char *optstr = "i:T:SNFh";
> > +       const char *optstr = "i:T:P:SNFh";
> >         struct bpf_prog_info info = {};
> >         __u32 info_len = sizeof(info);
> >         unsigned int kill_after_s = 0;
> >         int i, prog_fd, map_fd, opt;
> >         struct bpf_object *obj;
> > -       struct bpf_map *map;
> > +       __u32 max_pckt_size = 0;
> > +       __u32 key = 0;
> >         char filename[256];
> >         int err;
> >
> > @@ -110,6 +113,9 @@ int main(int argc, char **argv)
> >                 case 'T':
> >                         kill_after_s = atoi(optarg);
> >                         break;
> > +               case 'P':
> > +                       max_pckt_size = atoi(optarg);
> > +                       break;
> >                 case 'S':
> >                         xdp_flags |= XDP_FLAGS_SKB_MODE;
> >                         break;
> > @@ -150,15 +156,25 @@ int main(int argc, char **argv)
> >         if (bpf_prog_load_xattr(&prog_load_attr, &obj, &prog_fd))
> >                 return 1;
> >
> > -       map = bpf_map__next(NULL, obj);
> > -       if (!map) {
> > -               printf("finding a map in obj file failed\n");
> > +       if (prog_fd < 0) {
> > +               printf("load_bpf_file: %s\n", strerror(errno));
>
> program load error is checked above (bpf_prog_load_xattr check), not
> clear why check this again... Maybe just print helpful message there?
>

Oh, It won't be necessary since it gets error from 'bpf_prog_load_xattr'.
I'll resend the patch with this nit fixed.

Thanks for the review!

Thanks,
Daniel

> >                 return 1;
> >         }
> > -       map_fd = bpf_map__fd(map);
> >
> > -       if (!prog_fd) {
> > -               printf("load_bpf_file: %s\n", strerror(errno));
> > +       /* static global var 'max_pcktsz' is accessible from .data section */
> > +       if (max_pckt_size) {
> > +               map_fd = bpf_object__find_map_fd_by_name(obj, "xdp_adju.data");
> > +               if (map_fd < 0) {
> > +                       printf("finding a max_pcktsz map in obj file failed\n");
> > +                       return 1;
> > +               }
> > +               bpf_map_update_elem(map_fd, &key, &max_pckt_size, BPF_ANY);
> > +       }
> > +
> > +       /* fetch icmpcnt map */
> > +       map_fd = bpf_object__find_map_fd_by_name(obj, "icmpcnt");
> > +       if (map_fd < 0) {
> > +               printf("finding a icmpcnt map in obj file failed\n");
> >                 return 1;
> >         }
> >
> > --
> > 2.20.1
> >