[PATCH 3.2 085/106] powerpc/kprobes: Pause function_graph tracing during jprobes handling

[Ben Hutchings]

3.2.93-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx>

commit a9f8553e935f26cb5447f67e280946b0923cd2dc upstream.

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: 6794c78243bf ("powerpc64: port of the function graph tracer")
Signed-off-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
Acked-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Acked-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
[bwh: Backported to 3.2: include <linux/ftrace.h>, which apparently gets
 included indirectly upstream]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/arch/powerpc/kernel/kprobes.c
+++ b/arch/powerpc/kernel/kprobes.c
@@ -32,6 +32,7 @@
 #include <linux/module.h>
 #include <linux/kdebug.h>
 #include <linux/slab.h>
+#include <linux/ftrace.h>
 #include <asm/cacheflush.h>
 #include <asm/sstep.h>
 #include <asm/uaccess.h>
@@ -530,6 +531,15 @@ int __kprobes setjmp_pre_handler(struct
 	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;
 }
 
@@ -552,6 +562,8 @@ int __kprobes longjmp_break_handler(stru
 	 * 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;
 }