[PATCH] md/raid1: Replace zero-length array with flexible-array

Gustavo A. R. Silva
2020-05-07T19:17:45Z

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>
---
drivers/md/md-linear.h | 2 +-
drivers/md/raid1.h | 2 +-
drivers/md/raid10.h | 2 +-
3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/md/md-linear.h b/drivers/md/md-linear.h
index 8381d651d4ed..24e97db50ebb 100644
--- a/drivers/md/md-linear.h
+++ b/drivers/md/md-linear.h
@@ -12,6 +12,6 @@ struct linear_conf
struct rcu_head rcu;
sector_t array_sectors;
int raid_disks; /* a copy of mddev->raid_disks */
- struct dev_info disks[0];
+ struct dev_info disks[];
};
#endif
diff --git a/drivers/md/raid1.h b/drivers/md/raid1.h
index e7ccad898736..b7eb09e8c025 100644
--- a/drivers/md/raid1.h
+++ b/drivers/md/raid1.h
@@ -180,7 +180,7 @@ struct r1bio {
* if the IO is in WRITE direction, then multiple bios are used.
* We choose the number when they are allocated.
*/
- struct bio *bios[0];
+ struct bio *bios[];
/* DO NOT PUT ANY NEW FIELDS HERE - bios array is contiguously alloced*/
};

diff --git a/drivers/md/raid10.h b/drivers/md/raid10.h
index d3eaaf3eb1bc..79cd2b7d3128 100644
--- a/drivers/md/raid10.h
+++ b/drivers/md/raid10.h
@@ -153,7 +153,7 @@ struct r10bio {
};
sector_t addr;
int devnum;
- } devs[0];
+ } devs[];
};

/* bits for r10bio.state */