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ext4: use flexible-array member for xattr structs 

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 7649773293 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20200309180813.GA3347@embeddedor
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Gustavo A. R. Silva 2020-03-09 13:08:13 -05:00 committed by Theodore Ts'o
parent e32ac2459c
commit 47b1030612
1 changed files with 2 additions and 2 deletions
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4
fs/ext4/xattr.h
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@ -48,7 +48,7 @@ struct ext4_xattr_entry {
__le32 e_value_inum; /* inode in which the value is stored */
__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 EXT4_XATTR_PAD_BITS 2
@ -118,7 +118,7 @@ struct ext4_xattr_ibody_find {
struct ext4_xattr_inode_array {
unsigned int count; /* # of used items in the array */
struct inode *inodes[0];
struct inode *inodes[];
};
extern const struct xattr_handler ext4_xattr_user_handler;