linux-kernel - [PATCH] Squashfs: Replace zero-length array with flexible-array

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Date:   Thu, 7 May 2020 14:24:00 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Phillip Lougher <phillip@...ashfs.org.uk>
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH] Squashfs: Replace zero-length array with flexible-array

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@...nel.org>
---
 fs/squashfs/squashfs_fs.h |   16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/fs/squashfs/squashfs_fs.h b/fs/squashfs/squashfs_fs.h
index 7187bd1a30ea..8d64edb80ebf 100644
--- a/fs/squashfs/squashfs_fs.h
+++ b/fs/squashfs/squashfs_fs.h
@@ -262,7 +262,7 @@ struct squashfs_dir_index {
 	__le32			index;
 	__le32			start_block;
 	__le32			size;
-	unsigned char		name[0];
+	unsigned char		name[];
 };

 struct squashfs_base_inode {
@@ -327,7 +327,7 @@ struct squashfs_symlink_inode {
 	__le32			inode_number;
 	__le32			nlink;
 	__le32			symlink_size;
-	char			symlink[0];
+	char			symlink[];
 };

 struct squashfs_reg_inode {
@@ -341,7 +341,7 @@ struct squashfs_reg_inode {
 	__le32			fragment;
 	__le32			offset;
 	__le32			file_size;
-	__le16			block_list[0];
+	__le16			block_list[];
 };

 struct squashfs_lreg_inode {
@@ -358,7 +358,7 @@ struct squashfs_lreg_inode {
 	__le32			fragment;
 	__le32			offset;
 	__le32			xattr;
-	__le16			block_list[0];
+	__le16			block_list[];
 };

 struct squashfs_dir_inode {
@@ -389,7 +389,7 @@ struct squashfs_ldir_inode {
 	__le16			i_count;
 	__le16			offset;
 	__le32			xattr;
-	struct squashfs_dir_index	index[0];
+	struct squashfs_dir_index	index[];
 };

 union squashfs_inode {
@@ -410,7 +410,7 @@ struct squashfs_dir_entry {
 	__le16			inode_number;
 	__le16			type;
 	__le16			size;
-	char			name[0];
+	char			name[];
 };

 struct squashfs_dir_header {
@@ -428,12 +428,12 @@ struct squashfs_fragment_entry {
 struct squashfs_xattr_entry {
 	__le16			type;
 	__le16			size;
-	char			data[0];
+	char			data[];
 };

 struct squashfs_xattr_val {
 	__le32			vsize;
-	char			value[0];
+	char			value[];
 };

 struct squashfs_xattr_id {