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

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Date:   Fri, 23 Sep 2016 18:24:07 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     linux-kernel@...r.kernel.org
Cc:     Mark Rutland <mark.rutland@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>
Subject: [PATCH] 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@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Kees Cook <keescook@...omium.org>
Cc: linux-kernel@...r.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.

diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index f3837c7..a2c2f88 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -13,7 +13,7 @@

 #ifdef CONFIG_THREAD_INFO_IN_TASK
 struct thread_info {
-	u32			flags;		/* low level flags */
+	unsigned long		flags;		/* low level flags */
 };

 #define INIT_THREAD_INFO(tsk)			\
-- 
1.9.1