sk_filter_trim_cap(9)

✨ sk_filter_trim_cap ✨

okay bestie, welcome to the sk_filter_trim_cap page 💅 this is a kernel API manpage (like, from the Kernel API Manual, iykyk) and it's lowkey about running a packet through a socket filter. let's get into it 🫶

---

🌸 SYNOPSIS 🌸

enum skb_drop_reason sk_filter_trim_cap (struct sock *sk , struct
sk_buff *skb , unsigned int cap );

literally the whole vibe in one line, no cap 🎀

---

🦋 ARGUMENTS 🦋

• sk — the sock that's associated with the sk_buff, like its little home base 💖
• skb — the buffer you're filtering, the main character of this whole operation ✨
• cap — a limit on how short the eBPF program is allowed to trim the packet. so the program can trim, but it can't go shorter than this, periodt 🚫

---

👑 DESCRIPTION 👑

so here's what happens, bestie: it runs the eBPF program, and then it cuts skb->data down to the correct size that the program returned ✂️✨

a couple things to know:

• if pkt_len is 0, we toss the packet. bye bye 👋💀
• if skb->len is smaller than pkt_len, we just keep the whole skb->data — no trimming, we're not gonna invent bytes that don't exist 🙈

this is the socket level wrapper to bpf_prog_run 🧚 and it returns 0 if the packet should be accepted, or a drop_reason if the packet should be tossed 🔥

ngl the return value is doing double duty here — 0 means "keep it", a drop_reason means "yeet it", so always check before you celebrate 🎀

---

🌈 SEE ALSO 🌈

• the kernel file ./net/core/filter.c 👀
• bpf_convert_filter(9)
• bpf_check_classic(9)
• sk_filter_release_rcu(9)
• sk_filter_release(9)
• bpf_prog_create(9)
• bpf_prog_create_from_user(9)
• sk_attach_filter(9)
• xdp redirect(9)
• bpf_skb_meta_pointer(9)
• bpf_dynptr_from_skb_meta(9)
• bpf_xdp_pull_data(9)
• bpf_icmp_send(9)

like, go read those too, they're all part of the friend group fr fr 💜

---

okay that's the whole page bestie!! this lil gem lives over in ./net/core and it's dated October 2026, and yeah — it's still calling itself sk_filter_trim_cap(9) at the bottom too. iconic behavior honestly 💅⭐ say hi to your packets for me 🦋