Re: [patch 05/12] rseq: Add prctl() to enable time slice extensions

Mathieu Desnoyers
2025-09-11T15:50:06Z

On 2025-09-08 18:59, Thomas Gleixner wrote:
> Implement a prctl() so that tasks can enable the time slice extension
> mechanism. This fails, when time slice extensions are disabled at compile
> time or on the kernel command line and when no rseq pointer is registered
> in the kernel.
>
> That allows to implement a single trivial check in the exit to user mode
> hotpath, to decide whether the whole mechanism needs to be invoked.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> ---
> include/linux/rseq.h | 9 +++++++
> include/uapi/linux/prctl.h | 10 ++++++++
> kernel/rseq.c | 52 +++++++++++++++++++++++++++++++++++++++++++++
> kernel/sys.c | 6 +++++
> 4 files changed, 77 insertions(+)
>
> --- a/include/linux/rseq.h
> +++ b/include/linux/rseq.h
> @@ -190,4 +190,13 @@ void rseq_syscall(struct pt_regs *regs);
> static inline void rseq_syscall(struct pt_regs *regs) { }
> #endif /* !CONFIG_DEBUG_RSEQ */
>
> +#ifdef CONFIG_RSEQ_SLICE_EXTENSION
> +int rseq_slice_extension_prctl(unsigned long arg2, unsigned long arg3);
> +#else /* CONFIG_RSEQ_SLICE_EXTENSION */
> +static inline int rseq_slice_extension_prctl(unsigned long arg2, unsigned long arg3)
> +{
> + return -EINVAL;
> +}
> +#endif /* !CONFIG_RSEQ_SLICE_EXTENSION */
> +
> #endif /* _LINUX_RSEQ_H */
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -376,4 +376,14 @@ struct prctl_mm_map {
> # define PR_FUTEX_HASH_SET_SLOTS 1
> # define PR_FUTEX_HASH_GET_SLOTS 2
>
> +/* RSEQ time slice extensions */
> +#define PR_RSEQ_SLICE_EXTENSION 79
> +# define PR_RSEQ_SLICE_EXTENSION_GET 1
> +# define PR_RSEQ_SLICE_EXTENSION_SET 2
> +/*
> + * Bits for RSEQ_SLICE_EXTENSION_GET/SET
> + * PR_RSEQ_SLICE_EXT_ENABLE: Enable
> + */
> +# define PR_RSEQ_SLICE_EXT_ENABLE 0x01
> +
> #endif /* _LINUX_PRCTL_H */
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -71,6 +71,7 @@
> #define RSEQ_BUILD_SLOW_PATH
>
> #include <linux/debugfs.h>
> +#include <linux/prctl.h>
> #include <linux/ratelimit.h>
> #include <linux/rseq_entry.h>
> #include <linux/sched.h>
> @@ -490,6 +491,57 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
> #ifdef CONFIG_RSEQ_SLICE_EXTENSION
> DEFINE_STATIC_KEY_TRUE(rseq_slice_extension_key);
>
> +int rseq_slice_extension_prctl(unsigned long arg2, unsigned long arg3)
> +{
> + switch (arg2) {
> + case PR_RSEQ_SLICE_EXTENSION_GET:
> + if (arg3)
> + return -EINVAL;
> + return current->rseq.slice.state.enabled ? PR_RSEQ_SLICE_EXT_ENABLE : 0;
> +
> + case PR_RSEQ_SLICE_EXTENSION_SET: {
> + u32 rflags, valid = RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
> + bool enable = !!(arg3 & PR_RSEQ_SLICE_EXT_ENABLE);
> +
> + if (arg3 & ~PR_RSEQ_SLICE_EXT_ENABLE)
> + return -EINVAL;
> + if (!rseq_slice_extension_enabled())
> + return -ENOTSUPP;
> + if (!current->rseq.usrptr)
> + return -ENXIO;
> +
> + /* No change? */
> + if (enable == !!current->rseq.slice.state.enabled)
> + return 0;
> +
> + if (get_user(rflags, &current->rseq.usrptr->flags))
> + goto die;
> +
> + if (current->rseq.slice.state.enabled)
> + valid |= RSEQ_CS_FLAG_SLICE_EXT_ENABLED;
> +
> + if ((rflags & valid) != valid)
> + goto die;
> +
> + rflags &= ~RSEQ_CS_FLAG_SLICE_EXT_ENABLED;
> + rflags |= RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
> + if (enable)
> + rflags |= RSEQ_CS_FLAG_SLICE_EXT_ENABLED;
> +
> + if (put_user(rflags, &current->rseq.usrptr->flags))
> + goto die;
> +
> + current->rseq.slice.state.enabled = enable;

What should happen to this enabled state if rseq is unregistered
after this prctl ?

Thanks,

Mathieu

> + return 0;
> + }
> + default:
> + return -EINVAL;
> + }
> +die:
> + force_sig(SIGSEGV);
> + return -EFAULT;
> +}
> +
> static int __init rseq_slice_cmdline(char *str)
> {
> bool on;
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -53,6 +53,7 @@
> #include <linux/time_namespace.h>
> #include <linux/binfmts.h>
> #include <linux/futex.h>
> +#include <linux/rseq.h>
>
> #include <linux/sched.h>
> #include <linux/sched/autogroup.h>
> @@ -2805,6 +2806,11 @@ SYSCALL_DEFINE5(prctl, int, option, unsi
> case PR_FUTEX_HASH:
> error = futex_hash_prctl(arg2, arg3, arg4);
> break;
> + case PR_RSEQ_SLICE_EXTENSION:
> + if (arg4 || arg5)
> + return -EINVAL;
> + error = rseq_slice_extension_prctl(arg2, arg3);
> + break;
> default:
> trace_task_prctl_unknown(option, arg2, arg3, arg4, arg5);
> error = -EINVAL;
>

--
Mathieu Desnoyers
EfficiOS Inc.