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

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

Em 29/05/2026 14:16, Thomas Weißschuh escreveu:
> 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.
> 

OK, I will separated them for a next version. But hopefully those 
changes won't be needed for the next one.

>> 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.
> 

Ok, I don't think it moves the vDSO indeed.

> 
> 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/
> 

I believe this is true for x86, but not for aarch64. Currently x86 apps 
running on top of arm64 have no way to register a 32 bit list, but this 
will change with the upcoming multi list feature.
>>   	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.

Yes, you are right, thanks for the review!

> 
>>   	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;
> 
> (...)