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

From: Thomas Gleixner <[email-masked]>
Date: Wed, 22 Apr 2026 21:47:00 +0200

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.

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);