[PATCH] mm: Replace zero-length array with flexible-array member

[qiwuchen55@gmail.com]

From: chenqiwu <chenqiwu@xiaomi.com>

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: chenqiwu <chenqiwu@xiaomi.com>
---
 include/linux/mm.h     | 2 +-
 include/linux/mmzone.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index c54fb96..8dc699a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1528,7 +1528,7 @@ struct frame_vector {
 	unsigned int nr_frames;	/* Number of frames stored in ptrs array */
 	bool got_ref;		/* Did we pin pages by getting page ref? */
 	bool is_pfns;		/* Does array contain pages or pfns? */
-	void *ptrs[0];		/* Array of pinned pfns / pages. Use
+	void *ptrs[];		/* Array of pinned pfns / pages. Use
 				 * pfns_vector_pages() or pfns_vector_pfns()
 				 * for access */
 };
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 462f687..0e3c685 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1187,7 +1187,7 @@ static inline unsigned long section_nr_to_pfn(unsigned long sec)
 struct mem_section_usage {
 	DECLARE_BITMAP(subsection_map, SUBSECTIONS_PER_SECTION);
 	/* See declaration of similar field in struct zone */
-	unsigned long pageblock_flags[0];
+	unsigned long pageblock_flags[];
 };
 
 void subsection_map_init(unsigned long pfn, unsigned long nr_pages);
-- 
1.9.1