PR_GET_SPECULATION_CTRL(2const)

💅 PR_GET_SPECULATION_CTRL (2const) ✨

okay bestie, welcome to the girls' guide to asking your CPU "hey queen, what's the vibe with your speculation mitigations?" — it's giving very niche, very kernel-coded, but honestly? iconic.

Manual name: PR_GET_SPECULATION_CTRL — a Linux prctl() constant. yes, 2const means it lives in the constants corner of section 2. iykyk. 🎀

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🌸 NAME 🌸

PR_GET_SPECULATION_CTRL — get the state of a speculation misfeature for the calling thread

like, for this specific thread, not your whole process. main character energy but make it per-thread. 💖

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📚 LIBRARY 📚

Standard C library — link it with -lc

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✨ SYNOPSIS ✨

#include <linux/prctl.h>  /* Definition of PR_* constants */
#include <sys/prctl.h>

int prctl(PR_GET_SPECULATION_CTRL, long misfeature, 0L, 0L, 0L);

not me remembering the 0L, 0L, 0L every single time 😭 it's literally always zeros here, bestie.

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🦋 DESCRIPTION 🦋

So basically, this returns the state of the speculation misfeature you named in misfeature.

Currently, misfeature has to be one of these two divas:

• PR_SPEC_STORE_BYPASS — get the state of the speculative store bypass misfeature. the classic. 💅
• PR_SPEC_INDIRECT_BRANCH (since Linux 4.20) — get the state of the indirect branch speculation misfeature. younger sister, joined in 4.20. 🌸

Now the return value? it's literally a bitfield using bits 0-4, and each bit is telling you a whole story:

• PR_SPEC_PRCTL — the mitigation can be controlled per thread by PR_SET_SPECULATION_CTRL(2const). you have agency, bestie. 👑
• PR_SPEC_ENABLE — the speculation feature is enabled, mitigation is disabled. it's giving unbothered. 😌
• PR_SPEC_DISABLE — the speculation feature is disabled, mitigation is enabled. locked down, protected, safe. 🛡️
• PR_SPEC_FORCE_DISABLE — same as PR_SPEC_DISABLE but it cannot be undone. no take-backs. periodt. 💀
• PR_SPEC_DISABLE_NOEXEC (since Linux 5.1) — same as PR_SPEC_DISABLE, but the state gets cleared on execve(2). so it resets itself when you exec. clean. ✨

If all bits are 0, then the CPU is not affected by the speculation misfeature. your CPU said "not my problem" and honestly we respect it. 🙈

And listen, this part matters: if PR_SPEC_PRCTL is set, then per-thread control of the mitigation is available. if it is not set, then PR_SET_SPECULATION_CTRL(2const) for the speculation misfeature will fail. so like, vibe check the PR_SPEC_PRCTL bit before you go trying to set anything, fr fr. 🫶

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⭐ RETURN VALUE ⭐

On success, PR_GET_SPECULATION_CTRL returns the nonnegative value described above (the bitfield, bestie).

On error, -1 is returned, and errno is set to indicate the error. rude but standard. 😭

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🔥 ERRORS 🔥

• ENODEV — the kernel or CPU does not support the requested speculation misfeature. not every CPU is built for this life, and that's fine. 💜

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🌈 STANDARDS 🌈

Linux.

just Linux. no POSIX girly here, this is a kernel-only situation. 🐧

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👑 HISTORY 👑

Linux 4.17.

ancient in internet years (which is like, a decade-ish of discourse). 🎀

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🎯 SEE ALSO 🎯

• prctl(2)
• PR_SET_SPECULATION_CTRL(2const)

the "set" version is the sister page — go read her too, they're a matching set. 💕

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mwah, that's the whole page bestie 💖 go forth and query your bits responsibly.

sending you off with love — this was Linux man-pages 6.20, dated 2026-02-08. slay, stay speculative, and always check errno 🫶✨