| Message ID | 20200309180441.GA2992@embeddedor |
|---|---|
| State | Not Applicable |
| Headers | show |
| Series | [next] ext2: xattr.h: Replace zero-length array with flexible-array member | expand |
On Mon 09-03-20 13:04:41, 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 <gustavo@embeddedor.com> Thanks. I've added the patch to my tree. Honza > --- > fs/ext2/xattr.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/fs/ext2/xattr.h b/fs/ext2/xattr.h > index cee888cdc235..16272e6ddcf4 100644 > --- a/fs/ext2/xattr.h > +++ b/fs/ext2/xattr.h > @@ -39,7 +39,7 @@ struct ext2_xattr_entry { > __le32 e_value_block; /* disk block attribute is stored on (n/i) */ > __le32 e_value_size; /* size of attribute value */ > __le32 e_hash; /* hash value of name and value */ > - char e_name[0]; /* attribute name */ > + char e_name[]; /* attribute name */ > }; > > #define EXT2_XATTR_PAD_BITS 2 > -- > 2.25.0 >
diff --git a/fs/ext2/xattr.h b/fs/ext2/xattr.h index cee888cdc235..16272e6ddcf4 100644 --- a/fs/ext2/xattr.h +++ b/fs/ext2/xattr.h @@ -39,7 +39,7 @@ struct ext2_xattr_entry { __le32 e_value_block; /* disk block attribute is stored on (n/i) */ __le32 e_value_size; /* size of attribute value */ __le32 e_hash; /* hash value of name and value */ - char e_name[0]; /* attribute name */ + char e_name[]; /* attribute name */ }; #define EXT2_XATTR_PAD_BITS 2
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 <gustavo@embeddedor.com> --- fs/ext2/xattr.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)