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Date: Sun, 15 Sep 2019 12:24:11 +0900
From: "Daniel T. Lee" <danieltimlee@...il.com>
To: Yonghong Song <yhs@...com>
Cc: Daniel Borkmann <daniel@...earbox.net>,
Alexei Starovoitov <ast@...nel.org>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
"bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [bpf-next,v3] samples: bpf: add max_pckt_size option at xdp_adjust_tail
On Sat, Sep 14, 2019 at 7:34 AM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 9/11/19 8:02 PM, Daniel T. Lee 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,
> > +};
>
> We have new map definition format like in
> tools/testing/selftests/bpf/progs/bpf_flow.c.
> But looks like most samples/bpf still use SEC("maps").
> I guess we can leave it for now, and if needed,
> later on a massive conversion for all samples/bpf/
> bpf programs can be done.
>
Thanks for the detailed review!
Didn't notice there was an update with map definition.
This new map definition format looks very neat.
> > +
> > 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);
> > }
> >
> > -static __always_inline int send_icmp4_too_big(struct xdp_md *xdp)
> > +static __always_inline int send_icmp4_too_big(struct xdp_md *xdp,
> > + __u32 max_pckt_size)
> > {
> > int headroom = (int)sizeof(struct iphdr) + (int)sizeof(struct icmphdr);
> >
> > @@ -92,7 +100,7 @@ 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_pckt_size - sizeof(struct ethhdr));
> > icmp_hdr->checksum = 0;
> > ipv4_csum(icmp_hdr, ICMP_TOOBIG_PAYLOAD_SIZE, &csum);
> > icmp_hdr->checksum = csum;
> > @@ -118,14 +126,21 @@ static __always_inline int handle_ipv4(struct xdp_md *xdp)
> > {
> > void *data_end = (void *)(long)xdp->data_end;
> > void *data = (void *)(long)xdp->data;
> > + __u32 max_pckt_size = MAX_PCKT_SIZE;
> > + __u32 *pckt_sz;
> > + __u32 key = 0;
>
> The above two new definitions may the code not in
> reverse Christmas definition order, could you fix it?
>
I'll fix it right away!
> > int pckt_size = data_end - data;
> > int offset;
> >
> > - if (pckt_size > MAX_PCKT_SIZE) {
> > + pckt_sz = bpf_map_lookup_elem(&pcktsz, &key);
> > + if (pckt_sz && *pckt_sz)
> > + max_pckt_size = *pckt_sz;
> > +
> > + if (pckt_size > max_pckt_size) {
> > offset = pckt_size - ICMP_TOOBIG_SIZE;
> > if (bpf_xdp_adjust_tail(xdp, 0 - offset))
> > return XDP_PASS;
>
> We could have the following scenario:
> max_pckt_size = 1
> pckt_size = 2
> offset = -96
> bpf_xdp_adjust_tail return -EINVAL
> so we return XDP_PASS now
>
> Maybe you want to do
> if (pckt_size > max(max_pckt_size, ICMP_TOOBIG_SIZE)) {
> ...
> }
> as in original code, bpf_xdp_adjust_tail(...) already succeeds.
>
I'll try to fix this way.
> > - return send_icmp4_too_big(xdp);
> > + return send_icmp4_too_big(xdp, max_pckt_size);
> > }
> > return XDP_PASS;
> > }
> > diff --git a/samples/bpf/xdp_adjust_tail_user.c b/samples/bpf/xdp_adjust_tail_user.c
> > index a3596b617c4c..aef6c69a48a7 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);
> > + __u32 max_pckt_size = 0;
> > + __u32 key = 0;
> > unsigned int kill_after_s = 0;
> > int i, prog_fd, map_fd, opt;
> > struct bpf_object *obj;
> > - struct bpf_map *map;
> > 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;
> > @@ -150,12 +156,22 @@ 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");
> > + /* update pcktsz map */
> > + if (max_pckt_size) {
> > + map_fd = bpf_object__find_map_fd_by_name(obj, "pcktsz");
> > + if (!map_fd) {
>
> Let us test map_fd and below prog_fd with '< 0" instead of "!= 0'.
> In this particular sample, "! = 0" is okay since we did not close
> stdin. But in programs if stdin is closed, the fd 0 may be reused
> for map_fd. Let us just keep good coding practice here.
>
I didn't think of those details. I'll update this right away!
Once again, I really appreciate your time and effort for the review.
Thank you.
Best,
Daniel
> > + printf("finding a 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) {
> > + printf("finding a icmpcnt map in obj file failed\n");
> > return 1;
> > }
> > - map_fd = bpf_map__fd(map);
> >
> > if (!prog_fd) {
> > printf("load_bpf_file: %s\n", strerror(errno));
> >
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