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: KSPP/linux#78
Build-tested-by: kernel test robot <[email protected]>
Link: https://lore.kernel.org/lkml/62b675ec.wKX6AOZ6cbE71vtF%[email protected]/
Acked-by: Dan Williams <[email protected]> # For ndctl.h
Signed-off-by: Gustavo A. R. Silva <[email protected]>

Author: GustavoARSilva

arch/m68k/include/uapi/asm/bootinfo.h

@@ -34,7 +34,7 @@
 struct bi_record {
 	__be16 tag;			/* tag ID */
 	__be16 size;			/* size of record (in bytes) */
-	__be32 data[0];			/* data */
+	__be32 data[];			/* data */
 };
 
 
@@ -168,7 +168,7 @@ struct bootversion {
 	struct {
 		__be32 machtype;
 		__be32 version;
-	} machversions[0];
+	} machversions[];
 } __packed;
 
 #endif /* __ASSEMBLY__ */

arch/mips/include/uapi/asm/ucontext.h

@@ -60,7 +60,7 @@ struct ucontext {
 	sigset_t		uc_sigmask;
 
 	/* Extended context structures may follow ucontext */
-	unsigned long long	uc_extcontext[0];
+	unsigned long long	uc_extcontext[];
 };
 
 #endif /* __MIPS_UAPI_ASM_UCONTEXT_H */

arch/s390/include/uapi/asm/hwctrset.h

@@ -30,18 +30,18 @@ struct s390_ctrset_start {		/* Set CPUs to operate on */
 struct s390_ctrset_setdata {		/* Counter set data */
 	__u32 set;			/* Counter set number */
 	__u32 no_cnts;			/* # of counters stored in cv[] */
-	__u64 cv[0];			/* Counter values (variable length) */
+	__u64 cv[];			/* Counter values (variable length) */
 };
 
 struct s390_ctrset_cpudata {		/* Counter set data per CPU */
 	__u32 cpu_nr;			/* CPU number */
 	__u32 no_sets;			/* # of counters sets in data[] */
-	struct s390_ctrset_setdata data[0];
+	struct s390_ctrset_setdata data[];
 };
 
 struct s390_ctrset_read {		/* Structure to get all ctr sets */
 	__u64 no_cpus;			/* Total # of CPUs data taken from */
-	struct s390_ctrset_cpudata data[0];
+	struct s390_ctrset_cpudata data[];
 };
 
 #define S390_HWCTR_MAGIC	'C'	/* Random magic # for ioctls */

arch/x86/include/uapi/asm/bootparam.h

@@ -52,7 +52,7 @@ struct setup_data {
 	__u64 next;
 	__u32 type;
 	__u32 len;
-	__u8 data[0];
+	__u8 data[];
 };
 
 /* extensible setup indirect data node */

arch/x86/include/uapi/asm/kvm.h

@@ -198,13 +198,13 @@ struct kvm_msrs {
 	__u32 nmsrs; /* number of msrs in entries */
 	__u32 pad;
 
-	struct kvm_msr_entry entries[0];
+	struct kvm_msr_entry entries[];
 };
 
 /* for KVM_GET_MSR_INDEX_LIST */
 struct kvm_msr_list {
 	__u32 nmsrs; /* number of msrs in entries */
-	__u32 indices[0];
+	__u32 indices[];
 };
 
 /* Maximum size of any access bitmap in bytes */
@@ -241,7 +241,7 @@ struct kvm_cpuid_entry {
 struct kvm_cpuid {
 	__u32 nent;
 	__u32 padding;
-	struct kvm_cpuid_entry entries[0];
+	struct kvm_cpuid_entry entries[];
 };
 
 struct kvm_cpuid_entry2 {
@@ -263,7 +263,7 @@ struct kvm_cpuid_entry2 {
 struct kvm_cpuid2 {
 	__u32 nent;
 	__u32 padding;
-	struct kvm_cpuid_entry2 entries[0];
+	struct kvm_cpuid_entry2 entries[];
 };
 
 /* for KVM_GET_PIT and KVM_SET_PIT */
@@ -389,7 +389,7 @@ struct kvm_xsave {
 	 * the contents of CPUID leaf 0xD on the host.
 	 */
 	__u32 region[1024];
-	__u32 extra[0];
+	__u32 extra[];
 };
 
 #define KVM_MAX_XCRS	16
@@ -516,7 +516,7 @@ struct kvm_pmu_event_filter {
 	__u32 fixed_counter_bitmap;
 	__u32 flags;
 	__u32 pad[4];
-	__u64 events[0];
+	__u64 events[];
 };
 
 #define KVM_PMU_EVENT_ALLOW 0

include/uapi/drm/i915_drm.h

@@ -2123,7 +2123,7 @@ struct i915_context_engines_load_balance {
 
 	__u64 mbz64; /* reserved for future use; must be zero */
 
-	struct i915_engine_class_instance engines[0];
+	struct i915_engine_class_instance engines[];
 } __attribute__((packed));
 
 #define I915_DEFINE_CONTEXT_ENGINES_LOAD_BALANCE(name__, N__) struct { \
@@ -2161,7 +2161,7 @@ struct i915_context_engines_bond {
 	__u64 flags; /* all undefined flags must be zero */
 	__u64 mbz64[4]; /* reserved for future use; must be zero */
 
-	struct i915_engine_class_instance engines[0];
+	struct i915_engine_class_instance engines[];
 } __attribute__((packed));
 
 #define I915_DEFINE_CONTEXT_ENGINES_BOND(name__, N__) struct { \
@@ -2288,7 +2288,7 @@ struct i915_context_engines_parallel_submit {
 	 * length = width (i) * num_siblings (j)
 	 * index = j + i * num_siblings
 	 */
-	struct i915_engine_class_instance engines[0];
+	struct i915_engine_class_instance engines[];
 
 } __packed;
 

include/uapi/linux/blkzoned.h

@@ -130,7 +130,7 @@ struct blk_zone_report {
 	__u64		sector;
 	__u32		nr_zones;
 	__u32		flags;
-	struct blk_zone zones[0];
+	struct blk_zone zones[];
 };
 
 /**

include/uapi/linux/bpf.h

@@ -79,7 +79,7 @@ struct bpf_insn {
 /* Key of an a BPF_MAP_TYPE_LPM_TRIE entry */
 struct bpf_lpm_trie_key {
 	__u32	prefixlen;	/* up to 32 for AF_INET, 128 for AF_INET6 */
-	__u8	data[0];	/* Arbitrary size */
+	__u8	data[];	/* Arbitrary size */
 };
 
 struct bpf_cgroup_storage_key {

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 {
@@ -644,15 +644,15 @@ 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 {
 	__u32	bytes_left;	/* out -- bytes not needed to deliver output */
 	__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 {

include/uapi/linux/btrfs_tree.h

@@ -575,7 +575,7 @@ struct btrfs_inode_extref {
 	__le64 parent_objectid;
 	__le64 index;
 	__le16 name_len;
-	__u8   name[0];
+	__u8   name[];
 	/* name goes here */
 } __attribute__ ((__packed__));