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

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Message-ID: <20200507190452.GA15449@embeddedor>
Date:   Thu, 7 May 2020 14:04:52 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>
Cc:     linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: [PATCH] lightnvm: 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>
---
 drivers/nvme/host/lightnvm.c |    2 +-
 include/linux/nvme.h         |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/nvme/host/lightnvm.c b/drivers/nvme/host/lightnvm.c
index ec46693f6b64..3002bf972c6b 100644
--- a/drivers/nvme/host/lightnvm.c
+++ b/drivers/nvme/host/lightnvm.c
@@ -171,7 +171,7 @@ struct nvme_nvm_bb_tbl {
 	__le32	tdresv;
 	__le32	thresv;
 	__le32	rsvd2[8];
-	__u8	blk[0];
+	__u8	blk[];
 };

 struct nvme_nvm_id20_addrf {
diff --git a/include/linux/nvme.h b/include/linux/nvme.h
index 3d5189f46cb1..6ee80a44ed4f 100644
--- a/include/linux/nvme.h
+++ b/include/linux/nvme.h
@@ -1177,7 +1177,7 @@ struct nvmf_disc_rsp_page_hdr {
 	__le64		numrec;
 	__le16		recfmt;
 	__u8		resv14[1006];
-	struct nvmf_disc_rsp_page_entry entries[0];
+	struct nvmf_disc_rsp_page_entry entries[];
 };

 enum {