[REGRESSION] rseq: refactoring in v6.19 broke everyone on arm64 and tcmalloc everywhere

[Dmitry Vyukov]

On Thu, 23 Apr 2026 at 03:48, Jinjie Ruan <ruanjinjie at huawei.com> wrote:
>
> On 4/23/2026 3:47 AM, Thomas Gleixner wrote:
> > On Wed, Apr 22 2026 at 19:11, Mark Rutland wrote:
> >> On Wed, Apr 22, 2026 at 07:49:30PM +0200, Thomas Gleixner wrote:
> >> Conceptually we just need to use syscall_enter_from_user_mode() and
> >> irqentry_enter_from_user_mode() appropriately.
> >
> > Right. I figured that out.
> >
> >> In practice, I can't use those as-is without introducing the exception
> >> masking problems I just fixed up for irqentry_enter_from_kernel_mode(),
> >> so I'll need to do some similar refactoring first.
> >
> > See below.
> >
> >> I haven't paged everything in yet, so just to cehck, is there anything
> >> that would behave incorrectly if current->rseq.event.user_irq were set
> >> for syscall entry? IIUC it means we'll effectively do the slow path, and
> >> I was wondering if that might be acceptable as a one-line bodge for
> >> stable.
> >
> > It might work, but it's trivial enough to avoid that. See below. That on
> > top of 6.19.y makes the selftests pass too.
>
> This aligns with my thoughts when convert arm64 to generic syscall
> entry. Currently, the arm64 entry code does not distinguish between IRQ
> and syscall entries. It fails to call rseq_note_user_irq_entry() for IRQ
> entries as the generic entry framework does, because arm64 uses
> enter_from_user_mode() exclusively instead of
> irqentry_enter_from_user_mode().
>
> https://lore.kernel.org/all/20260320102620.1336796-10-ruanjinjie@huawei.com/
>
> >
> > Thanks,
> >
> >         tglx
> > ---
> >  arch/arm64/kernel/entry-common.c |   14 ++++++++++----
> >  1 file changed, 10 insertions(+), 4 deletions(-)
> >
> > --- a/arch/arm64/kernel/entry-common.c
> > +++ b/arch/arm64/kernel/entry-common.c
> > @@ -58,6 +58,12 @@ static void noinstr exit_to_kernel_mode(
> >       irqentry_exit(regs, state);
> >  }
> >
> > +static __always_inline void arm64_enter_from_user_mode_syscall(struct pt_regs *regs)
> > +{
> > +     enter_from_user_mode(regs);
> > +     mte_disable_tco_entry(current);
> > +}
> > +
> >  /*
> >   * Handle IRQ/context state management when entering from user mode.
> >   * Before this function is called it is not safe to call regular kernel code,
> > @@ -65,8 +71,8 @@ static void noinstr exit_to_kernel_mode(
> >   */
> >  static __always_inline void arm64_enter_from_user_mode(struct pt_regs *regs)
> >  {
> > -     enter_from_user_mode(regs);
> > -     mte_disable_tco_entry(current);
> > +     arm64_enter_from_user_mode_syscall(regs);
> > +     rseq_note_user_irq_entry();
> >  }
> >
> >  /*
> > @@ -717,7 +723,7 @@ static void noinstr el0_brk64(struct pt_
> >
> >  static void noinstr el0_svc(struct pt_regs *regs)
> >  {
> > -     arm64_enter_from_user_mode(regs);
> > +     arm64_enter_from_user_mode_syscall(regs);
> >       cortex_a76_erratum_1463225_svc_handler();
> >       fpsimd_syscall_enter();
> >       local_daif_restore(DAIF_PROCCTX);
> > @@ -869,7 +875,7 @@ static void noinstr el0_cp15(struct pt_r
> >
> >  static void noinstr el0_svc_compat(struct pt_regs *regs)
> >  {
> > -     arm64_enter_from_user_mode(regs);
> > +     arm64_enter_from_user_mode_syscall(regs);
> >       cortex_a76_erratum_1463225_svc_handler();
> >       local_daif_restore(DAIF_PROCCTX);
> >       do_el0_svc_compat(regs);


+linux-man

This part of the rseq man page needs to be fixed as well I think. The
kernel no longer reliably provides clearing of rseq_cs on preemption,
right?

https://git.kernel.org/pub/scm/libs/librseq/librseq.git/tree/doc/man/rseq.2#n241

"and set to NULL by the kernel when it restarts an assembly
instruction sequence block,
as well as when the kernel detects that it is preempting or delivering
a signal outside of the range targeted by the rseq_cs."