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

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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>
This commit is contained in:
Jesper Dangaard Brouer 2018-01-17 11:17:37 +01:00 committed by Daniel Borkmann
parent cda18e9726
commit e2e3224122
1 changed files with 5 additions and 3 deletions
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samples/bpf/xdp2skb_meta_kern.c
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@ -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;