[051/127] mm: replace zero-length array with flexible-array member

Commit Message

Andrew Morton June 4, 2020, 11:48 p.m. UTC

From: chenqiwu <[e-mail]>
Subject: mm: replace zero-length array with flexible-array member

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")

[[e-mail]: fix build]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: chenqiwu <[e-mail]>
Reviewed-by: Andrew Morton <[e-mail]>
Reviewed-by: Wei Yang <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Pankaj Gupta <[e-mail]>
Cc: Yang Shi <[e-mail]>
Cc: Qian Cai <[e-mail]>
Cc: Baoquan He <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 include/linux/mm.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch

--- a/include/linux/mm.h~mm-replace-zero-length-array-with-flexible-array-member
+++ a/include/linux/mm.h
@@ -1726,7 +1726,7 @@  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 */
 };