[PATCH v4 0/5] arm64: vdso: Implement __vdso_futex_robust_try_unlock()

["André Almeida" <andrealmeid@igalia.com>]

Hi folks,

This is my take on implementing the new vDSO for unlocking a robust futex in
arm64. If you don't know what's that, Thomas wrote a good summary,
including the motivation for this work and the x86 implementation:

   https://lore.kernel.org/lkml/878qb89g7b.ffs@tglx/

* Testing

There's one selftest proposed [1] that tests precisely if the task is
interrupted during the critical section, if the kernel will clear op_pending
pointer. I've adapted to arm64 [2] and it works as expected. This test is not
being upstreamed right now because it depends on a better way to expose
vdso.so.dbg [3].

I also used gdb to manually check if the address is cleared when the kernel
interrupts the critical section.

* The patchset

As explained in the Testing section above, we developed a test that puts
breakpoints in the code to test if a user code being interrupted really clears
the op_pending pointer. This test wasn't initially working because of this check
at futex_fixup_robust_unlock():

        /*
         * Avoid dereferencing current->mm if not returning from interrupt.
         * current->rseq.event is going to be used subsequently, so bringing the
         * cache line in is not a big deal.
         */
        if (!current->rseq.event.user_irq)
		return;

rseq.event.user_irq was always false during my tests, and it prevents the fixup
to happen. I figured out that arm64_syscall_enter_from_user_mode() was the
issue because it doesn't set user_irq to true when the task comes from a
syscall. I dropped arm64_syscall_enter_from_user_mode() and replaced with
arm64_enter_from_user_mode() and the test now works fine. I honestly don't know
if this solution is the correct one here, so I would like to hear from the arm64
folks what's the best approach here.

Thanks!
	André

[1] https://lore.kernel.org/lkml/20260404093939.7XgeW_54@linutronix.de/
[2] https://lore.kernel.org/lkml/20260529-tonyk-robust_arm-v3-3-a6f02684d4fe@igalia.com/
[3] https://lore.kernel.org/lkml/20260602090536.045586688@kernel.org/

Changes in v4:
 - Added commit "arm64/entry: Unify user mode handling"
 - Added missing ifdef FUTEX_ROBUST_UNLOCK guards
 - Fixed the position of _start and _success labels in the critical section
 - Instead of checking the zero flag, check the result register to decide if the
 op_pending needs to be cleared
v3: https://patch.msgid.link/20260529-tonyk-robust_arm-v3-0-a6f02684d4fe@igalia.com

Changes in v3:
 - Change asm to always use x2 to store *pop
 - Fix clang asm errors
 - Moved 32 bit entry point to vdso32/ and use littlearm asm
 - Adapted Sebastians test for arm
v2: https://patch.msgid.link/20260424-tonyk-robust_arm-v2-0-db4e46f752cf@igalia.com

Changes in v2:
 - s/CONFIG_COMPAT/CONFIG_COMPAT_VDSO (Thomas Weißschuh)
 - Fixed linker not finding the symbols (Thomas Weißschuh)
v1: https://patch.msgid.link/20260417-tonyk-robust_arm-v1-0-03aa64e2ff1a@igalia.com

---
André Almeida (5):
      arm64/entry: Unify user mode handling
      arm64: vdso: Prepare for robust futex unlock support
      arm64: vdso: Implement __vdso_futex_robust_try_unlock()
      arm64: vdso32: Bring vdso32-offsets.h back
      arm64: vdso32: Implement __vdso_futex_robust_try_unlock()

 arch/arm64/Kconfig                    |  1 +
 arch/arm64/Makefile                   |  2 +-
 arch/arm64/include/asm/futex_robust.h | 19 +++++++++++++++++++
 arch/arm64/include/asm/vdso.h         |  3 +++
 arch/arm64/kernel/entry-common.c      | 11 ++---------
 arch/arm64/kernel/vdso.c              | 32 ++++++++++++++++++++++++++++++++
 arch/arm64/kernel/vdso/Makefile       | 10 ++++++++++
 arch/arm64/kernel/vdso/vdso.lds.S     |  9 +++++++++
 arch/arm64/kernel/vdso/vfutex.c       | 35 +++++++++++++++++++++++++++++++++++
 arch/arm64/kernel/vdso32/Makefile     | 12 ++++++++++++
 arch/arm64/kernel/vdso32/vdso.lds.S   |  9 +++++++++
 arch/arm64/kernel/vdso32/vfutex.c     | 33 +++++++++++++++++++++++++++++++++
 12 files changed, 166 insertions(+), 10 deletions(-)
---
base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
change-id: 20260416-tonyk-robust_arm-54ff77d2c4e4

Best regards,
--  
André Almeida <andrealmeid@igalia.com>