linux-kernel - Re: [PATCH 2/3] x86/fpu: Remove the thread::fpu pointer

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ZmQHrHqQUVTRtjSz@gmail.com>
Date: Sat, 8 Jun 2024 09:26:36 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: 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>, "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 2/3] x86/fpu: Remove the thread::fpu pointer


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Ingo Molnar <mingo@...nel.org> wrote:
> 
> > 
> > * Oleg Nesterov <oleg@...hat.com> wrote:
> > 
> > > On 06/05, Ingo Molnar wrote:
> > > >
> > > > @@ -591,13 +591,11 @@ int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal,
> > > >  	 * This is safe because task_struct size is a multiple of cacheline size.
> > > >  	 */
> > > >  	struct fpu *dst_fpu = (void *)dst + sizeof(*dst);
> > > > -	struct fpu *src_fpu = current->thread.fpu;
> > > > +	struct fpu *src_fpu = x86_task_fpu(current);
> > > 
> > > I think this patch can also change
> > > 
> > > 	struct fpu *dst_fpu = (void *)dst + sizeof(*dst);
> > > 
> > > above to use x86_task_fpu(dst).
> > 
> > Yeah, so I'd prefer to keep it open coded, because of the comment and the 
> > debug check makes a lot more sense if the pointer calculation is visible:
> 
> On a second thought I changed it to your suggested variant:
> 
>         struct fpu *src_fpu = x86_task_fpu(current);
>         struct fpu *dst_fpu = x86_task_fpu(dst);
> 
> because you are right, it's in fact easier to read this way.

On a third thought, while more readable, this doesn't work in practice with 
the current scheme, because x86_task_fpu() gets called on kthreads in 
fpu_clone(), which trips up the new debugging code.

We could resolve it by special-casing PF_KTHREAD here too, but that weakens 
the whole readability argument. I'll leave it as-is for now.

Thanks,

	Ingo