linux-kernel - [RFC PATCH 2/8] thread_info: allow custom in-task thread

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Message-Id: <1473947349-14521-3-git-send-email-mark.rutland@arm.com>
Date:   Thu, 15 Sep 2016 14:49:03 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     linux-arm-kernel@...ts.infradead.org
Cc:     akpm@...ux-foundation.org, ard.biesheuvel@...aro.org,
        catalin.marinas@....com, james.morse@....com,
        keescook@...omium.org, linux-kernel@...r.kernel.org,
        lorenzo.pieralisi@....com, luto@...nel.org, mark.rutland@....com,
        suzuki.poulose@....com, takahiro.akashi@...aro.org,
        will.deacon@....com, kernel-hardening@...ts.openwall.com
Subject: [RFC PATCH 2/8] thread_info: allow custom in-task thread_info

Currently, task_struct is defined in <linux/sched.h>, which (indirectly)
pulls in a number of low-level arch headers such as <asm/preempt.h>
through a number of other headers. Thus, code and structures in these
headers can't rely on the definition of task_struct. Some of these
headers are necessary for the definition of task_struct, so moving
task_struct into its own header is insufficient tio avoid circular
includes.

With CONFIG_THREAD_INFO_IN_TASK, arch code needs to implement its own
get_current() for the generic get_thread_info(). To avoid header
dependency issues, this relies on thread_info being the first member of
task_struct. This can be used by low-level arch code, as it doesn't
depend on the definition of task_struct.

For architectures without preempt-safe this_cpu ops, some data required
by low-level arch code (e.g. preempt_count) has to be stored per-thread,
and for the reasons above, we cannot place this in task_struct (or
thread_struct, since we cannot know its offset from within task_struct).
The only practical location for these is thread_info.

This patch allows architectures with CONFIG_ARCH_HAS_OWN_THREAD_INFO to
define their own thread_info, avoiding the problems described above.

Signed-off-by: Mark Rutland <mark.rutland@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Will Deacon <will.deacon@....com>
Cc: linux-kernel@...r.kernel.org
---
 include/linux/thread_info.h | 3 ++-
 init/Kconfig                | 3 +++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index d9622f7..7984f87 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -13,7 +13,8 @@
 struct timespec;
 struct compat_timespec;

-#ifdef CONFIG_THREAD_INFO_IN_TASK
+#if defined(CONFIG_THREAD_INFO_IN_TASK) && \
+    !defined(CONFIG_ARCH_HAS_OWN_THREAD_INFO)
 struct thread_info {
 	u32			flags;		/* low level flags */
 };
diff --git a/init/Kconfig b/init/Kconfig
index 3b9a47f..f812098 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -26,6 +26,9 @@ config IRQ_WORK
 config BUILDTIME_EXTABLE_SORT
 	bool

+config ARCH_HAS_OWN_THREAD_INFO
+	bool
+
 config THREAD_INFO_IN_TASK
 	bool
 	help
-- 
1.9.1