Re: [PATCH RFC 1/2] arm64: vdso: Prepare for robust futex unlock support

[André Almeida]

Em 22/04/2026 10:17, Thomas Weißschuh escreveu:
> On 2026-04-22 15:02:45+0200, Thomas Weißschuh wrote:
> > On 2026-04-17 11:56:10-0300, 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@xxxxxxxxxx>
> > > ---
> > > RFC: Those symbols can't be found by the linker after patch 2/2, it fails with:
> > >
> > > ld: arch/arm64/kernel/vdso.o: in function `vdso_futex_robust_unlock_update_ips':
> > > arch/arm64/kernel/vdso.c:72:(.text+0x200): undefined reference to `__futex_list64_try_unlock_cs_success'
> > > ld: arch/arm64/kernel/vdso.o: relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `__futex_list64_try_unlock_cs_success' which may bind externally can not be used when making a shared object; recompile with -fPIC
> > > arch/arm64/kernel/vdso.c:72:(.text+0x200): dangerous relocation: unsupported relocation
> >
> > arch/arm64/kernel/vdso.o is a kernel object.
> > __futex_list64_try_unlock_cs_success is a vDSO symbol.
> > They live in wholly different objects, which are never linked together.
> > Look at VDSO_SYMBOL() to get the offset of the vDSO symbol in kernel code.
> >
> > (...)
>
> The diff below shows the idea. Compat code is not fixed, and I didn't
> even try to run it. Also please use CONFIG_COMPAT_VDSO over plain
> CONFIG_COMPAT.


VDSO_SYMBOL() did the tricky, I fixed this for the v2, thanks Thomas!

	André