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btrfs: delayed-inode: Replace zero-length array with flexible-array member 

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>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Gustavo A. R. Silva 2020-03-06 10:49:12 -06:00 committed by David Sterba
parent 29566c9c77
commit 17b238acf7
1 changed files with 1 additions and 1 deletions
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2
fs/btrfs/delayed-inode.h
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@ -70,7 +70,7 @@ struct btrfs_delayed_item {
refcount_t refs; refcount_t refs;
int ins_or_del; int ins_or_del;
u32 data_len; u32 data_len;
char data[0]; char data[];
}; };
static inline void btrfs_init_delayed_root( static inline void btrfs_init_delayed_root(