[PATCH v2 1/4] powerpc/kprobes: Pause function_graph tracing during jprobes handling

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

This fixes a crash when function_graph and jprobes are used together.
This is essentially commit 237d28db036e ("ftrace/jprobes/x86: Fix
conflict between jprobes and function graph tracing"), but for powerpc.

Jprobes breaks function_graph tracing since the jprobe hook needs to use
jprobe_return(), which never returns back to the hook, but instead to
the original jprobe'd function. The solution is to momentarily pause
function_graph tracing before invoking the jprobe hook and re-enable it
when returning back to the original jprobe'd function.

Fixes: 6794c78243bfd ("powerpc64: port of the function graph tracer")
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
---
v2: No changes since v1.

 arch/powerpc/kernel/kprobes.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
index e1fb95385a9e..0554d6e66194 100644
--- a/arch/powerpc/kernel/kprobes.c
+++ b/arch/powerpc/kernel/kprobes.c
@@ -610,6 +610,15 @@ int setjmp_pre_handler(struct kprobe *p, struct pt_regs *regs)
 	regs->gpr[2] = (unsigned long)(((func_descr_t *)jp->entry)->toc);
 #endif
 
+	/*
+	 * jprobes use jprobe_return() which skips the normal return
+	 * path of the function, and this messes up the accounting of the
+	 * function graph tracer.
+	 *
+	 * Pause function graph tracing while performing the jprobe function.
+	 */
+	pause_graph_tracing();
+
 	return 1;
 }
 NOKPROBE_SYMBOL(setjmp_pre_handler);
@@ -635,6 +644,8 @@ int longjmp_break_handler(struct kprobe *p, struct pt_regs *regs)
 	 * saved regs...
 	 */
 	memcpy(regs, &kcb->jprobe_saved_regs, sizeof(struct pt_regs));
+	/* It's OK to start function graph tracing again */
+	unpause_graph_tracing();
 	preempt_enable_no_resched();
 	return 1;
 }
-- 
2.12.2