linux-kernel - Re: [PATCH 10/9] x86/fpu: Fix 'struct fpu' misalignment on 32-bit kernels

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Message-ID: <20240615102352.GA18384@redhat.com>
Date: Sat, 15 Jun 2024 12:23:53 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>, linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave@...1.net>, Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>, Brian Gerst <brgerst@...il.com>,
	"H . Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Uros Bizjak <ubizjak@...il.com>
Subject: Re: [PATCH 10/9] x86/fpu: Fix 'struct fpu' misalignment on 32-bit
 kernels

On 06/14, Oleg Nesterov wrote:
>
> On 06/13, Ingo Molnar wrote:
> >
> > > --- a/include/linux/sched.h
> > > +++ b/include/linux/sched.h
> > > @@ -1562,7 +1562,7 @@ struct task_struct {
> > >  	 * they are included in the randomized portion of task_struct.
> > >  	 */
> > >  	randomized_struct_fields_end
> > > -};
> > > +} __attribute__ ((aligned (64)));
>
> I guess __aligned(64) will look a bit better, but this is minor.

...

> But Ingo, it was a shot in the dark ;) I still don't really understand
> what exactly should be aligned. Is it the fpstate->regs member? Or what?
> If yes, perhaps this member needs __aligned(64) too to be safe?

Ah, I didn't notice that fpregs_state->xregs_state has
__attribute__ ((packed, aligned (64))), so everything is clear.

>From your previous email:

	 - The extra alignment attribute in <linux/sched.h> will affect other
	   architecture as well, although in practice the alignment of init_task is
	   not critical, and is very likely at least 32 bytes, probably more.
	   Still, it's a bit ugly in its current form.

Agreed, but afaics we need to align task_struct only to ensure that

	(void *)task + sizeof(*task);

doesn't break the alignment.

So perhaps we can (later) change x86_task_fpu(), fpu_clone(), and
fpu__init_task_struct_size() to use

	ALIGN(sizeof(struct task_struct), 64)

and remove the alignment attribute in sched.h?

Or use ARCH_MIN_TASKALIGN == __alignof__(union fpregs_state) which is
also used in fork_init()->kmem_cache_create().

Oleg.