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

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 <[email protected]> know about it.

From: Will Deacon <[email protected]>
Date: Fri, 26 Aug 2016 11:36:39 +0100
Subject: arm64: debug: avoid resetting stepping state machine when TIF_SINGLESTEP

From: Will Deacon <[email protected]>

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.

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

@@ -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 [email protected] are

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