bpf: Differentiate skb tracking reasons #384
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By differentiating skb track reason (filter/skb/stackid), we can avoid a mismatching issue that leads to confusing output. I saw an issue when --filter-track-skb and --filter-track-skb-by-stackid are used together: 1. At func1, an skb matches the pcap filter, we then fill the map stackid_skb[stackid] = skb for --filter-track-skb-by-stackid; 2. At func2, the skb arg doesn't match the pcap filter, but stackid can be found in map stackid_skb. We therefore store the map entry skb_addresses[skb] = true for --filter-track-skb; 3. Due to some reasons, this skb never gets freed by kfree_skbmem(), so pwru keeps matching "wrong" packets due to permanent entry in map skb_addresses for --filter-track-skb; This patch mitigates these risks by not storing new entry in skb_addresses if the skb is tracked by stackid. This doesn't harm the original purpose of --filter-track-skb-by-stack, which is designed for non-skb function tracing on the same stack. On the otherhand, calling bpf_update_map() only for specific track reasons improves performance a little bit. For example, if an skb is already in map skb_addresses, now we don't need to put it in the map again. Signed-off-by: gray <gray.liang@isovalent.com>
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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 tracking &skb to tracking skb->head. So imagine an skb whose original skb->head = 0xa, whose value is updated to 0xb after a while. The first time pwru sees this skb, skb_heads map will insert 0xa entry. However, after skb->head is set to 0xb, pwru sees the skb being tracked due to "by_stackid", we won't insert 0xb entry into skb_heads map. Then when the skb is on veth, we can't find an entry by looking up 0xb from skb_heads map, ending up with 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 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 <gray.liang@isovalent.com>
jschwinger233
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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 <gray.liang@isovalent.com>
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By differentiating skb track reason (filter/skb/stackid), we can avoid a mismatching issue that leads to confusing output.
I saw an issue when --filter-track-skb and --filter-track-skb-by-stackid are used together:
This patch mitigates these risks by not storing new entry in skb_addresses if the skb is tracked by stackid. This doesn't harm the original purpose of --filter-track-skb-by-stack, which is designed for non-skb function tracing on the same stack.
On the otherhand, calling bpf_update_map() only for specific track reasons improves performance a little bit. For example, if an skb is already in map skb_addresses, now we don't need to put it in the map again.