Re: [linus:master] [mseal] 8be7258aad: stress-ng.pagemove.page_remaps_per_sec -4.4% regression

[Michael Ellerman <mpe@ellerman.id.au>]

Linus Torvalds <torvalds@linux-foundation.org> writes:
> On Mon, 5 Aug 2024 at 16:25, Nicholas Piggin <npiggin@gmail.com> wrote:
>>
>> Can userspace on other archs not unmap their vdsos?
>
> I think they can, and nobody cares. The "context.vdso" value stays at
> some stale value, and anybody who tries to use it will just fail.
>
> So what makes powerpc special is not "you can unmap the vdso", but
> "powerpc cares".
>
> I just don't quite know _why_ powerpc cares.

AFAIK for CRIU the problem is signal delivery:

arch/powerpc/kernel/signal_64.c:

int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
		struct task_struct *tsk)
{
        ...
	/* Set up to return from userspace. */
	if (tsk->mm->context.vdso) {
		regs_set_return_ip(regs, VDSO64_SYMBOL(tsk->mm->context.vdso, sigtramp_rt64));


ie. if the VDSO is moved but mm->context.vdso is not updated, signal
delivery will crash in userspace.

x86-64 always uses SA_RESTORER, and arm64 & s390 can use SA_RESTORER, so
I think CRIU uses that to avoid problems with signal delivery when the
VDSO is moved.

riscv doesn't support SA_RESTORER but I guess CRIU doesn't support riscv
yet so it's not become a problem.

There was a patch to support SA_RESTORER on powerpc, but I balked at
merging it because I couldn't find anyone on the glibc side to say
whether they wanted it or not. I guess I should have just merged it.

There was an attempt to unify all the vdso stuff and handle the
VDSO mremap case in generic code:

  https://lore.kernel.org/lkml/20210611180242.711399-17-dima@arista.com/

But I think that series got a bit big and complicated and Dmitry had to
move on to other things.

cheers