ocfs2: ocfs2_fs.h: 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://urldefense.com/v3/__https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html__;!!GqivPVa7Brio!OKPotRhYhHbCG2kibo8Q6_6CuKaa28d_74h1svxyR6rbshrK2L_BdrQpNbvJWBWb40QCkg$
[2] https://urldefense.com/v3/__https://github.com/KSPP/linux/issues/21__;!!GqivPVa7Brio!OKPotRhYhHbCG2kibo8Q6_6CuKaa28d_74h1svxyR6rbshrK2L_BdrQpNbvJWBUhNn9M6g$
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Gang He <ghe@suse.com>
Cc: Jun Piao <piaojun@huawei.com>
Link: http://lkml.kernel.org/r/20200309202155.GA8432@embeddedor
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/ocfs2/ocfs2_fs.h b/fs/ocfs2/ocfs2_fs.h
index 0db4a7e..0dd8c41 100644 (file)
--- 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 */
 };