Commit b18ae8dd authored May 07, 2020 by Gustavo A. R. Silva Committed by Mike Snitzer May 20, 2020

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

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

parent bd5c4031

drivers/md/dm-crypt.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -214,7 +214,7 @@ struct crypt_config {
		struct mutex bio_alloc_lock;

		u8 authenc_key; / space for keys in authenc() format (if used) */
		u8 key[0];
		u8 key[];
		};

		#define MIN_IOS 64

drivers/md/dm-integrity.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -92,7 +92,7 @@ struct journal_entry {
		} s;
		__u64 sector;
		} u;
		commit_id_t last_bytes[0];
		commit_id_t last_bytes[];
		/* __u8 tag[0]; */
		};

drivers/md/dm-log-writes.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -127,7 +127,7 @@ struct pending_block {
		char *data;
		u32 datalen;
		struct list_head list;
		struct bio_vec vecs[0];
		struct bio_vec vecs[];
		};

		struct per_bio_data {

drivers/md/dm-raid.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -254,7 +254,7 @@ struct raid_set {
		int mode;
		} journal_dev;

		struct raid_dev dev[0];
		struct raid_dev dev[];
		};

		static void rs_config_backup(struct raid_set rs, struct rs_layout l)

drivers/md/dm-raid1.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -83,7 +83,7 @@ struct mirror_set {
		struct work_struct trigger_event;

		unsigned nr_mirrors;
		struct mirror mirror[0];
		struct mirror mirror[];
		};

		DECLARE_DM_KCOPYD_THROTTLE_WITH_MODULE_PARM(raid1_resync_throttle,

Admin message