Re: [PATCH v3 1/5] arm64: vdso: Prepare for robust futex unlock support

["Thomas Weißschuh" <linux@weissschuh.net>]

On 2026-05-29 13:33:53-0300, André Almeida wrote:
> There will be a VDSO function to unlock non-contended robust futexes in
> user space. The unlock sequence is racy vs. clearing the list_pending_op
> pointer in the task's robust list head. To plug this race the kernel needs
> to know the critical section window so it can clear the pointer when the
> task is interrupted within that race window. The window is determined by
> labels in the inline assembly.
> 
> Signed-off-by: André Almeida <andrealmeid@igalia.com>
> ---
> Notes:
>  - The diff futex_set_vdso_cs_range() should happen in the commit that
>  introduced it, and rebase will clear it from here
>  - So far I couldn't figure out why current->rseq.event.user_irq is never set in
>  aarch64

Why not put these unrelated changes into their own commits?
It makes reviewing and integrating it into the original series easier.

> v3:
>  - Fix adding vdso base addr twice
>  - Call vdso_futex_robust_unlock_update_ips() on remap as well
> v2:
>  - Fixed linker not finding VDSO symbols
> ---
>  arch/arm64/kernel/vdso.c          | 25 +++++++++++++++++++++++++
>  arch/arm64/kernel/vdso/vdso.lds.S |  5 +++++
>  arch/x86/entry/vdso/vma.c         |  4 ++--
>  include/linux/futex.h             | 13 ++-----------
>  4 files changed, 34 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
> index 592dd8668de4..76f22ea8e181 100644
> --- a/arch/arm64/kernel/vdso.c
> +++ b/arch/arm64/kernel/vdso.c
> @@ -11,6 +11,7 @@
>  #include <linux/clocksource.h>
>  #include <linux/elf.h>
>  #include <linux/err.h>
> +#include <linux/futex.h>
>  #include <linux/errno.h>
>  #include <linux/gfp.h>
>  #include <linux/kernel.h>
> @@ -57,11 +58,31 @@ static struct vdso_abi_info vdso_info[] __ro_after_init = {
>  #endif /* CONFIG_COMPAT_VDSO */
>  };
>  
> +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK
> +static void vdso_futex_robust_unlock_update_ips(enum vdso_abi abi, struct mm_struct *mm)
> +{
> +	unsigned long vdso = (unsigned long) mm->context.vdso;
> +	struct futex_mm_data *fd = &mm->futex;
> +	uintptr_t success, end;
> +
> +	if (abi == VDSO_ABI_AA64) {
> +		success = (uintptr_t) VDSO_SYMBOL(vdso, futex_list64_try_unlock_cs_success);
> +		end = (uintptr_t) VDSO_SYMBOL(vdso, futex_list64_try_unlock_cs_end);
> +
> +		futex_set_vdso_cs_range(fd, 0, success, end, false);
> +	}
> +}
> +#else
> +static inline void vdso_futex_robust_unlock_update_ips(enum vdso_abi abi, struct mm_struct *mm) { }
> +#endif /* CONFIG_FUTEX_ROBUST_UNLOCK */
> +
>  static int vdso_mremap(const struct vm_special_mapping *sm,
>  		struct vm_area_struct *new_vma)
>  {
>  	current->mm->context.vdso = (void *)new_vma->vm_start;
>  
> +	vdso_futex_robust_unlock_update_ips(VDSO_ABI_AA64, current->mm);
> +
>  	return 0;
>  }
>  
> @@ -134,6 +155,8 @@ static int __setup_additional_pages(enum vdso_abi abi,
>  	if (IS_ERR(ret))
>  		goto up_fail;
>  
> +	vdso_futex_robust_unlock_update_ips(abi, mm);
> +
>  	return 0;
>  
>  up_fail:
> @@ -159,6 +182,8 @@ static int aarch32_sigpage_mremap(const struct vm_special_mapping *sm,
>  {
>  	current->mm->context.sigpage = (void *)new_vma->vm_start;
>  
> +	vdso_futex_robust_unlock_update_ips(VDSO_ABI_AA32, current->mm);

This is for the sigpage remap, not the vDSO, is it really necessary?
If yes it should be part of the later VDSO_ABI_AA32 patch IMO.


If there is a way for a 64-bit application to call 32-bit syscalls then 
the 64-bit vDSO also needs the 32-bit functions. See:
[0] https://lore.kernel.org/lkml/875x4zw4bp.ffs@tglx/

>  	return 0;
>  }
>  
> diff --git a/arch/arm64/kernel/vdso/vdso.lds.S b/arch/arm64/kernel/vdso/vdso.lds.S
> index 52314be29191..8633aafe6b81 100644
> --- a/arch/arm64/kernel/vdso/vdso.lds.S
> +++ b/arch/arm64/kernel/vdso/vdso.lds.S
> @@ -104,6 +104,7 @@ VERSION
>  		__kernel_clock_gettime;
>  		__kernel_clock_getres;
>  		__kernel_getrandom;
> +		__vdso_futex_robust_list64_try_unlock;

Guard behind CONFIG_FUTEX_ROBUST_UNLOCK ?

ld.lld fails when a function mentioned in the linker script is missing.

>  	local: *;
>  	};
>  }
> @@ -112,3 +113,7 @@ VERSION
>   * Make the sigreturn code visible to the kernel.
>   */
>  VDSO_sigtramp		= __kernel_rt_sigreturn;
> +
> +VDSO_futex_list64_try_unlock_cs_start = __futex_list64_try_unlock_cs_start;
> +VDSO_futex_list64_try_unlock_cs_success = __futex_list64_try_unlock_cs_success;
> +VDSO_futex_list64_try_unlock_cs_end = __futex_list64_try_unlock_cs_end;

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