insn_fetch_from_user(9)

💅 insn_fetch_from_user — a kernel API manual moment

okay bestie, welcome to the Kernel API Manual entry for insn_fetch_from_user(9) — yes the (9) is the manual section, we love a properly filed page ✨

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🌸 NAME 🌸

insn_fetch_from_user — literally copies instruction bytes straight outta user-space memory 🫶

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🎀 SYNOPSIS 🎀

int insn_fetch_from_user (struct pt_regs *regs , unsigned char
buf[MAX_INSN_SIZE] );

that's the whole vibe, no cap 🔥

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✨ ARGUMENTS ✨

• regs — the struct stuffed with register values, exactly as they looked when the kernel mode got entered 👑 so like, the receipts of the moment.
• buf — the array where the fetched instruction goes to live its best life 🎀

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🦋 DESCRIPTION 🦋

so what it does is: it gets the linear address of the instruction, and then it copies the instruction bytes into buf. that's it, that's the whole plot 💖

and then it gives you back one of these:

• the number of instruction bytes copied ✅ slay, we got bytes
• 0 if nothing was copied 🙈 awkward but valid
• -EINVAL if the linear address of the instruction could not be calculated 💀 yeah that one's a no

ngl bestie, -EINVAL means the address math just didn't math. that's the whole warning, please respect it 🙏

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🌈 SEE ALSO 🌈

all the siblings and cousins, fr fr — the kernel file ./arch/x86/lib/insn-eval.c is the source of truth, and here's the entire family reunion:

is_string_insn(9), insn_has_rep_prefix(9), get_seg_reg_override_idx(9), check_seg_overrides(9), resolve_default_seg(9), resolve_seg_reg(9), get_segment_selector(9), get_reg_offset_16(9), get_desc(9), insn_get_seg_base(9), get_seg_limit(9), insn_get_code_seg_params(9), insn_get_modrm_rm_off(9), insn_get_modrm_reg_off(9), insn_get_modrm_reg_ptr(9), get_seg_base_limit(9), get_eff_addr_reg(9), get_eff_addr_modrm(9), get_eff_addr_modrm_16(9), get_eff_addr_sib(9), get_addr_ref_16(9), get_addr_ref_32(9), get_addr_ref_64(9), insn_get_addr_ref(9), insn_fetch_from_user_inatomic(9), insn_decode_from_regs(9), insn_decode_mmio(9)

like, that's the entire group chat and every single one of them is invited 💅

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okay and finally the sign-off bestie: this page lives under ./arch/x86/lib and it's dated October 2026 — so yeah, technically a little time-traveller 🕰️✨ periodt, read responsibly 🧚💜