Collect and output skb->cb when --filter-trace-tc #461
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jschwinger233
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Asphaltt
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Dec 5, 2024
bpf/kprobe_pwru.c
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| @@ -443,6 +444,10 @@ set_output(void *ctx, struct sk_buff *skb, struct event_t *event) { | |||
| if (cfg->output_stack) { | |||
| event->print_stack_id = bpf_get_stackid(ctx, &print_stack_map, BPF_F_FAST_STACK_CMP); | |||
| } | |||
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| if (cfg->output_cb) { | |||
| bpf_probe_read_kernel(&event->meta.cb, sizeof(event->meta.cb), (void *)&skb->cb[8]); | |||
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It's hard to understand skb->cb[8] here. It's better to convert skb->cb as struct qdisc_skb_cb *, and then read data from cb->data like:
struct qdisc_skb_cb *cb = (typeof(cb))(void *) skb->cb;
bpf_probe_read_kernel(&event->meta.cb, sizeof(event->meta.cb), (void *) &cb->data);
bpf/kprobe_pwru.c
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| @@ -54,7 +54,7 @@ struct skb_meta { | |||
| u32 len; | |||
| u32 mtu; | |||
| u16 protocol; | |||
| u16 pad; | |||
| u8 cb[20]; | |||
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It's OK to define as u32 cb[5] like cb in struct __sk_buff, I think.
According to kernel verifier implementation[1], __sk_buff->cb will be mapped to ((struct qdisc_skb_cb*)&sk_buff->cb)->data, let's collect 20 bytes from there and output cb as u32[5] when --filter-trace-tc is turned on. [1] https://elixir.bootlin.com/linux/v6.8/source/net/core/filter.c#L9593 Signed-off-by: gray <[email protected]>
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According to kernel verifier implementation[1], __sk_buff->cb will be mapped to ((struct qdisc_skb_cb*)&sk_buff->cb)->data, let's collect 20 bytes from there and output cb as u32[5] when --filter-trace-tc is turned on.
[1] https://elixir.bootlin.com/linux/v6.8/source/net/core/filter.c#L9593
Fixes: #295