get_seg_limit(9)

💅 get_seg_limit — the kernel API manual, but make it make sense ✨

okay so bestie, this page is get_seg_limit(9) from the Kernel API Manual, and it's literally all about getting the limit of a segment descriptor 🎀 it's giving "x86 arch girlie reads the descriptor table" and honestly? iconic.

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🌸 NAME 🌸

get_seg_limit — obtain the limit of a segment descriptor. that's it. that's the whole vibe. 💖

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🦋 SYNOPSIS 🦋

unsigned long get_seg_limit (struct pt_regs *regs , int seg_reg_idx );

no cap, that's the whole signature. two args, one unsigned long coming back at you. slay. ✨

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🎀 ARGUMENTS 🎀

• regs — the register values exactly as they looked when you entered kernel mode. like a lil snapshot of the state, you know? 📸
• seg_reg_idx — the index of the segment register that points to the seg descriptor. that's your map, bestie, it's how we find the descriptor we care about. 🗺️

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💅 DESCRIPTION 💅

so what does it actually do? it obtains the limit of the segment as indicated by the segment descriptor pointed to by the segment selector. and where does the segment selector come from? from the input segment register index, seg_reg_idx. easy. iykyk. 💜

now here's the part where the modes matter, and lowkey this is the main character energy of the whole page:

• in protected mode, you get the limit of the segment descriptor in bytes. bytes, bestie. actual real numbers. 🍓
• in long mode and virtual-8086 mode, segment limits are not enforced. so instead it returns the limit as -1L, which implies a limit-less segment. so extra. so free. 🦋

and if something goes wrong? zero is returned on error. that's your signal, fr fr. 💀

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🔥 SEE ALSO 🔥

okay this list is long but we do NOT skip options in this house, so here's every single one, and the kernel file is ./arch/x86/lib/insn-eval.c:

• 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)
• 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(9)
• insn_fetch_from_user_inatomic(9)
• insn_decode_from_regs(9)
• insn_decode_mmio(9)

like, that's the whole squad. 🫶 go read them. they're all in it together. 🌈

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✨ and that's the page ✨ this manual came from ./arch/x86/lib, dated October 2026, and it's get_seg_limit(9) — obsessed, honestly. 💖🎀💅