strncpy_from_user_nofault(9)

💅 strncpy_from_user_nofault — Kernel API Manual ✨

okay so bestie, buckle up, because we're diving into strncpy_from_user_nofault(9) from the Kernel API Manual and honestly? she's a little dangerous and that's the whole point 🫶

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🎀 NAME 🎀

strncpy_from_user_nofault — copy a NUL-terminated string from an unsafe user address. yeah. unsafe. we said what we said 💀

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✨ SYNOPSIS ✨

long strncpy_from_user_nofault (char *dst , const void __user *unsafe_addr , long count );

literally the whole vibe in one line, no cap 🔥

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🌸 ARGUMENTS 🌸

• dst — the destination address, and it lives in kernel space. this buffer must be at least count bytes long, like that's not a suggestion, that's a requirement, periodt 👑
• unsafe_addr — the unsafe user address. yes, it's literally named unsafe. the name is doing a lot of heavy lifting here and we respect it 🙈
• count — maximum number of bytes to copy, including the trailing NUL. count her in, don't forget the NUL bestie 💅

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🦋 DESCRIPTION 🦋

so what does she actually do? she copies a NUL-terminated string from an unsafe user address into a kernel buffer. that's the whole gig ✨

on success, she returns the length of the string INCLUDING the trailing NUL. so if you were expecting the length without it… no ma'am, she counts the NUL too 🎀

if the access fails, she returns -EFAULT. and listen — some data may have already been copied, and the trailing NUL added. so like, it's not a clean slate situation, okay? just so you know 😭

and if count is smaller than the length of the string? she copies count-1 bytes, sets the last byte of the dst buffer to NUL, and returns count. truncation slay, but respectfully 💖

⚠️ reminder that this is the nofault version and the address is unsafe — the manpage says the buffer must be at least count bytes long, so please, for the love of everything, size your buffer correctly 🫶

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🌈 SEE ALSO 🌈

• kernel file ./mm/maccess.c
• copy_from_user_nofault(9)
• copy_to_user_nofault(9)
• strnlen_user_nofault(9)

go read those too, iykyk 👑

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and that's the whole page, bestie 💜 this iconic little number comes to you from ./mm, dated October 2026 — main character energy from the kernel source to your terminal, mwah 🧚⭐