start_userspace(9)

💅 start_userspace(9) — from the Kernel API Manual, bestie 🎀

okay so picture this: you're deep in the kernel, you want a whole new userspace process to just exist, and this little function is the girlboss that sets the stage for it. it's giving backstage-pass energy. ✨

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🦋 NAME 🦋

start_userspace — the one that prepares a brand-new userspace process. literally its whole job. no cap. 💖

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🌸 SYNOPSIS 🌸

int start_userspace (struct mm_id *mm_id );

yes bestie, that's the whole signature. int out, one pointer in. clean, iconic, minimal. 👑

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🎀 ARGUMENTS 🎀

• mm_id — the corresponding struct mm_id. that's it, that's the argument. it's the matching struct mm_id for this whole operation, so don't go handing it some random other one, iykyk. 🫶

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✨ DESCRIPTION ✨

so what actually happens? two things, and they're both important so don't skip:

1. it setups a new temporary stack page — yes, the manpage says "setups", we're keeping it, it's canon — and that page is the one getting used while userspace_tramp runs. temporary stack, main character energy, does its bit then dips. 🌈
2. it clones the kernel process into a new userspace process — with FDs only. that's the whole vibe: file descriptors only, nothing extra invited to the party. 💅

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🔥 RETURN 🔥

• when positive → you get the process id of the new userspace process. slay, a fresh PID, a fresh little guy. 🐣
• when negative → you get an error number. not slay, but informative. 😭

and then there's this: FIXME: can PIDs become negative?! — genuinely one of the great unanswered questions of our time, and the manpage is just as confused as we are. 💀 it's a real note in the original, we do not resolve it, we simply sit with it. 🙈

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🧚 SEE ALSO 🧚

• Kernel file ./arch/um/os-Linux/skas/process.c — go read it, that's where the lore lives. 📖

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and that's the page, bestie!! 🍓 this one's from the ./arch/um/os-Linux/skas corner of the source tree, dated October 2026 — so like, future-dated and mysterious and honestly kind of iconic for it. mwah, go forth and start some userspace 💜🦋✨