strnlen_user_nofault(9)

💅 strnlen_user_nofault 💅

okay bestie, welcome to the kernel API manual for strnlen_user_nofault — yes that name is a mouthful, yes it's iconic, and yes we're gonna get through it together 🫶

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✨ NAME ✨

strnlen_user_nofault — get the size of a user string INCLUDING the final NUL 🎀

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🌸 SYNOPSIS 🌸

long strnlen_user_nofault (const void __user *unsafe_addr , long count );

that's the whole signature, no cap. take it in. 📝

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🦋 ARGUMENTS 🦋

• unsafe_addr — the string we're measuring, literally. give it the address, it gives you the length 🧚
• count — the maximum count, and yes that includes the NUL bestie, don't forget the NUL 💀

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💖 DESCRIPTION 💖

So what does it do? It gets the size of a NUL-terminated string in user space without pagefault ✨ like, it just does it. no faulting, no drama.

It returns the size of the string INCLUDING the terminating NUL, periodt 👑

Now here's the part where you gotta pay attention: if the string is too long, it returns a number larger than count. so you, the caller, have to check the return value against "> count" yourself. that's on you, bestie 🙈

And on exception (or an invalid count), it returns 0. zero. zilch. 😭

Unlike strnlen_user, this one can be used from an IRQ handler etc. because it disables pagefaults 🔥 that's the whole glow-up, iykyk. (and also, lowkey, the reason it's "nofault" — the name is giving exactly what it should 💅)

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👑 SEE ALSO 👑

• the kernel file ./mm/maccess.c — go read it, bestie, it's where the magic lives 📂
• copy_from_user_nofault(9)
• copy_to_user_nofault(9)
• strncpy_from_user_nofault(9)

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and that's the page!! 🌈 this lil' beauty lives in ./mm and it's dated October 2026 — so yeah, mark your calendar, screenshot it, send it to the group chat, whatever you gotta do 🍓💜 mwah, go measure those strings responsibly 💅✨