register_ftrace_direct(9)

🌸 register_ftrace_direct(9) — like, the kernel API manual, bestie 🌸

okay so real talk, this is a page straight out of the Kernel API Manual (register_ftrace_direct(9), section 9, kernel vibes only) and it's genuinely one of those "small function, huge consequences" situations. sit down, grab a lil drink 🍓, and let's get into it.

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✨ SYNOPSIS ✨

int register_ftrace_direct (struct ftrace_ops *ops , unsigned long addr
);

like yes that's the whole signature, she's cute, she's compact 💅

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🎀 ARGUMENTS 🎀

• ops — the address of the struct ftrace_ops object. that's your lil registry of which functions we're messing with 🫶
• addr — the address of the trampoline that gets called at the ops functions. the destination of the jump, iykyk

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🦋 DESCRIPTION 🦋

so what this actually does: it hooks up direct calls to addr from the nop locations of the functions registered in ops — and those functions get registered via the ftrace_set_filter_ip function, no cap.

the way that this works is lowkey important though: the location it calls (addr) must be able to handle a direct call. not optional, not a suggestion, it's giving requirement ✨. and it has to save the parameters of the function being traced, and restore them (or inject new ones if needed 😳) before returning. that's the whole deal. miss that and it's a whole situation.

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🔥 RETURN 🔥

• 0 — success, we ate 💅
• -EINVAL — the ops object was already registered with this call, or there are no functions in the ops object. either way, invalid, periodt.
• -EBUSY — another direct function is already attached. and bestie, there can be only one. that's the rule.
• -ENODEV — ip does not point to a ftrace nop location (or it's not supported). wrong address, wrong vibe 😭
• -ENOMEM — there was an allocation failure. the memory said no 🙈

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💜 SEE ALSO 💜

okay this list is LONG and honestly iconic for it. first up, the source file:

• Kernel file ./kernel/trace/ftrace.c

and then the whole friend group (all section 9, all linked):

• ftrace_lookup_ip(9)
• ftrace_free_filter(9)
• ftrace_hash_move(9)
• ftrace_location_range(9)
• ftrace_location(9)
• ftrace_text_reserved(9)
• ftrace_bug(9)
• ftrace_update_record(9)
• ftrace_test_record(9)
• ftrace_get_addr_new(9)
• ftrace_get_addr_curr(9)
• ftrace_rec_iter_start(9)
• ftrace_rec_iter_next(9)
• ftrace_rec_iter_record(9)
• ftrace_run_stop_machine(9)
• arch_ftrace_update_code(9)
• ftrace_startup_subops(9)
• ftrace_shutdown_subops(9)
• ftrace_regex_open(9)
• allocate_ftrace_func_mapper(9)
• ftrace_func_mapper_find_ip(9)
• ftrace_func_mapper_add_ip(9)
• ftrace_func_mapper_remove_ip(9)
• free_ftrace_func_mapper(9)
• unregister_ftrace_direct(9)
• modify_ftrace_direct_nolock(9)
• modify_ftrace_direct(9)
• hash_add(9)
• update_ftrace_direct_add(9)
• hash_sub(9)
• update_ftrace_direct_del(9)
• update_ftrace_direct_mod(9)
• ftrace_set_filter_ip(9)
• ftrace_set_filter_ips(9)
• ftrace_ops_set_global_filter(9)
• ftrace_set_filter(9)
• ftrace_set_notrace(9)
• ftrace_set_global_filter(9)
• ftrace_set_global_notrace(9)
• ftrace_ops_get_func(9)
• ftrace_kill(9)
• ftrace_is_dead(9)
• register_ftrace_function(9)
• unregister_ftrace_function(9)
• ftrace_lookup_symbols(9)

like. forty-five entries. she's thorough and honestly? obsessed 🧚

ngl this whole page lives under ./kernel/trace, dated October 2026, and it's still signing off as register_ftrace_direct(9) at the bottom. main character energy from start to finish 👑

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and that's the page, bestie. go forth, register your direct calls responsibly, and remember: addr has to handle the call. no shortcuts, no vibes-based guessing 🌈⭐