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

[André Almeida]

Em 17/09/2026 07:01, Mark Rutland escreveu:
> 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?

Yes, given __vdso_futex_robust_list64_try_unlock() would write a 64-bit 
zero word in a 32-bit pointer, I probably need someway to expose 
__vdso_futex_robust_list32_try_unlock().

However, this will be need when a x86-32 bit app is running on top of 
the emulator, where the 32-bit glibc would eventually call the x86-32 
version of the VDSO. I will check what the emulator does with the other 
VDSOs, if they redirect to the native arm versions or what.

> 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@xxxxxxxxxx/
> >
> > [2] https://lore.kernel.org/lkml/878q9vwote.ffs@tglx/