KEYCTL_UPDATE(2const)

✨ KEYCTL_UPDATE 💅

okay bestie, so this whole page is from the Linux man-pages set, and it's a 2const page — that means it's not its own syscall, it's one of the constants you shove into the keyctl syscall to tell it what to do. This one is KEYCTL_UPDATE, and fr fr it's giving "change the contents of a key". Let's get into it. 🎀

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🌸 NAME 🌸

KEYCTL_UPDATE — update a key's data payload

Like, you've got a key, it's got some data in it, and you want to swap that data out. That's the whole vibe. 🦋

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📚 LIBRARY 📚

Standard C library (libc, -lc)

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🌈 SYNOPSIS 🌈

#include <linux/keyctl.h>  /* Definition of KEY* constants */
#include <sys/syscall.h>   /* Definition of SYS_* constants */
#include <unistd.h>

long syscall(size_t size;
             SYS_keyctl, KEYCTL_UPDATE, key_serial_t key,
             void payload[size], size_t size);

no cap, that's exactly how you call it. SYS_keyctl + KEYCTL_UPDATE + your args, and the libc syscall() wrapper does the rest. ✨

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💖 DESCRIPTION 💖

Update a key's data payload. Simple as that. 🫶

• key — the ID of the key you're updating. This is your target, your main character.
• payload — points to the new payload you want to put in there.
• size — the new payload size, in bytes.

The caller must have write permission on the key specified, okay? Like you can't just go editing other people's keys. Rude. 🙈 And the key type must support updating — some types literally do not, and they will not be taking notes.

Also super important cutie: a negatively instantiated key (aka the sad rejected kind, see KEYCTL_REJECT(2const)) can be positively instantiated with this operation. Glow-up arc, honestly. 💅

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🎀 RETURN VALUE 🎀

• On success, 0 is returned. 🥳
• On error, -1 is returned, and errno is set to indicate the error. 😭

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💀 ERRORS 💀

• The key type does not support updating.

That's it. That's the list. If your key type is one of the no-update-allowed ones, this is the wall you hit. 🧱

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⭐ VERSIONS ⭐

A wrapper is provided in the libkeyutils library: keyctl_update(3). So like, if you don't wanna rawdog the syscall, use that. 👑

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🌸 STANDARDS 🌸

Linux.

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🦋 HISTORY 🦋

Linux 2.6.10.

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👑 SEE ALSO 👑

• keyctl(2)
• keyctl_update(3)

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okay that's the whole page bestie, go forth and update those payloads responsibly. And just so you know where all this lore came from: it's Linux man-pages 6.20, dated 2026-02-08, and the page title is KEYCTL_UPDATE(2const). slay 💜✨