From: Gustavo A. R. Silva Date: Fri, 6 Mar 2020 16:51:05 +0000 (-0600) Subject: btrfs: rcu-string: Replace zero-length array with flexible-array member X-Git-Url: http://git.lede-project.org./?a=commitdiff_plain;h=7593f4c53c699699c6968a5fdd795d0fdf99a65d;p=openwrt%2Fstaging%2Fblogic.git btrfs: rcu-string: 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 76497732932f ("cxgb3/l2t: Fix undefined behaviour") Signed-off-by: Gustavo A. R. Silva Reviewed-by: David Sterba Signed-off-by: David Sterba --- diff --git a/fs/btrfs/rcu-string.h b/fs/btrfs/rcu-string.h index a97dc74a4d3d..5c1a617eb25d 100644 --- a/fs/btrfs/rcu-string.h +++ b/fs/btrfs/rcu-string.h @@ -8,7 +8,7 @@ struct rcu_string { struct rcu_head rcu; - char str[0]; + char str[]; }; static inline struct rcu_string *rcu_string_strdup(const char *src, gfp_t mask)