linux-kernel - [PATCH][next] isofs: rock.h: Replace zero-length array with flexible-array member

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Message-ID: <20200309181527.GA4134@embeddedor>
Date:   Mon, 9 Mar 2020 13:15:27 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] isofs: rock.h: 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 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
---
 fs/isofs/rock.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

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 */
-- 
2.25.0