[PATCH] tracing: Add *iter check for NULL

[zhengjun.xing@intel.com]

From: xingzhen <zhengjun.xing@intel.com>

3debb0a9ddb adding a "__used" to the variable in the
__trace_printk_fmt section. Sometimes it will cause
*iter to be NULL, then strcmp in lookup_format and
strcpy in hold_module_trace_bprintk_format will panic.

Signed-off-by: xingzhen <zhengjun.xing@intel.com>
---
 kernel/trace/trace_printk.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/trace/trace_printk.c b/kernel/trace/trace_printk.c
index f96f038..82ecfb5 100644
--- a/kernel/trace/trace_printk.c
+++ b/kernel/trace/trace_printk.c
@@ -55,6 +55,8 @@ void hold_module_trace_bprintk_format(const char **start, const char **end)
 
 	mutex_lock(&btrace_mutex);
 	for (iter = start; iter < end; iter++) {
+		if (!*iter)
+			goto err;
 		struct trace_bprintk_fmt *tb_fmt = lookup_format(*iter);
 		if (tb_fmt) {
 			*iter = tb_fmt->fmt;
@@ -75,6 +77,7 @@ void hold_module_trace_bprintk_format(const char **start, const char **end)
 		*iter = fmt;
 
 	}
+err:
 	mutex_unlock(&btrace_mutex);
 }
 
-- 
1.9.1