[PATCH] fs/binfmt_elf.c: Replace zero-length array with flexible-array

[Gustavo A. R. Silva]

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]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

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 <gustavoars@xxxxxxxxxx>
---
 fs/binfmt_elf.c         |    2 +-
 fs/hfs/btree.h          |    2 +-
 fs/hfsplus/hfsplus_fs.h |    2 +-
 fs/isofs/rock.h         |    8 ++++----
 fs/select.c             |    4 ++--
 include/linux/fs.h      |    4 ++--
 6 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 13f25e241ac4..6a7f1fc26eb1 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1664,7 +1664,7 @@ struct elf_thread_core_info {
 	struct elf_thread_core_info *next;
 	struct task_struct *task;
 	struct elf_prstatus prstatus;
-	struct memelfnote notes[0];
+	struct memelfnote notes[];
 };
 
 struct elf_note_info {
diff --git a/fs/hfs/btree.h b/fs/hfs/btree.h
index dcc2aab1b2c4..4ba45caf5939 100644
--- a/fs/hfs/btree.h
+++ b/fs/hfs/btree.h
@@ -60,7 +60,7 @@ struct hfs_bnode {
 	wait_queue_head_t lock_wq;
 	atomic_t refcnt;
 	unsigned int page_offset;
-	struct page *page[0];
+	struct page *page[];
 };
 
 #define HFS_BNODE_ERROR		0
diff --git a/fs/hfsplus/hfsplus_fs.h b/fs/hfsplus/hfsplus_fs.h
index 3b03fff68543..a92de5199ec3 100644
--- a/fs/hfsplus/hfsplus_fs.h
+++ b/fs/hfsplus/hfsplus_fs.h
@@ -117,7 +117,7 @@ struct hfs_bnode {
 	wait_queue_head_t lock_wq;
 	atomic_t refcnt;
 	unsigned int page_offset;
-	struct page *page[0];
+	struct page *page[];
 };
 
 #define HFS_BNODE_LOCK		0
diff --git a/fs/isofs/rock.h b/fs/isofs/rock.h
index 1558cf22ef8a..ee9660e9671c 100644
--- a/fs/isofs/rock.h
+++ b/fs/isofs/rock.h
@@ -22,7 +22,7 @@ struct SU_ER_s {
 	__u8 len_des;
 	__u8 len_src;
 	__u8 ext_ver;
-	__u8 data[0];
+	__u8 data[];
 } __attribute__ ((packed));
 
 struct RR_RR_s {
@@ -44,7 +44,7 @@ struct RR_PN_s {
 struct SL_component {
 	__u8 flags;
 	__u8 len;
-	__u8 text[0];
+	__u8 text[];
 } __attribute__ ((packed));
 
 struct RR_SL_s {
@@ -54,7 +54,7 @@ struct RR_SL_s {
 
 struct RR_NM_s {
 	__u8 flags;
-	char name[0];
+	char name[];
 } __attribute__ ((packed));
 
 struct RR_CL_s {
@@ -71,7 +71,7 @@ struct stamp {
 
 struct RR_TF_s {
 	__u8 flags;
-	struct stamp times[0];	/* Variable number of these beasts */
+	struct stamp times[];	/* Variable number of these beasts */
 } __attribute__ ((packed));
 
 /* Linux-specific extension for transparent decompression */
diff --git a/fs/select.c b/fs/select.c
index 11d0285d46b7..f38a8a7480f7 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -97,7 +97,7 @@ u64 select_estimate_accuracy(struct timespec64 *tv)
 struct poll_table_page {
 	struct poll_table_page * next;
 	struct poll_table_entry * entry;
-	struct poll_table_entry entries[0];
+	struct poll_table_entry entries[];
 };
 
 #define POLL_TABLE_FULL(table) \
@@ -826,7 +826,7 @@ SYSCALL_DEFINE1(old_select, struct sel_arg_struct __user *, arg)
 struct poll_list {
 	struct poll_list *next;
 	int len;
-	struct pollfd entries[0];
+	struct pollfd entries[];
 };
 
 #define POLLFD_PER_PAGE  ((PAGE_SIZE-sizeof(struct poll_list)) / sizeof(struct pollfd))
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 4f6f59b4f22a..f14635d58813 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -983,7 +983,7 @@ struct file_handle {
 	__u32 handle_bytes;
 	int handle_type;
 	/* file identifier */
-	unsigned char f_handle[0];
+	unsigned char f_handle[];
 };
 
 static inline struct file *get_file(struct file *f)
@@ -3475,7 +3475,7 @@ static inline ino_t parent_ino(struct dentry *dentry)
  */
 struct simple_transaction_argresp {
 	ssize_t size;
-	char data[0];
+	char data[];
 };
 
 #define SIMPLE_TRANSACTION_LIMIT (PAGE_SIZE - sizeof(struct simple_transaction_argresp))