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

[Peter Zijlstra <peterz@infradead.org>]

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.