UFFDIO_WRITEPROTECT(2const)

💅 UFFDIO_WRITEPROTECT (that's a section 2const page, bestie, from the Linux man-pages crew)

okay so picture this: you've got memory registered with userfaultfd, and you wanna toggle the write permission on it like a little light switch. That's literally this ioctl. Sit down, grab a strawberry 🍓, and let's get into it.

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✨ NAME ✨

UFFDIO_WRITEPROTECT — write-protect or write-unprotect a userfaultfd-registered memory range

no cap, that's the whole job. protect it, unprotect it, repeat. 💫

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🎀 LIBRARY 🎀

Standard C library (libc, -lc)

yes babe, you link it the normal way, nothing fancy here.

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🦋 SYNOPSIS 🦋

#include <linux/userfaultfd.h>  /* Definition of UFFD* constants */
#include <sys/ioctl.h>

int ioctl(int fd, UFFDIO_WRITEPROTECT, ...);

#include <linux/userfaultfd.h>

struct uffdio_writeprotect {
    struct uffdio_range  range;  /* Range to change write permission*/
    __u64                mode;   /* Mode to change write permission */
};

look at that struct. she's little but she's doing a LOT. 👑

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🌸 DESCRIPTION 🌸

So the vibe is: this ioctl write-protects or write-unprotects a memory range that you already registered with userfaultfd using mode UFFDIO_REGISTER_MODE_WP. It's giving "I decide who gets to write here" energy.

There are two mode bits supported in this structure:

• UFFDIO_WRITEPROTECT_MODE_WP — when this bit is set, the ioctl is a write-protect operation on the memory range you specified in range. When it's not set? It's a write-unprotect operation on that range instead — which is the move you pull to resolve a userfaultfd write-protect page fault. ✨
• UFFDIO_WRITEPROTECT_MODE_DONTWAKE — when this bit is set, do not wake up any thread that's waiting around for page-fault resolution after the operation. 🛑 Heads up though: you can only specify this one if UFFDIO_WRITEPROTECT_MODE_WP is not specified. That's the rule, bestie, no negotiating.

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💖 RETURN VALUE 💖

• Success? 0 comes back. slay. ✨
• Error? -1 comes back and errno gets set to tell you what went wrong. 💀

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😭 ERRORS 😭

• EINVAL — the start or the len field of the ufdio_range structure wasn't a multiple of the system page size; or len was zero; or the specified range was otherwise invalid. like, invalid invalid. 🚫
• EAGAIN — the process was interrupted; just retry this call. she's just a little sleepy, try again. 😴
• ENOENT — the range specified in range is not valid. For example, the virtual address does not exist, or it's not registered with userfaultfd write-protect mode. ghosted, literally. 👻
• EFAULT — encountered a generic fault during processing. generic fault, generic heartbreak. 🥲

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👑 STANDARDS 👑

Linux. that's it, that's the standard. 🇱🇮🇳🇺🇽

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⭐ HISTORY ⭐

Linux 5.7. baby era for this one, and she's been iconic ever since. 🌈

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🧚 EXAMPLES 🧚

See userfaultfd(2). go read the big one, i believe in you 🫶

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🔥 SEE ALSO 🔥

• ioctl(2)
• ioctl_userfaultfd(2)
• userfaultfd(2)
• Documentation/admin-guide/mm/userfaultfd.rst in the Linux kernel source tree

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fr fr, this is a section 2const page, so if you were looking for the full story, the userfaultfd(2) page is your main character. 💅

okay that's the whole page, mwah. this was Linux man-pages 6.20, dated 2026-02-08 — and yes the UFFDIO_WRITEPROTECT(2const) header is right there at the top, iykyk. bye bestie, go protect some memory 🦋✨💜