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 <[email protected]>
Signed-off-by: Jesper Dangaard Brouer <[email protected]>
---
samples/bpf/xdp2skb_meta_kern.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/samples/bpf/xdp2skb_meta_kern.c b/samples/bpf/xdp2skb_meta_kern.c
index 12e1024069c2..0c12048ac79f 100644
--- 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;
