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

[Thomas Weißschuh]

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.

(...)


Thomas