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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

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.
https://www.efficios.com