Commit a4e439a6 authored Mar 09, 2020 by Gustavo A. R. Silva Committed by David Teigland May 12, 2020

dlm: dlm_internal: 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: David Teigland <teigland@redhat.com>

parent 1d2cc5ac

fs/dlm/dlm_internal.h

+3 −3

Original line number	Diff line number	Diff line
		@@ -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[];
		};

		/*

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