bpf: Fix veth tracing + --track-by-stackid #401
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cilium#391 allows --track-skb to track new skb on veth, it relies on a map lookup to decide whether to track or not: ``` SEC("kprobe/veth_convert_skb_to_xdp_buff") int kprobe_veth_convert_skb_to_xdp_buff(struct pt_regs *ctx) { [...] u64 skb_head = (u64) BPF_CORE_READ(skb, head); if (bpf_map_lookup_elem(&skb_heads, &skb_head)) { [...] } return BPF_OK; } ``` However, when --track-skb-by-stackid is used along with --track-skb, the tracked skbs have risks not being recorded in skb_heads map. This is because: 1. cilium#384 no more updates skb_heads map when track reason is "by_stackid". 2. cilium#339 changes --track-skb from using &skb to skb->head. So imagine an skb whose original skb->head = 0xa, the value is updated to 0xb after a while. The first time pwru sees this skb, skb_heads map will insert 0xa entry, this is correct. However, after skb->head being set to 0xb, pwru will verdict the skb of being tracked due to "by_stackid", we end up not inserting 0xb entry into skb_heads map. Then the skb reaches veth, pwru can't find an entry by looking up 0xb from skb_heads map, we are losing track of veth skb again. This patch fixes the issue by raising the priority of track_by_filter: if an skb can be defined as both tracked_by_filter and tracked_by_stackid, use tracked_by_filter over tracked_by_stackid. Another issue cilium#339 brings about is, an skb can have multiple skb->head stored in skb_heads map during its lifetime, but we only clean the latest value at kprobe_skb_lifetime_termination. This issue is beyond this patch. Signed-off-by: gray <[email protected]>
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Closed in favor of a better solution. |
jschwinger233
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cilium#339 changed how pwru tracks skb from using &skb to skb->head for xdp tracing. We found it causing problems such as: 1. An skb can have multiple different skb->head values during its lifetime, especically after (e.g. IPsec) encryption. Pwru now is likely to lose track of skb after encryption. 2. Some subtle issues like cilium#401. This is because some other tracking mechanism relies on &skb. This commit brings back tracking &skb instead of skb->head. As for XDP tracing, I'll introduce a new solution in the following patches. Signed-off-by: gray <[email protected]>
brb
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Aug 30, 2024
#339 changed how pwru tracks skb from using &skb to skb->head for xdp tracing. We found it causing problems such as: 1. An skb can have multiple different skb->head values during its lifetime, especically after (e.g. IPsec) encryption. Pwru now is likely to lose track of skb after encryption. 2. Some subtle issues like #401. This is because some other tracking mechanism relies on &skb. This commit brings back tracking &skb instead of skb->head. As for XDP tracing, I'll introduce a new solution in the following patches. Signed-off-by: gray <[email protected]>
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#391 allows --track-skb to track new skb on veth, it relies on a map lookup to decide whether to track or not:
However, when --track-skb-by-stackid is used along with --track-skb, the tracked skbs have risks not being recorded in skb_heads map.
This is because:
So imagine an skb whose original skb->head = 0xa, the value is updated to 0xb after a while. The first time pwru sees this skb, skb_heads map will insert 0xa entry, this is correct. However, after skb->head being set to 0xb, pwru will verdict the skb of being tracked due to "by_stackid", we end up not inserting 0xb entry into skb_heads map.
Then the skb reaches veth, pwru can't find an entry by looking up 0xb from skb_heads map, we are losing track of veth skb again.
This patch fixes the issue by raising the priority of track_by_filter: if an skb can be defined as both tracked_by_filter and tracked_by_stackid, use tracked_by_filter over tracked_by_stackid.
Another issue #339 brings about is, an skb can have multiple skb->head stored in skb_heads map during its lifetime, but we only clean the latest value at
kprobe_skb_lifetime_termination. This issue is beyond this patch.