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

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Message-ID: <20200226212612.GA4663@embeddedor>
Date:   Wed, 26 Feb 2020 15:26:12 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Cc:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] xen: 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 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
---
 drivers/xen/xen-pciback/pciback.h | 2 +-
 include/xen/interface/io/tpmif.h  | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/xen-pciback/pciback.h b/drivers/xen/xen-pciback/pciback.h
index ce1077e32466..7c95516a860f 100644
--- a/drivers/xen/xen-pciback/pciback.h
+++ b/drivers/xen/xen-pciback/pciback.h
@@ -52,7 +52,7 @@ struct xen_pcibk_dev_data {
 	unsigned int ack_intr:1; /* .. and ACK-ing */
 	unsigned long handled;
 	unsigned int irq; /* Saved in case device transitions to MSI/MSI-X */
-	char irq_name[0]; /* xen-pcibk[000:04:00.0] */
+	char irq_name[]; /* xen-pcibk[000:04:00.0] */
 };
 
 /* Used by XenBus and xen_pcibk_ops.c */
diff --git a/include/xen/interface/io/tpmif.h b/include/xen/interface/io/tpmif.h
index 28e7dcd75e82..f8aa8bac5196 100644
--- a/include/xen/interface/io/tpmif.h
+++ b/include/xen/interface/io/tpmif.h
@@ -46,7 +46,7 @@ struct vtpm_shared_page {
 	uint8_t pad;
 
 	uint8_t nr_extra_pages;  /* extra pages for long packets; may be zero */
-	uint32_t extra_pages[0]; /* grant IDs; length in nr_extra_pages */
+	uint32_t extra_pages[]; /* grant IDs; length in nr_extra_pages */
 };
 
 #endif
-- 
2.25.0