[PATCH] jffs2: Replace zero-length array with flexible-array

Gustavo A. R. Silva
2020-05-07T18:51:32Z

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@kernel.org>
---
fs/jffs2/nodelist.h | 2 +-
fs/jffs2/summary.h | 4 ++--
2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/jffs2/nodelist.h b/fs/jffs2/nodelist.h
index 0637271f3770..8ff4d1a1e774 100644
--- a/fs/jffs2/nodelist.h
+++ b/fs/jffs2/nodelist.h
@@ -259,7 +259,7 @@ struct jffs2_full_dirent
uint32_t ino; /* == zero for unlink */
unsigned int nhash;
unsigned char type;
- unsigned char name[0];
+ unsigned char name[];
};

/*
diff --git a/fs/jffs2/summary.h b/fs/jffs2/summary.h
index 60207a2ae952..e4131cb1f1d4 100644
--- a/fs/jffs2/summary.h
+++ b/fs/jffs2/summary.h
@@ -61,7 +61,7 @@ struct jffs2_sum_dirent_flash
jint32_t ino; /* == zero for unlink */
uint8_t nsize; /* dirent name size */
uint8_t type; /* dirent type */
- uint8_t name[0]; /* dirent name */
+ uint8_t name[]; /* dirent name */
} __attribute__((packed));

struct jffs2_sum_xattr_flash
@@ -117,7 +117,7 @@ struct jffs2_sum_dirent_mem
jint32_t ino; /* == zero for unlink */
uint8_t nsize; /* dirent name size */
uint8_t type; /* dirent type */
- uint8_t name[0]; /* dirent name */
+ uint8_t name[]; /* dirent name */
} __attribute__((packed));

struct jffs2_sum_xattr_mem