Patch "MIPS: Only change $28 to thread_info if coming from user mode" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

MIPS: Only change $28 to thread_info if coming from user mode

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mips-only-change-28-to-thread_info-if-coming-from-user-mode.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Matt Redfearn <[email protected]>
Date: Mon, 19 Dec 2016 14:20:58 +0000
Subject: MIPS: Only change $28 to thread_info if coming from user mode

From: Matt Redfearn <[email protected]>

commit 510d86362a27577f5ee23f46cfb354ad49731e61 upstream.

The SAVE_SOME macro is used to save the execution context on all exceptions.
If an exception occurs while executing user code, the stack is switched to the kernel's stack for the current task, and register $28 is switched to point to the current_thread_info, which is at the bottom of the stack region.
If the exception occurs while executing kernel code, the stack is left, and this change ensures that register $28 is not updated. This is the correct behaviour when the kernel can be executing on the separate irq stack, because the thread_info will not be at the base of it.

With this change, register $28 is only switched to it's kernel conventional usage of the currrent thread info pointer at the point at which execution enters kernel space. Doing it on every exception was redundant, but OK without an IRQ stack, but will be erroneous once that is introduced.

Patchwork: https://patchwork.linux-mips.org/patch/14742/

---
 arch/mips/include/asm/stackframe.h |    7 +++++++
 1 file changed, 7 insertions(+)

@@ -216,12 +216,19 @@
 		LONG_S	$25, PT_R25(sp)
 		LONG_S	$28, PT_R28(sp)
 		LONG_S	$31, PT_R31(sp)
+
+		/* Set thread_info if we're coming from user mode */
+		mfc0	k0, CP0_STATUS
+		sll	k0, 3		/* extract cu0 bit */
+		bltz	k0, 9f
+
 		ori	$28, sp, _THREAD_MASK
 		xori	$28, _THREAD_MASK
 #ifdef CONFIG_CPU_CAVIUM_OCTEON
 		.set    mips64
 		pref    0, 0($28)       /* Prefetch the current pointer */
 #endif
+9:
 		.set	pop
 		.endm
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/mips-only-change-28-to-thread_info-if-coming-from-user-mode.patch queue-4.4/mips-stack-unwinding-while-on-irq-stack.patch queue-4.4/mips-introduce-irq_stack.patch