linux-kernel - Re: [PATCH] thread_info: use unsigned long for flags

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Date:   Sat, 24 Sep 2016 00:48:34 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] thread_info: use unsigned long for flags

[Adding Ingo, so this doesn't get lost -- please see the end of the mail]

On Fri, Sep 23, 2016 at 11:08:46AM -0700, Andy Lutomirski wrote:
> On Fri, Sep 23, 2016 at 10:24 AM, Mark Rutland <mark.rutland@....com> wrote:
> > The generic THREAD_INFO_IN_TASK definition of thread_info::flags is a
> > u32, matching x86 prior to the introduction of THREAD_INFO_IN_TASK.
> >
> > However, common helpers like test_ti_thread_flag() implicitly assume
> > that thread_info::flags has at least the size and alignment of unsigned
> > long, and relying on padding and alignment provided by other elements of
> > task_struct is somewhat fragile. Additionally, some architectures use
> > more that 32 bits for thread_info::flags, and others may need to in
> > future.
> >
> > With THREAD_INFO_IN_TASK, task struct follows thread_info with a long
> > field, and thus we no longer save any space as we did back in commit
> > affa219b60a11b32 ("x86: change thread_info's flag field back to 32
> > bits").
> >
> > Given all this, it makes more sense for the generic thread_info::flags
> > to be an unsigned long. Make it so.
> 
> I have only one problem with this, and it's a general objection that's
> mostly off topic: why the [expletive] do the arch-independent bitfield
> helpers think in units of variable size?  It's *absurd*, especially on
> big-endian architectures.
> 
> Now that that's out of my system, I think this patch is fine.
> Big-endian arches that opt in will have to deal with it somehow, but I
> don't see why making it 'unsigned long' is worse than anything else.
> x86 is fine with this change.

FWIW, given <linux/thread_info.h> contains/uses the helpers mentioned above, BE
arches *must* use unsigned long (or something of the same size) today, or they
wouldn't work. In v4.8-rc7 that is the case:

$ ls -l arch | wc -l
33

# Note the above includes Kconfig, so there are 32 to consider

$ git grep -W 'struct thread_info {' -- arch/*/include | \
  grep flags | grep 'unsigned long\s\+flags' | wc -l       
29

$ git grep -W 'struct thread_info {' -- arch/*/include | \
  grep flags | grep 'int\s\+flags'
arch/alpha/include/asm/thread_info.h-   unsigned int            flags;          /* low level flags */

$ git grep -W 'struct thread_info {' -- arch/*/include | \
  grep flags | grep 'u32\s\+flags' 
arch/ia64/include/asm/thread_info.h-    __u32 flags;                    /* thread_info flags (see TIF_*) */
arch/x86/include/asm/thread_info.h-     __u32                   flags;          /* low level flags */

> Acked-by: Andy Lutomirski <luto@...nel.org>
> 
> Ingo, can you apply this for 4.9 so that we can make this change
> before other arches might start depending on the field being u32?
> 
> --Andy

Thanks,
Mark.