fs: omfs: Use flexible-array member in struct omfs_extent

Memory for 'struct omfs_extent' and a 'e_extent_count' number of extent
entries is indirectly allocated through 'bh->b_data', which is a pointer
to data within the page. This implies that the member 'e_entry'
(which is the start of extent entries) functions more like an array than
a single object of type 'struct omfs_extent_entry'.

So we better turn this object into a proper array, in this case a
flexible-array member, and with that, fix the following
-Wstringop-overflow warning seen after building s390 architecture with
allyesconfig (GCC 13):

fs/omfs/file.c: In function 'omfs_grow_extent':
include/linux/fortify-string.h:57:33: warning: writing 16 bytes into a region of size 0 [-Wstringop-overflow=]
   57 | #define __underlying_memcpy     __builtin_memcpy
      |                                 ^
include/linux/fortify-string.h:648:9: note: in expansion of macro '__underlying_memcpy'
  648 |         __underlying_##op(p, q, __fortify_size);                        \
      |         ^~~~~~~~~~~~~
include/linux/fortify-string.h:693:26: note: in expansion of macro '__fortify_memcpy_chk'
  693 | #define memcpy(p, q, s)  __fortify_memcpy_chk(p, q, s,                  \
      |                          ^~~~~~~~~~~~~~~~~~~~
fs/omfs/file.c:170:9: note: in expansion of macro 'memcpy'
  170 |         memcpy(terminator, entry, sizeof(struct omfs_extent_entry));
      |         ^~~~~~
In file included from fs/omfs/omfs.h:8,
                 from fs/omfs/file.c:11:
fs/omfs/omfs_fs.h:80:34: note: at offset 16 into destination object 'e_entry' of size 16
   80 |         struct omfs_extent_entry e_entry;       /* start of extent entries */
      |                                  ^~~~~~~

There are some binary differences before and after changes, but this are
expected due to the change in the size of 'struct omfs_extent' and the
necessary adjusments.

This helps with the ongoing efforts to globally enable
-Wstringop-overflow.

Link: https://github.com/KSPP/linux/issues/330
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

fs/omfs/file.c

@@ -14,7 +14,7 @@ static u32 omfs_max_extents(struct omfs_sb_info *sbi, int offset)
 {
 	return (sbi->s_sys_blocksize - offset -
 		sizeof(struct omfs_extent)) /
-		sizeof(struct omfs_extent_entry) + 1;
+		sizeof(struct omfs_extent_entry);
 }
 
 void omfs_make_empty_table(struct buffer_head *bh, int offset)
@@ -24,8 +24,8 @@ void omfs_make_empty_table(struct buffer_head *bh, int offset)
 	oe->e_next = ~cpu_to_be64(0ULL);
 	oe->e_extent_count = cpu_to_be32(1),
 	oe->e_fill = cpu_to_be32(0x22),
-	oe->e_entry.e_cluster = ~cpu_to_be64(0ULL);
+	oe->e_entry[0].e_cluster = ~cpu_to_be64(0ULL);
-	oe->e_entry.e_blocks = ~cpu_to_be64(0ULL);
+	oe->e_entry[0].e_blocks = ~cpu_to_be64(0ULL);
 }
 
 int omfs_shrink_inode(struct inode *inode)
@@ -68,7 +68,7 @@ int omfs_shrink_inode(struct inode *inode)
 
 		last = next;
 		next = be64_to_cpu(oe->e_next);
-		entry = &oe->e_entry;
+		entry = oe->e_entry;
 
 		/* ignore last entry as it is the terminator */
 		for (; extent_count > 1; extent_count--) {
@@ -117,7 +117,7 @@ static int omfs_grow_extent(struct inode *inode, struct omfs_extent *oe,
 			u64 *ret_block)
 {
 	struct omfs_extent_entry *terminator;
-	struct omfs_extent_entry *entry = &oe->e_entry;
+	struct omfs_extent_entry *entry = oe->e_entry;
 	struct omfs_sb_info *sbi = OMFS_SB(inode->i_sb);
 	u32 extent_count = be32_to_cpu(oe->e_extent_count);
 	u64 new_block = 0;
@@ -245,7 +245,7 @@ static int omfs_get_block(struct inode *inode, sector_t block,
 
 		extent_count = be32_to_cpu(oe->e_extent_count);
 		next = be64_to_cpu(oe->e_next);
-		entry = &oe->e_entry;
+		entry = oe->e_entry;
 
 		if (extent_count > max_extents)
 			goto out_brelse;

fs/omfs/omfs_fs.h

@@ -77,7 +77,7 @@ struct omfs_extent {
 	__be64 e_next;			/* next extent table location */
 	__be32 e_extent_count;		/* total # extents in this table */
 	__be32 e_fill;
-	struct omfs_extent_entry e_entry;	/* start of extent entries */
+	struct omfs_extent_entry e_entry[];	/* start of extent entries */
 };
 
 #endif