resolve_seg_reg(9)

🌸 resolve_seg_reg(9) — the kernel API manual, but make it make sense 💅

okay so bestie, welcome to the resolve_seg_reg(9) page, straight out of the Kernel API Manual. this one's a little x86-flavored gem and it's literally about figuring out which segment register an instruction is talking about. buckle up 🎀

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✨ NAME ✨

resolve_seg_reg — obtain segment register index

like, that's the whole vibe. it hands you back an index. not the value, the index. we'll get there, i promise 🫶

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✨ SYNOPSIS ✨

int resolve_seg_reg (struct insn *insn , struct pt_regs *regs , int regoff );

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✨ ARGUMENTS ✨

• insn — the instruction, complete with its operands. this is your main character of the hour 👑
• regs — the register values exactly as they were when we entered kernel mode. like a lil snapshot 📸
• regoff — the operand offset, in pt_regs, used to determine the segment register

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✨ DESCRIPTION ✨

so the whole job here is: determine the segment register that's associated with the operands, and — where it applies — with the prefixes and the instruction that insn is pointing at. ezpz, right? 😭

here's the thing though. the segment register associated with an operand used in register-indirect addressing depends on a whole little stack of conditions:

a) whether we're running in long mode — because in that case segments are straight up ignored, except if FS or GS are used. icons only 💅

b) whether segment override prefixes can even be used — certain instructions and registers simply do not allow override prefixes. rude, but okay 🙈

c) whether segment override prefixes are actually found in the instruction prefixes

d) if there are no segment override prefixes, or they can't be used, then the default segment register associated with the operand register is what we go with. defaults to the rescue ✨

so, what does the function actually do? glad you asked, bestie. first it checks whether segment override prefixes can be used with the operand indicated by regoff. if that's allowed, it obtains that overridden segment register index. and then, lastly, if no prefixes were found or they can't be used, it resolves the segment register index to use based on the defaults described in the Intel documentation. and in long mode? all segment register indexes get ignored, except if overrides were found for FS or GS. the way that all of this is handled is via helper functions — she delegates, she's organized 🧚

the operand register, regoff, is represented as the offset from the base of pt_regs. noted 📝

so as established, the main use of this function is to determine the segment register index based on the instruction, its operands and prefixes — which means insn must be valid. however! if regoff indicates rIP, we don't need to inspect insn at all, because in that case CS is used in all cases. that check happens before we proceed any further. slay, an early return 🔥

⚠️ very important, do not skip this: this function does not return the value in the segment register (i.e. the segment selector). it returns our defined index. to actually get the segment selector you need to call get_segment_selector and pass it the segment register index that this function resolved. two-step process, that's just how she is 💖

an index identifying the segment register to use, among CS, SS, DS, ES, FS, or GS. INAT_SEG_REG_IGNORE is returned if running in long mode.

-EINVAL in case of error. 😭

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✨ SEE ALSO ✨

Kernel file ./arch/x86/lib/insn-eval.c — and honestly the whole friend group is here:

is_string_insn(9), insn_has_rep_prefix(9), get_seg_reg_override_idx(9), check_seg_overrides(9), resolve_default_seg(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(9), insn_fetch_from_user_inatomic(9), insn_decode_from_regs(9), insn_decode_mmio(9)

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and that's the page, bestie!! 🦋 this lil beauty lives in ./arch/x86/lib, and the version you're reading is the October 2026 one, page resolve_seg_reg(9). go forth and resolve those segment register indexes, no cap 💜✨