Commit 7aba5dcc authored Mar 09, 2020 by Gustavo A. R. Silva Committed by Dave Kleikamp Mar 09, 2020

jfs: Replace zero-length array with flexible-array member

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]

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 76497732 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>

parent 2c523b34

fs/jfs/jfs_dtree.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -2964,7 +2964,7 @@ struct jfs_dirent {
		loff_t position;
		int ino;
		u16 name_len;
		char name[0];
		char name[];
		};

		/*

fs/jfs/jfs_xattr.h

+2 −2

Original line number	Diff line number	Diff line
		@@ -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 */

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