Re: [PATCH v8 1/4] arm64: vdso: Prepare for robust futex unlock support

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Sep 16, 2026 at 10:05:38PM -0300, André Almeida wrote:
> Em 16/09/2026 11:24, Mark Rutland escreveu:
> > I reckon this might also be clearer as:
> > 
> > | 	if (regs->regs[3])
> > | 		return NULL;
> > | 	
> > | 	return (void __user *)regs->regs[2];
> > 
> > Do we need a __force cast here, or is sparse happy without that?
> 
> I've just re-compiled those patches with C=1, and didn't find any warning
> for this.

Perfect, thanks for confirming!

[...]

> > > +	futex_set_vdso_cs_range(fd, is_32bit ? 1 : 0, start, end, is_32bit);
> > 
> > On arm64 (and every architecture other than x86, AFAICT), the
> > native/compat VDSOs are mutually exclusive, and a single mm can only
> > have one of them.
> > 
> > Given that, I think we can make this:
> > 
> > 	futex_set_vdso_cs_range(fd, 0, start, end, is_32bit);
> > 
> > That way we'll avoid confusing/bikesheeding over 'is_32bit ? 1 : 0',
> > without having to add mnemnonics for the native/compat CS indices.
> 
> Perfect, I will simplify that then.
> 
> > That said, what's the plan for 32-bit robust lists on a 64-bit host?
> > IIUC you wanted that for emulation, and AFAICT you have no way to call
> > __vdso_futex_robust_list32_try_unlock() from a native task.
> 
> Yes, the plan is to be able to use the 32-bit robust lists on a 64-bit
> hosts, with a new interface (set/get_robust_list2[1]). That means that a
> 64-bit kernel will be able to process an user robust list made of 32-bit
> pointers. So the emulator will call set_robust_list2() one time for it's own
> list, and another time for the emulated app list.
> 
> However, what we have agreed for the moment is that, while there's no
> support for that, we will not expose the 32-bit functions yet [2]:

Ah, thanks for the pointer! I had mistakenly assumed that some of
that had already happened.

I guess the common logic for futex_set_vdso_cs_range() will need to
change at that point I guess if/when that happens, you'd expect to add
__vdso_futex_robust_list32_try_unlock() into the 64-bit VDSO, regardless
of compat?

Mark.

> "...that requires also the ability to register a 32-bit robust list for a
> 64-bit process, which is not supported right now. So no, we are not exposing
> something half functional..."
> 
> So yes, in the near future I will need to expose
> __vdso_futex_robust_list32_try_unlock() to 64-bit apps as well, to be able
> to do this handling with 32-bit pointers, but we are not doing it right now.
> 
> [1] https://lore.kernel.org/lkml/20251122-tonyk-robust_futex-v6-0-05fea005a0fd@igalia.com/
> 
> [2] https://lore.kernel.org/lkml/878q9vwote.ffs@tglx/
>