bpf_copy_from_user_str(9)

💅 bpf_copy_from_user_str(9) — Kernel API Manual 🎀

okay so bestie, welcome to the section 9 glow-up: we're talking bpf_copy_from_user_str, the kfunc that grabs a string out of unsafe user space and drops it safely into kernel space. no cap, this one's a little baddie. ✨

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🌸 SYNOPSIS 🌸

__bpf_kfunc int bpf_copy_from_user_str (void *dst , u32 dst__sz , const
       void __user *unsafe_ptr__ign , u64 flags );

literally the whole vibe in one line. let's break it down 💖

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🫶 ARGUMENTS 🫶

• dst — the destination address, and it lives in kernel space. heads up: this buffer must be at least dst__sz bytes long, no exceptions, she needs room to breathe.
• dst__sz — the maximum number of bytes to copy, and yes that count includes the trailing NUL. the NUL is invited to the party too 🎀
• unsafe_ptr__ign — the source address, and it's in user space. hence the name, bestie, it's unsafe — treat it with the respect (and suspicion) it deserves.
• flags — the only supported flag is BPF_F_PAD_ZEROS. that's it. that's the whole menu. 👑

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⭐ DESCRIPTION ⭐

so what does she actually do? she copies a NUL-terminated string from userspace into BPF space. simple, iconic, effective. 🌈

if the user string is too long, she'll still make sure there's zero termination in the dst buffer — the only exception being if the buffer size is 0, in which case, well, there's nowhere to put a NUL. 😭

and if the BPF_F_PAD_ZEROS flag is set? she goes full main character: memset the tail of dst to 0 on success, and memset all of dst on failure. zeroes everywhere, no crumbs left behind. 💅

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💜 SEE ALSO 💜

the kernel file ./kernel/bpf/helpers.c, plus this entire friend group right here (yes, all of them, we're listing every single one, nobody gets left on read 🫶):

• bpf_obj_new(9), bpf_percpu_obj_new(9), bpf_obj_drop(9), bpf_percpu_obj_drop(9)
• bpf_refcount_acquire(9), bpf_list_push_front(9), bpf_list_push_back(9), bpf_rbtree_add(9)
• bpf_task_acquire(9), bpf_task_release(9)
• bpf_cgroup_acquire(9), bpf_cgroup_release(9), bpf_cgroup_ancestor(9), bpf_cgroup_from_id(9), bpf_task_under_cgroup(9), bpf_task_get_cgroup1(9), bpf_task_from_pid(9), bpf_task_from_vpid(9)
• bpf_dynptr_slice(9), bpf_dynptr_slice_rdwr(9), bpf_dynptr_copy(9), bpf_dynptr_memset(9)
• bpf_iter_bits_new(9), bpf_iter_bits_next(9), bpf_iter_bits_destroy(9)
• bpf_copy_from_user_task_str(9)
• bpf_strcmp(9), bpf_strcasecmp(9), bpf_strnchr(9), bpf_strchr(9), bpf_strchrnul(9), bpf_strrchr(9), bpf_strnlen(9), bpf_strlen(9), bpf_strspn(9), bpf_strcspn(9), bpf_strstr(9), bpf_strcasestr(9), bpf_strnstr(9), bpf_strncasestr(9)
• bpf_lookup_user_key(9), bpf_lookup_system_key(9), bpf_key_put(9), bpf_verify_pkcs7_signature(9)
• bpf_task_work_schedule_signal(9), bpf_task_work_schedule_resume(9), bpf_timer_cancel_async(9)

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🍓 reminder bestie: unsafe_ptr__ign is a user pointer in a kernel function. the __ign is basically the kernel going "i'm ignoring the address-space annotation on purpose, don't @ me". stay safe out there. 💀

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okay that's the whole page, and now the cute sign-off: this little masterpiece lives in ./kernel/bpf, it's dated October 2026 (yes, future queen behavior ⏳), from the Kernel API Manual. and page bpf_copy_from_user_str(9)? she ate. periodt. 👑💖