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: 36e04a2d78
("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>
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@ -35,15 +35,17 @@ int _xdp_mark(struct xdp_md *ctx)
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void *data, *data_end;
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int ret;
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/* Reserve space in-front data pointer for our meta info.
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/* Reserve space in-front of data pointer for our meta info.
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* (Notice drivers not supporting data_meta will fail here!)
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*/
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ret = bpf_xdp_adjust_meta(ctx, -(int)sizeof(*meta));
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if (ret < 0)
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return XDP_ABORTED;
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/* For some unknown reason, these ctx pointers must be read
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* after bpf_xdp_adjust_meta, else verifier will reject prog.
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/* Notice: Kernel-side verifier requires that loading of
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* ctx->data MUST happen _after_ helper bpf_xdp_adjust_meta(),
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* as pkt-data pointers are invalidated. Helpers that require
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* this are determined/marked by bpf_helper_changes_pkt_data()
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*/
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data = (void *)(unsigned long)ctx->data;
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