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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

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?

>                 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
>