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

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Date:   Thu, 7 May 2020 13:52:05 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Christine Caulfield <ccaulfie@...hat.com>,
        David Teigland <teigland@...hat.com>
Cc:     cluster-devel@...hat.com, linux-kernel@...r.kernel.org
Subject: [PATCH] dlm: 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/dlm/dlm_internal.h |    6 +++---
 fs/dlm/user.c         |    2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/dlm/dlm_internal.h b/fs/dlm/dlm_internal.h
index 416d9de35679..d231ae5d2c65 100644
--- a/fs/dlm/dlm_internal.h
+++ b/fs/dlm/dlm_internal.h
@@ -421,7 +421,7 @@ struct dlm_message {
 	int			m_bastmode;
 	int			m_asts;
 	int			m_result;	/* 0 or -EXXX */
-	char			m_extra[0];	/* name or lvb */
+	char			m_extra[];	/* name or lvb */
 };

@@ -450,7 +450,7 @@ struct dlm_rcom {
 	uint64_t		rc_id;		/* match reply with request */
 	uint64_t		rc_seq;		/* sender's ls_recover_seq */
 	uint64_t		rc_seq_reply;	/* remote ls_recover_seq */
-	char			rc_buf[0];
+	char			rc_buf[];
 };

 union dlm_packet {
@@ -506,7 +506,7 @@ struct rcom_lock {
 	__le16			rl_wait_type;
 	__le16			rl_namelen;
 	char			rl_name[DLM_RESNAME_MAXLEN];
-	char			rl_lvb[0];
+	char			rl_lvb[];
 };

 /*
diff --git a/fs/dlm/user.c b/fs/dlm/user.c
index 5264bac75115..e5cefa90b1ce 100644
--- a/fs/dlm/user.c
+++ b/fs/dlm/user.c
@@ -46,7 +46,7 @@ struct dlm_lock_params32 {
 	__u32 bastaddr;
 	__u32 lksb;
 	char lvb[DLM_USER_LVB_LEN];
-	char name[0];
+	char name[];
 };

 struct dlm_write_request32 {