samples/bpf: xdp2skb_meta comment explain why pkt-data pointers are invalidated

Improve the 'unknown reason' comment, with an actual explaination of why
the ctx pkt-data pointers need to be loaded after the helper function
bpf_xdp_adjust_meta().  Based on the explaination Daniel gave.

Fixes: 36e04a2d78d9 ("samples/bpf: xdp2skb_meta shows transferring info from XDP to SKB")
Reported-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

diff --git a/samples/bpf/xdp2skb_meta_kern.c b/samples/bpf/xdp2skb_meta_kern.c
index 12e1024..0c12048 100644 (file)
--- a/samples/bpf/xdp2skb_meta_kern.c
+++ b/samples/bpf/xdp2skb_meta_kern.c
@@ -35,15 +35,17 @@ int _xdp_mark(struct xdp_md *ctx)
        void *data, *data_end;
        int ret;
 
-       /* Reserve space in-front data pointer for our meta info.
+       /* Reserve space in-front of data pointer for our meta info.
         * (Notice drivers not supporting data_meta will fail here!)
         */
        ret = bpf_xdp_adjust_meta(ctx, -(int)sizeof(*meta));
        if (ret < 0)
                return XDP_ABORTED;
 
-       /* For some unknown reason, these ctx pointers must be read
-        * after bpf_xdp_adjust_meta, else verifier will reject prog.
+       /* Notice: Kernel-side verifier requires that loading of
+        * ctx->data MUST happen _after_ helper bpf_xdp_adjust_meta(),
+        * as pkt-data pointers are invalidated.  Helpers that require
+        * this are determined/marked by bpf_helper_changes_pkt_data()
         */
        data = (void *)(unsigned long)ctx->data;