[PATCH][next] fs: omfs: Use flexible-array member in struct omfs_extent

Gustavo A. R. Silva
2023-07-14T19:55:52Z

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
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
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 de8f57ee39ec..6b580b9da8e3 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
--
2.34.1