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

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Date:   Thu, 7 May 2020 14:05:44 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] PCI: 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/pci/pci.c   |    2 +-
 include/linux/pci.h |    4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 595fcf59843f..bb78f580814e 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -1578,7 +1578,7 @@ EXPORT_SYMBOL(pci_restore_state);

 struct pci_saved_state {
 	u32 config_space[16];
-	struct pci_cap_saved_data cap[0];
+	struct pci_cap_saved_data cap[];
 };

 /**
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 83ce1cdf5676..0453ee458ab1 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -279,7 +279,7 @@ struct pci_cap_saved_data {
 	u16		cap_nr;
 	bool		cap_extended;
 	unsigned int	size;
-	u32		data[0];
+	u32		data[];
 };

 struct pci_cap_saved_state {
@@ -532,7 +532,7 @@ struct pci_host_bridge {
 			resource_size_t start,
 			resource_size_t size,
 			resource_size_t align);
-	unsigned long	private[0] ____cacheline_aligned;
+	unsigned long	private[] ____cacheline_aligned;
 };

 #define	to_pci_host_bridge(n) container_of(n, struct pci_host_bridge, dev)