compat_ptr_ioctl(9)

💅 compat_ptr_ioctl(9) — like, the Kernel API Manual, but make it cute ✨

okay so bestie, welcome to the compat_ptr_ioctl page from the Kernel API Manual 🎀 this one's short but she's important, so grab your little iced coffee and let's get into it 🍓

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🌸 NAME 🌸

compat_ptr_ioctl — the generic implementation of the .compat_ioctl file operation. that's it, that's the whole vibe ✨

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🦋 SYNOPSIS 🦋

long compat_ptr_ioctl (struct file *file , unsigned int cmd , unsigned long arg );

like, here's her whole signature, no cap 👑

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💖 ARGUMENTS 💖

• file — the file you're operating on. the main character of this whole story, literally 🫶
• cmd — the ioctl command number. yes, the number. keep track of it bestie 🔢
• arg — the argument to the ioctl. whatever you're passing in, this is it 💅

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🔥 DESCRIPTION 🔥

okay so hear me out, this one is not normally called as a function. it's not that kind of girl. instead, you set it in struct file_operations like this:

.compat_ioctl = compat_ptr_ioctl,

iconic, right? one line and you're done 💫

On most architectures, compat_ptr_ioctl literally just passes all the arguments straight through to the corresponding ->ioctl handler. no drama, no extra steps, just vibes 🌈

The exception — and yes there's always an exception — is arch/s390, where compat_ptr clears the top bit of a 32-bit pointer value. so user space pointers to the second 2GB end up aliasing the first 2GB, exactly like it works for native 32-bit s390 user space. that's just how s390 does it, we don't make the rules 🤷‍♀️

Because of all that, compat_ptr_ioctl must therefore be used only with ioctl functions that either ignore the argument entirely, or pass a pointer to a compatible data type. don't be out here using it for anything else, please, i'm begging 🙈

and now the serious part, bestie, pay attention 💀 if any ioctl command handled by fops->unlocked_ioctl passes a plain integer instead of a pointer, or if any of the passed data types is incompatible between 32-bit and 64-bit architectures, then a proper handler is required instead of compat_ptr_ioctl. like, an actual real one. not this shortcut. periodt ✋

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🧚 SEE ALSO 🧚

• Kernel file ./fs/ioctl.c — vfs_ioctl(9)
• fiemap_fill_next_extent(9)
• fiemap_prep(9)

go read those too, iykyk 📚✨

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aaaand that's the page!! 🎀

cute little sign-off for you: this one lives in ./fs, dated October 2026 — so it's giving future-file energy, and honestly? obsessed 💜🦋

bye bestie, go forth and handle your ioctls responsibly 💅💖