[PATCH] omfs: avoid gcc-10 stringop-overflow warning

[Arnd Bergmann <arnd@kernel.org>]

From: Arnd Bergmann <arnd@arndb.de>

gcc-10 correctly points out a memcpy beyond the end of the structure
it gets copied into:

fs/omfs/file.c: In function 'omfs_grow_extent':
cc1: warning: writing 16 bytes into a region of size 0 [-Wstringop-overflow=]
In file included from fs/omfs/omfs.h:8,
                 from fs/omfs/file.c:11:
fs/omfs/omfs_fs.h:80:27: note: at offset 0 to object 'e_entry' with size 16 declared here
   80 |  struct omfs_extent_entry e_entry; /* start of extent entries */
      |                           ^~~~~~~

This is not a bug however, as the file system layout contains an array
of extent entries instead of just a single one that was part of the
structure definition.

Changing the definition to use a c99 flexible array member makes
makes the code match the actual layout, which avoids the warning and
an odd '+1' in the calculation of the number of extent entries.

Fixes: 8f09e98768c1 ("omfs: add file routines")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 fs/omfs/file.c    | 12 ++++++------
 fs/omfs/omfs_fs.h |  2 +-
 2 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/fs/omfs/file.c b/fs/omfs/file.c
index d1304acf3ff8..4bd9d0c140e1 100644
--- a/fs/omfs/file.c
+++ b/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.e_blocks = ~cpu_to_be64(0ULL);
+	oe->e_entry[0].e_cluster = ~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;
diff --git a/fs/omfs/omfs_fs.h b/fs/omfs/omfs_fs.h
index caecb3d5a344..1ff6b9e41297 100644
--- a/fs/omfs/omfs_fs.h
+++ b/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
-- 
2.29.2