treewide: uapi: Replace zero-length arrays with flexible-array members

There is a regular need in the kernel to provide a way to declare
having a dynamically sized set of trailing elements in a structure.
Kernel code should always use “flexible array members”[1] for these
cases. The older style of one-element or zero-length arrays should
no longer be used[2].

This code was transformed with the help of Coccinelle:
(linux-5.19-rc2$ spatch --jobs $(getconf _NPROCESSORS_ONLN) --sp-file script.cocci --include-headers --dir . > output.patch)

@@
identifier S, member, array;
type T1, T2;
@@

struct S {
  ...
  T1 member;
  T2 array[
- 0
  ];
};

-fstrict-flex-arrays=3 is coming and we need to land these changes
to prevent issues like these in the short future:

../fs/minix/dir.c:337:3: warning: 'strcpy' will always overflow; destination buffer has size 0,
but the source string has length 2 (including NUL byte) [-Wfortify-source]
		strcpy(de3->name, ".");
		^

Since these are all [0] to [] changes, the risk to UAPI is nearly zero. If
this breaks anything, we can use a union with a new member name.

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.16/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/78
Build-tested-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/62b675ec.wKX6AOZ6cbE71vtF%25lkp@intel.com/
Acked-by: Dan Williams <dan.j.williams@intel.com> # For ndctl.h
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

1 files changed, 5 insertions, 5 deletions

diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h
index d956b2993970..3d0edbe3b991 100644
--- a/include/uapi/linux/btrfs.h
+++ b/include/uapi/linux/btrfs.h
@@ -93,7 +93,7 @@ struct btrfs_qgroup_inherit {
 	__u64	num_ref_copies;
 	__u64	num_excl_copies;
 	struct btrfs_qgroup_limit lim;
-	__u64	qgroups[0];
+	__u64	qgroups[];
 };
 
 struct btrfs_ioctl_qgroup_limit_args {
@@ -561,7 +561,7 @@ struct btrfs_ioctl_search_args_v2 {
 	__u64 buf_size;		   /* in - size of buffer
 					    * out - on EOVERFLOW: needed size
 					    *       to store item */
-	__u64 buf[0];                       /* out - found items */
+	__u64 buf[];                       /* out - found items */
 };
 
 struct btrfs_ioctl_clone_range_args {
@@ -632,7 +632,7 @@ struct btrfs_ioctl_same_args {
 	__u16 dest_count;	/* in - total elements in info array */
 	__u16 reserved1;
 	__u32 reserved2;
-	struct btrfs_ioctl_same_extent_info info[0];
+	struct btrfs_ioctl_same_extent_info info[];
 };
 
 struct btrfs_ioctl_space_info {
@@ -644,7 +644,7 @@ struct btrfs_ioctl_space_info {
 struct btrfs_ioctl_space_args {
 	__u64 space_slots;
 	__u64 total_spaces;
-	struct btrfs_ioctl_space_info spaces[0];
+	struct btrfs_ioctl_space_info spaces[];
 };
 
 struct btrfs_data_container {
@@ -652,7 +652,7 @@ struct btrfs_data_container {
 	__u32	bytes_missing;	/* out -- additional bytes needed for result */
 	__u32	elem_cnt;	/* out */
 	__u32	elem_missed;	/* out */
-	__u64	val[0];		/* out */
+	__u64	val[];		/* out */
 };
 
 struct btrfs_ioctl_ino_path_args {