thread_info: use unsigned long for flags

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.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: linux-kernel@vger.kernel.org
---
 include/linux/thread_info.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

As discussed in the arm64 RFC.

Andy, I'm assuming that as with affa219b60a11b32 no x86 assembly fixups are
required. An x86_64 defconfig built fine for me.

I've based this on your x86/vmap_stack branch. I'm not sure what the plan is
for merging that. If I should be using a different branch to base the arm64
work atop of, please let me know!

Thanks,
Mark.

-- 
1.9.1

[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@arm.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@kernel.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.