Patch "arm64: debug: avoid resetting stepping state machine when TIF_SINGLESTEP" has been added to the 4.8-stable tree

Patch "arm64: debug: avoid resetting stepping state machine when TIF_SINGLESTEP" has been added to the 4.8-stable tree

[gregkh]

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

    arm64: debug: avoid resetting stepping state machine when TIF_SINGLESTEP

to the 4.8-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:
     arm64-debug-avoid-resetting-stepping-state-machine-when-tif_singlestep.patch
and it can be found in the queue-4.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 3a402a709500c5a3faca2111668c33d96555e35a Mon Sep 17 00:00:00 2001
From: Will Deacon <will.deacon@arm.com>
Date: Fri, 26 Aug 2016 11:36:39 +0100
Subject: arm64: debug: avoid resetting stepping state machine when TIF_SINGLESTEP

From: Will Deacon <will.deacon@arm.com>

commit 3a402a709500c5a3faca2111668c33d96555e35a upstream.

When TIF_SINGLESTEP is set for a task, the single-step state machine is
enabled and we must take care not to reset it to the active-not-pending
state if it is already in the active-pending state.

Unfortunately, that's exactly what user_enable_single_step does, by
unconditionally setting the SS bit in the SPSR for the current task.
This causes failures in the GDB testsuite, where GDB ends up missing
expected step traps if the instruction being stepped generates another
trap, e.g. PTRACE_EVENT_FORK from an SVC instruction.

This patch fixes the problem by preserving the current state of the
stepping state machine when TIF_SINGLESTEP is set on the current thread.

Cc: <stable@vger.kernel.org>
Reported-by: Yao Qi <yao.qi@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/arm64/kernel/debug-monitors.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -435,8 +435,10 @@ NOKPROBE_SYMBOL(kernel_active_single_ste
 /* ptrace API */
 void user_enable_single_step(struct task_struct *task)
 {
-	set_ti_thread_flag(task_thread_info(task), TIF_SINGLESTEP);
-	set_regs_spsr_ss(task_pt_regs(task));
+	struct thread_info *ti = task_thread_info(task);
+
+	if (!test_and_set_ti_thread_flag(ti, TIF_SINGLESTEP))
+		set_regs_spsr_ss(task_pt_regs(task));
 }
 NOKPROBE_SYMBOL(user_enable_single_step);
 


Patches currently in stable-queue which might be from will.deacon@arm.com are

queue-4.8/arm64-debug-avoid-resetting-stepping-state-machine-when-tif_singlestep.patch