PR_SET_SPECULATION_CTRL(2const)

✨ PR_SET_SPECULATION_CTRL ✨

okay bestie, so this one is PR_SET_SPECULATION_CTRL from the (2const) girlies — it lives in the land of prctl(2) constants, and it's literally the "set the state of a speculation misfeature for the calling thread" page. yes, per-thread. yes, that matters. 💅

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🌸 LIBRARY 🌸

Standard C library (libc, -lc). the classic. the icon. 🎀

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🦋 SYNOPSIS 🦋

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

int prctl(PR_SET_SPECULATION_CTRL, long misfeature, long val, 0L, 0L);

note how those last two args are just 0L and 0L, they're not doing anything, they're just there for the vibe. okay not for the vibe, for the API. whatever. ✨

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💖 DESCRIPTION 💖

So what it does: it sets the state of the speculation misfeature that you name in misfeature. And the important lil detail — these speculation-misfeature settings are per-thread attributes. not global. not per-process. per. thread. okay? periodt. 👑

Right now, misfeature has to be one of these two besties:

• PR_SPEC_STORE_BYPASS — sets the state of the speculative store bypass misfeature.
• PR_SPEC_INDIRECT_BRANCH (since Linux 4.20) — sets the state of the indirect branch speculation misfeature.

Then there's the val argument, which is where you hand in the actual control value. and val is one of these:

• PR_SPEC_ENABLE — the speculation feature is enabled, mitigation is disabled. (living dangerously, we love a bold choice 🫶)
• PR_SPEC_DISABLE — the speculation feature is disabled, mitigation is enabled. (safe queen behavior 💅)
• PR_SPEC_FORCE_DISABLE — same as PR_SPEC_DISABLE, but it cannot be undone. no takebacks. no regrets. it's giving irreversible. 💀
• PR_SPEC_DISABLE_NOEXEC (since Linux 5.1) — same as PR_SPEC_DISABLE, but the state will be cleared on execve(2). And heads up — this one is currently only supported for PR_SPEC_STORE_BYPASS.

Alsooo, the speculation feature can also be controlled by the spec_store_bypass_disable boot parameter. And plot twist: this parameter may enforce a read-only policy, which will result in the prctl() call failing with the error ENXIO. For further details, see the kernel source file Documentation/admin-guide/kernel-parameters.rst.

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🌈 RETURN VALUE 🌈

Slay: on success, 0 is returned. On error, -1 is returned, and errno is set to indicate the error. that's the whole story bestie. 😌

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💜 ERRORS 💜

• ENODEV — the kernel or CPU does not support the requested speculation misfeature. (they simply don't have it in them 🙈)
• ENXIO — the control of the selected speculation misfeature is not possible. See PR_GET_SPECULATION_CTRL for the bit fields to determine which option is available. 👀
• EPERM — the speculation was disabled with PR_SPEC_FORCE_DISABLE and the caller tried to enable it again. yeah… that's the irreversible thing from earlier. told you. 💀
• ERANGE — misfeature is not a valid value.

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⭐ STANDARDS ⭐

Linux. that's it, that's the standard. 🐧

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🎀 HISTORY 🎀

Linux 4.17.

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🧚 SEE ALSO 🧚

• prctl(2)
• PR_GET_SPECULATION_CTRL(2const)

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okay that's the whole page bestie, you ate that 🫶 stay speculative, stay per-thread, and remember: PR_SPEC_FORCE_DISABLE is forever. mwah. 💖

this has been a Linux man-pages 6.20 moment, dated 2026-02-08, brought to you by the PR_SET_SPECULATION_CTRL(2const) girlies. no cap. ✨