Re: [RESEND PATCH 1/3] mm: Replace zero-length array with flexible-array member

[Wei Yang <richard.weiyang@gmail.com>]

On Sat, Apr 11, 2020 at 06:11:54PM +0800, qiwuchen55@gmail.com wrote:
>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>


The change looks good to me.

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>

>---
> include/linux/mm.h     | 2 +-
> include/linux/mmzone.h | 2 +-
> include/linux/swap.h   | 2 +-
> 3 files changed, 3 insertions(+), 3 deletions(-)
>
>diff --git a/include/linux/mm.h b/include/linux/mm.h
>index 5a32342..9831bb5 100644
>--- a/include/linux/mm.h
>+++ b/include/linux/mm.h
>@@ -1718,7 +1718,7 @@ 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 1b9de7d..f6b9fa9 100644
>--- a/include/linux/mmzone.h
>+++ b/include/linux/mmzone.h
>@@ -1147,7 +1147,7 @@ struct mem_section_usage {
> 	DECLARE_BITMAP(subsection_map, SUBSECTIONS_PER_SECTION);
> #endif
> 	/* 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);
>diff --git a/include/linux/swap.h b/include/linux/swap.h
>index b835d8d..e1bbf7a 100644
>--- a/include/linux/swap.h
>+++ b/include/linux/swap.h
>@@ -275,7 +275,7 @@ struct swap_info_struct {
> 					 */
> 	struct work_struct discard_work; /* discard worker */
> 	struct swap_cluster_list discard_clusters; /* discard clusters list */
>-	struct plist_node avail_lists[0]; /*
>+	struct plist_node avail_lists[]; /*
> 					   * entries in swap_avail_heads, one
> 					   * entry per node.
> 					   * Must be last as the number of the
>-- 
>1.9.1

-- 
Wei Yang
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