Re: [PATCH v2 0/3] futex: Create set_robust_list2

Peter Zijlstra
2024-11-04T11:57:02Z

On Mon, Nov 04, 2024 at 12:32:40PM +0100, Peter Zijlstra wrote:
> On Sat, Nov 02, 2024 at 10:58:42PM +0100, Florian Weimer wrote:
>
> > QEMU hints towards further problems (in linux-user/syscall.c):
> >
> > case TARGET_NR_set_robust_list:
> > case TARGET_NR_get_robust_list:
> > /* The ABI for supporting robust futexes has userspace pass
> > * the kernel a pointer to a linked list which is updated by
> > * userspace after the syscall; the list is walked by the kernel
> > * when the thread exits. Since the linked list in QEMU guest
> > * memory isn't a valid linked list for the host and we have
> > * no way to reliably intercept the thread-death event, we can't
> > * support these. Silently return ENOSYS so that guest userspace
> > * falls back to a non-robust futex implementation (which should
> > * be OK except in the corner case of the guest crashing while
> > * holding a mutex that is shared with another process via
> > * shared memory).
> > */
> > return -TARGET_ENOSYS;
>
> I don't think we can sanely fix that. Can't QEMU track the robust thing
> itself and use waitpid() to discover the thread is gone and fudge things
> from there?

Hmm, what about we mandate 'natural' alignement of the structure such
that it is always inside a single page, then QEMU can do the translation
here and hand the kernel the 'real' address.