[PATCH v3 0/5] arm64: vdso: Implement __vdso_futex_robust_try_unlock()

[André Almeida]

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/

This patchset depends on this series:

   https://lore.kernel.org/lkml/20260402151131.876492985@xxxxxxxxxx/

The only part that I didn't figure out about this implementation is
something that I have removed from patch 01:

        /*
         * 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;

On arm64 _it seems_ that user_irq is never true. I had to remove this if
to make the code reach __futex_fixup_robust_unlock() and make the test work.

On x86, this is the callstack that sets user_irq to true:

	rseq_note_user_irq_entry
	irqentry_enter_from_user_mode
	exc_debug_user
	noist_exc_debug
	asm_exc_debug

So I'm not sure if there's something missing on arm64 and it should also hit 
irqentry_enter_from_user_mode(), or if we should look into other flag to check
if the task is returing from interrupt.

* Testing

This patchset works fine with the tests proposed at
https://lore.kernel.org/lkml/20260330120118.012924430@xxxxxxxxxx/

In the series there's also a patch to adapt Sebastian's test to arm64, and it
worked:
https://lore.kernel.org/lkml/20260404093939.7XgeW_54@xxxxxxxxxxxxx/

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

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@xxxxxxxxxx

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@xxxxxxxxxx

---
André Almeida (5):
      arm64: vdso: Prepare for robust futex unlock support
      arm64: vdso: Implement __vdso_futex_robust_try_unlock()
      selftests: futex: Add support for aarch64 in robust_list_critical
      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              | 20 +++++
 arch/arm64/include/asm/vdso.h                      |  3 +
 arch/arm64/kernel/vdso.c                           | 34 ++++++++
 arch/arm64/kernel/vdso/Makefile                    |  9 ++-
 arch/arm64/kernel/vdso/vdso.lds.S                  |  5 ++
 arch/arm64/kernel/vdso/vfutex.c                    | 34 ++++++++
 arch/arm64/kernel/vdso32/Makefile                  | 10 ++-
 arch/arm64/kernel/vdso32/vdso.lds.S                |  5 ++
 arch/arm64/kernel/vdso32/vfutex.c                  | 33 ++++++++
 arch/x86/entry/vdso/vma.c                          |  4 +-
 include/linux/futex.h                              | 13 +--
 include/vdso/futex.h                               |  1 +
 .../futex/functional/robust_list_critical.c        | 92 ++++++++++++++++++++--
 15 files changed, 245 insertions(+), 21 deletions(-)
---
base-commit: 78333d5371a5fe6f3eaec5e6e4c16d22fe386cbe
change-id: 20260416-tonyk-robust_arm-54ff77d2c4e4

Best regards,
--  
André Almeida <andrealmeid@xxxxxxxxxx>