[PATCH] ring-buffer: Clearly check null ptr returned by rb_set_head_page()

[Zheng Yejian <zhengyejian1@huawei.com>]

In error case, 'buffer_page' returned by rb_set_head_page() is NULL,
currently check '&buffer_page->list' is equivalent to check 'buffer_page'
due to 'list' is the first member of 'buffer_page', but suppose it is not
some time, 'head_page' would be wild memory while check would be bypassed.

Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
---
 kernel/trace/ring_buffer.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 76a2d91eecad..025324df1f96 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2060,10 +2060,11 @@ rb_insert_pages(struct ring_buffer_per_cpu *cpu_buffer)
 		struct list_head *head_page, *prev_page, *r;
 		struct list_head *last_page, *first_page;
 		struct list_head *head_page_with_bit;
+		struct buffer_page *hpage = rb_set_head_page(cpu_buffer);
 
-		head_page = &rb_set_head_page(cpu_buffer)->list;
-		if (!head_page)
+		if (!hpage)
 			break;
+		head_page = &hpage->list;
 		prev_page = head_page->prev;
 
 		first_page = pages->next;
-- 
2.25.1