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

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Message-ID: <20200507185550.GA14715@embeddedor>
Date:   Thu, 7 May 2020 13:55:50 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Dave Kleikamp <shaggy@...nel.org>
Cc:     jfs-discussion@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: [PATCH] JFS: 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/jfs/jfs_dtree.c |    2 +-
 fs/jfs/jfs_xattr.h |    4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/jfs/jfs_dtree.c b/fs/jfs/jfs_dtree.c
index 3acc954f7c04..837d42f61464 100644
--- a/fs/jfs/jfs_dtree.c
+++ b/fs/jfs/jfs_dtree.c
@@ -2964,7 +2964,7 @@ struct jfs_dirent {
 	loff_t position;
 	int ino;
 	u16 name_len;
-	char name[0];
+	char name[];
 };

 /*
diff --git a/fs/jfs/jfs_xattr.h b/fs/jfs/jfs_xattr.h
index f0558b3348da..c50167a7bc50 100644
--- a/fs/jfs/jfs_xattr.h
+++ b/fs/jfs/jfs_xattr.h
@@ -17,12 +17,12 @@ struct jfs_ea {
 	u8 flag;	/* Unused? */
 	u8 namelen;	/* Length of name */
 	__le16 valuelen;	/* Length of value */
-	char name[0];	/* Attribute name (includes null-terminator) */
+	char name[];	/* Attribute name (includes null-terminator) */
 };			/* Value immediately follows name */

 struct jfs_ea_list {
 	__le32 size;		/* overall size */
-	struct jfs_ea ea[0];	/* Variable length list */
+	struct jfs_ea ea[];	/* Variable length list */
 };

 /* Macros for defining maxiumum number of bytes supported for EAs */