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Message-ID: <CALCETrV6G2DU1d7qr=ADas7nk5a77=J6X7ezKxw2_kzdmJGDXg@mail.gmail.com>
Date: Thu, 22 Sep 2016 15:23:59 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Mark Rutland <mark.rutland@....com>
Cc: james.morse@....com, AKASHI Takahiro <takahiro.akashi@...aro.org>,
Kees Cook <keescook@...omium.org>, Andrew Morton <akpm@...ux-foundation.org>,
"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, lorenzo.pieralisi@....com,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, suzuki.poulose@....com,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Catalin Marinas <catalin.marinas@....com>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>, Will Deacon <will.deacon@....com>
Subject: Re: [RFC PATCH 2/8] thread_info: allow custom in-task thread_info
On Sep 21, 2016 12:28 AM, "Mark Rutland" <mark.rutland@....com> wrote:
>
> Hi Andy,
>
> On Fri, Sep 16, 2016 at 08:11:14AM -0700, Andy Lutomirski wrote:
> > > On Thu, Sep 15, 2016 at 11:37:47AM -0700, Andy Lutomirski wrote:
> > > Just to check, what do you mean to happen with the flags field? Should
> > > that always be in the generic thread_info? e.g.
> > >
> > > struct thread_info {
> > > u32 flags;
> > > #ifdef arch_thread_info
> > > struct arch_thread_info arch_ti;
> > > #endif
> > > };
> >
> > Exactly. Possibly with a comment that using thread_struct should be
> > preferred and that arch_thread_info should be used only if some header
> > file requires access via current_thread_info() or task_thread_info().
>
> While fixing up these patches, I realised that I'm somewhat concerned by
> flags becoming a u32 (where it was previously an unsigned long for
> arm64).
>
> The generic {test,set,*}_ti_thread_flag() helpers use the usual bitops,
> which perform accesses of sizeof(unsigned long) at a time, and for arm64
> these need to be naturally-aligned.
>
> We happen to get that alignment from subsequent fields in task_struct
> and/or thread_info, and for arm64 we don't seem to have a problem with
> tearing, but it feels somewhat fragile, and leaves me uneasy.
>
> Looking at the git log, it seems that x86 also use unsigned long until
> commit affa219b60a11b32 ("x86: change thread_info's flag field back to
> 32 bits"), where if I'm reading correctly, this was done to get rid of
> unnecessary padding. With THREAD_INFO_IN_STACK, thread_info::flags is
> immediately followed by a long on x86, so we save no padding.
>
> Given all that, can we make the generic thread_info::flags an unsigned
> long, matching what the thread flag helpers implicitly assume?
>
Yes. Want to send the patch or should I?
--Andy
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