Re: [PATCH v2 1/2] sched: move stack_canary field at the top of task_struct

Le 19/09/2018 à 13:58, Peter Zijlstra a écrit :

On Wed, Sep 19, 2018 at 11:14:43AM +0000, Christophe Leroy wrote:

quoted

In order to allow the use of non global stack protector canary,
the stack canary needs to be located at a know offset defined
in Makefile via -mstack-protector-guard-offset.

On powerpc/32, register r2 points to current task_struct at
all time, the stack_canary located inside task_struct can be
used directly if it is located in a known place.

In order to allow that, this patch moves the stack_canary field
out of the randomized area of task_struct.

And you cannot use something like asm-offsets to extract this?

I have not been able to find a way to define the compilation flags AFTER 
building asm-offsets.h, see https://patchwork.ozlabs.org/patch/971521/

If you have a suggestion, it is welcomed.

quoted

Signed-off-by: Christophe Leroy <redacted>
---
  include/linux/sched.h | 8 ++++----
  1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 977cb57d7bc9..1d977b8a4bac 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h

@@ -601,6 +601,10 @@ struct task_struct {
  	/* -1 unrunnable, 0 runnable, >0 stopped: */
  	volatile long			state;
  
+#ifdef CONFIG_STACKPROTECTOR
+	/* Canary value for the -fstack-protector GCC feature: */
+	unsigned long			stack_canary;
+#endif
  	/*
  	 * This begins the randomizable portion of task_struct. Only
  	 * scheduling-critical items should be added above here.

Might as well put it before state, right after the task_info thing.

Yes, it doesn't make much difference, don't any arch expect state at 
offset 0 ?

Christophe