linux-kernel - Re: [PATCH v2] drm/ttm: Silence randstruct warning about casting struct file

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Message-ID: <20250502053303.GX2023217@ZenIV>
Date: Fri, 2 May 2025 06:33:03 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Matthew Brost <matthew.brost@...el.com>
Cc: Kees Cook <kees@...nel.org>,
	Thomas Hellström <thomas.hellstrom@...ux.intel.com>,
	Christian Koenig <christian.koenig@....com>,
	Somalapuram Amaranath <Amaranath.Somalapuram@....com>,
	Huang Rui <ray.huang@....com>,
	Matthew Auld <matthew.auld@...el.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH v2] drm/ttm: Silence randstruct warning about casting
 struct file

On Thu, May 01, 2025 at 09:52:08PM -0700, Matthew Brost wrote:
> On Fri, May 02, 2025 at 05:31:49AM +0100, Al Viro wrote:
> > On Thu, May 01, 2025 at 09:26:25PM -0700, Matthew Brost wrote:

> > And what is the lifecycle of that thing?  E.g. what is guaranteed about
> > ttm_backup_fini() vs. functions accessing the damn thing?  Are they
> > serialized on something/tied to lifecycle stages of struct ttm_tt?
> 
> I believe the life cycle is when ttm_tt is destroyed or api allows
> overriding the old backup with a new one (currently unused).

Umm...  So can ttm_tt_setup_backup() be called in the middle of
e.g. ttm_backup_drop() or ttm_backup_{copy,backup}_page(), etc.?

I mean, if they had been called by ttm_backup.c internals, it would
be an internal business of specific implementation, with all
serialization, etc. warranties being its responsibility;
but if it's called by other code that is supposed to be isolated
from details of what ->backup is pointing to...

Sorry for asking dumb questions, but I hadn't seen the original
threads.  Basically, what prevents the underlying shmem file getting
torn apart while another operation is using it?  It might very well
be simple, but I had enough "it's because of... oh, bugger" moments
on the receiving end of such questions...