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SubjectRe: [PATCH][next] ocfs2: ocfs2_fs.h: Replace zero-length array with flexible-array member
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On 2020/3/10 04:21, Gustavo A. R. Silva wrote:
> 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: Gustavo A. R. Silva <gustavo@embeddedor.com>

Acked-by: Joseph Qi <joseph.qi@linux.alibaba.com>
> ---
> fs/ocfs2/ocfs2_fs.h | 18 +++++++++---------
> 1 file changed, 9 insertions(+), 9 deletions(-)
>
> diff --git a/fs/ocfs2/ocfs2_fs.h b/fs/ocfs2/ocfs2_fs.h
> index 0db4a7ec58a2..0dd8c41bafd4 100644
> --- a/fs/ocfs2/ocfs2_fs.h
> +++ b/fs/ocfs2/ocfs2_fs.h
> @@ -470,7 +470,7 @@ struct ocfs2_extent_list {
> __le16 l_reserved1;
> __le64 l_reserved2; /* Pad to
> sizeof(ocfs2_extent_rec) */
> -/*10*/ struct ocfs2_extent_rec l_recs[0]; /* Extent records */
> +/*10*/ struct ocfs2_extent_rec l_recs[]; /* Extent records */
> };
>
> /*
> @@ -484,7 +484,7 @@ struct ocfs2_chain_list {
> __le16 cl_count; /* Total chains in this list */
> __le16 cl_next_free_rec; /* Next unused chain slot */
> __le64 cl_reserved1;
> -/*10*/ struct ocfs2_chain_rec cl_recs[0]; /* Chain records */
> +/*10*/ struct ocfs2_chain_rec cl_recs[]; /* Chain records */
> };
>
> /*
> @@ -496,7 +496,7 @@ struct ocfs2_truncate_log {
> /*00*/ __le16 tl_count; /* Total records in this log */
> __le16 tl_used; /* Number of records in use */
> __le32 tl_reserved1;
> -/*08*/ struct ocfs2_truncate_rec tl_recs[0]; /* Truncate records */
> +/*08*/ struct ocfs2_truncate_rec tl_recs[]; /* Truncate records */
> };
>
> /*
> @@ -640,7 +640,7 @@ struct ocfs2_local_alloc
> __le16 la_size; /* Size of included bitmap, in bytes */
> __le16 la_reserved1;
> __le64 la_reserved2;
> -/*10*/ __u8 la_bitmap[0];
> +/*10*/ __u8 la_bitmap[];
> };
>
> /*
> @@ -653,7 +653,7 @@ struct ocfs2_inline_data
> * for data, starting at id_data */
> __le16 id_reserved0;
> __le32 id_reserved1;
> - __u8 id_data[0]; /* Start of user data */
> + __u8 id_data[]; /* Start of user data */
> };
>
> /*
> @@ -798,7 +798,7 @@ struct ocfs2_dx_entry_list {
> * possible in de_entries */
> __le16 de_num_used; /* Current number of
> * de_entries entries */
> - struct ocfs2_dx_entry de_entries[0]; /* Indexed dir entries
> + struct ocfs2_dx_entry de_entries[]; /* Indexed dir entries
> * in a packed array of
> * length de_num_used */
> };
> @@ -935,7 +935,7 @@ struct ocfs2_refcount_list {
> __le16 rl_used; /* Current number of used records */
> __le32 rl_reserved2;
> __le64 rl_reserved1; /* Pad to sizeof(ocfs2_refcount_record) */
> -/*10*/ struct ocfs2_refcount_rec rl_recs[0]; /* Refcount records */
> +/*10*/ struct ocfs2_refcount_rec rl_recs[]; /* Refcount records */
> };
>
>
> @@ -1021,7 +1021,7 @@ struct ocfs2_xattr_header {
> buckets. A block uses
> xb_check and sets
> this field to zero.) */
> - struct ocfs2_xattr_entry xh_entries[0]; /* xattr entry list. */
> + struct ocfs2_xattr_entry xh_entries[]; /* xattr entry list. */
> };
>
> /*
> @@ -1207,7 +1207,7 @@ struct ocfs2_local_disk_dqinfo {
> /* Header of one chunk of a quota file */
> struct ocfs2_local_disk_chunk {
> __le32 dqc_free; /* Number of free entries in the bitmap */
> - __u8 dqc_bitmap[0]; /* Bitmap of entries in the corresponding
> + __u8 dqc_bitmap[]; /* Bitmap of entries in the corresponding
> * chunk of quota file */
> };
>
>

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