[bpf-next PATCH] samples/bpf: xdp2skb_meta comment explain why pkt-data pointers are invalidated
From: Jesper Dangaard Brouer <hidden>
Date: 2018-01-17 10:17:42
Subsystem:
bpf [general] (safe dynamic programs and tools), the rest, xdp (express data path) · Maintainers:
Alexei Starovoitov, Daniel Borkmann, Andrii Nakryiko, Eduard Zingerman, Kumar Kartikeya Dwivedi, Linus Torvalds, David S. Miller, Jakub Kicinski, Jesper Dangaard Brouer, John Fastabend
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 <redacted>
---
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;