linux-kernel - Re: [PATCH v2 0/3] futex: Create set_robust

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Message-ID: <20241104115656.GW33184@noisy.programming.kicks-ass.net>
Date: Mon, 4 Nov 2024 12:56:56 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Florian Weimer <fweimer@...hat.com>
Cc: André Almeida <andrealmeid@...lia.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>, Arnd Bergmann <arnd@...db.de>,
	sonicadvance1@...il.com, linux-kernel@...r.kernel.org,
	kernel-dev@...lia.com, linux-api@...r.kernel.org,
	Nathan Chancellor <nathan@...nel.org>
Subject: Re: [PATCH v2 0/3] futex: Create set_robust_list2

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.