STRUCT HID_BPF_CTX(9)

💅 struct hid_bpf_ctx(9) — Kernel API Manual, but make it cute

okay bestie, so this is struct hid_bpf_ctx — it's literally the user-accessible data for all HID programs ✨. Like, every single one. That's the whole vibe.

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✨ SYNOPSIS ✨

struct hid_bpf_ctx {
    struct hid_device *hid;
    __u32 allocated_size;
    union {
      __s32 retval;
      __s32 size;
    };
 };

no cap, that's exactly it, copy it character for character 🫶

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🎀 Members 🎀

• hid — the struct hid_device that represents the device itself. main character energy, honestly 👑
• allocated_size — how much data is allocated. the size of the data. that's it, that's the tweet 💅
• {unnamed_union} — anonymous. yes really, it's just chillin' there unnamed 🙈
• retval — the return value of the previous program. lowkey important 👀
• size — the valid data in the data field. the actual usable amount 🔥

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🦋 Description 🦋

so here's the thing bestie — data is not directly accessible from the context. 😭 like you can't just grab it. you gotta issue a call to hid_bpf_get_data in order to get a pointer to that field. that's the whole move, iykyk.

allocated_size is how much memory is available and can be requested by the HID program. and important note, fr fr: for HID_BPF_RDESC_FIXUP, that memory is set to 4096 (4 KB). periodt.

programs can get the available valid size in data by fetching this field. programs can also change this value by returning a positive number in the program. and to discard the event? return a negative error code. simple as that 💀

⚠️ size must always be less or equal than allocated_size — and yes it's enforced once all BPF programs have been run. so don't try it, the kernel will vibe check you 😤

and for the read/write tea: hid and allocated_size are read-only, while size and retval are read-write. slay, now you know 🌸

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🌈 SEE ALSO 🌈

• Kernel file ./include/linux/hid_bpf.h
• struct hid_bpf_ops(9)

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and that's the whole page, bestie! 💖 this absolute icon came from ./include/linux, dated October 2026, and it's filed under struct hid_bpf_ctx(9) — go off, king 🧚✨🎀