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 <[email protected]>
---
fs/jfs/jfs_dtree.c | 2 +-
fs/jfs/jfs_xattr.h | 4 ++--
2 files changed, 3 insertions(+), 3 deletions(-)
diff --git a/fs/jfs/jfs_dtree.c b/fs/jfs/jfs_dtree.c
index 3acc954f7c04..837d42f61464 100644
--- a/fs/jfs/jfs_dtree.c
+++ b/fs/jfs/jfs_dtree.c
@@ -2964,7 +2964,7 @@ struct jfs_dirent {
loff_t position;
int ino;
u16 name_len;
- char name[0];
+ char name[];
};
/*
diff --git a/fs/jfs/jfs_xattr.h b/fs/jfs/jfs_xattr.h
index f0558b3348da..c50167a7bc50 100644
--- a/fs/jfs/jfs_xattr.h
+++ b/fs/jfs/jfs_xattr.h
@@ -17,12 +17,12 @@ struct jfs_ea {
u8 flag; /* Unused? */
u8 namelen; /* Length of name */
__le16 valuelen; /* Length of value */
- char name[0]; /* Attribute name (includes null-terminator) */
+ char name[]; /* Attribute name (includes null-terminator) */
}; /* Value immediately follows name */
struct jfs_ea_list {
__le32 size; /* overall size */
- struct jfs_ea ea[0]; /* Variable length list */
+ struct jfs_ea ea[]; /* Variable length list */
};
/* Macros for defining maxiumum number of bytes supported for EAs */
--
2.25.0
Looks good. I'll queue this for linux-next.
Shaggy
On 3/9/20 1:23 PM, 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 <[email protected]>
> ---
> fs/jfs/jfs_dtree.c | 2 +-
> fs/jfs/jfs_xattr.h | 4 ++--
> 2 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/fs/jfs/jfs_dtree.c b/fs/jfs/jfs_dtree.c
> index 3acc954f7c04..837d42f61464 100644
> --- a/fs/jfs/jfs_dtree.c
> +++ b/fs/jfs/jfs_dtree.c
> @@ -2964,7 +2964,7 @@ struct jfs_dirent {
> loff_t position;
> int ino;
> u16 name_len;
> - char name[0];
> + char name[];
> };
>
> /*
> diff --git a/fs/jfs/jfs_xattr.h b/fs/jfs/jfs_xattr.h
> index f0558b3348da..c50167a7bc50 100644
> --- a/fs/jfs/jfs_xattr.h
> +++ b/fs/jfs/jfs_xattr.h
> @@ -17,12 +17,12 @@ struct jfs_ea {
> u8 flag; /* Unused? */
> u8 namelen; /* Length of name */
> __le16 valuelen; /* Length of value */
> - char name[0]; /* Attribute name (includes null-terminator) */
> + char name[]; /* Attribute name (includes null-terminator) */
> }; /* Value immediately follows name */
>
> struct jfs_ea_list {
> __le32 size; /* overall size */
> - struct jfs_ea ea[0]; /* Variable length list */
> + struct jfs_ea ea[]; /* Variable length list */
> };
>
> /* Macros for defining maxiumum number of bytes supported for EAs */
>