Re: [PATCH v16 02/18] syscall_user_dispatch: Introduce a weak fallback for arch_syscall_is_vdso_sigreturn()

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

On Mon, Jun 29, 2026 at 09:06:00PM +0800, Jinjie Ruan wrote:
> Currently, multiple architectures (LoongArch, RISC-V, S390, Powerpc)
> provide identical stubs for arch_syscall_is_vdso_sigreturn() that simply
> return false. This results in redundant boilerplate code across the tree.
> 
> Introduce a default __weak implementation of
> arch_syscall_is_vdso_sigreturn() directly in syscall_user_dispatch.c that
> returns false. This allows architectures that do not utilize a vDSO
> sigreturn to entirely drop their redundant inline definitions.
> 
> Architectures requiring a specialized check (such as x86) will continue to
> override this fallback with their strong symbol definitions.
> 
> Clean up the redundant implementations in loongarch, riscv, s390
> and powerpc.

> +bool __weak arch_syscall_is_vdso_sigreturn(struct pt_regs *regs)
> +{
> +	return false;
> +}

If we need this, please make it:

#ifndef arch_syscall_is_vdso_sigreturn
static inline bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs)
{
	return false;
}
#endif

... and require that architectures which need this provide a CPP
definition.

The use of __weak is generally problematic, as it prevents the compiler
form being able to elide code, and gets in the way of symbol resolution.
It's perfectly fine to require that architectures need to provide a CPP
definition alongside their own implementation of this function.

That said, as per my comment on v15, I'd prefer that for now we DO NOT
enable syscall user dispatch on arm64, and we first make it possible for
architecture to express whether or not they support that, even if they
use GENERIC_ENTRY. That might mean this patch isn't necessary right now.

[1] https://lore.kernel.org/linux-arm-kernel/akZgV0Y4YAmB43_g@J2N7QTR9R3.cambridge.arm.com/

Mark.