[PATCH v2 01/60][SRU][OEM-5.6] iommu: Use C99 flexible array in fwspec

[You-Sheng Yang]

From: Robin Murphy <robin.murphy at arm.com>

BugLink: https://bugs.launchpad.net/bugs/1876707

Although the 1-element array was a typical pre-C99 way to implement
variable-length structures, and indeed is a fundamental construct in the
APIs of certain other popular platforms, there's no good reason for it
here (and in particular the sizeof() trick is far too "clever" for its
own good). We can just as easily implement iommu_fwspec's preallocation
behaviour using a standard flexible array member, so let's make it look
the way most readers would expect.

Signed-off-by: Robin Murphy <robin.murphy at arm.com>
Signed-off-by: Joerg Roedel <jroedel at suse.de>
(cherry picked from commit 098accf2da940189f4d62d3514d17f8bb05dc6e1)
Signed-off-by: You-Sheng Yang <vicamo.yang at canonical.com>
---
 drivers/iommu/iommu.c | 15 ++++++++-------
 include/linux/iommu.h |  2 +-
 2 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 22b28076d48e..273f35dddaa9 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -2410,7 +2410,8 @@ int iommu_fwspec_init(struct device *dev, struct fwnode_handle *iommu_fwnode,
 	if (fwspec)
 		return ops == fwspec->ops ? 0 : -EINVAL;
 
-	fwspec = kzalloc(sizeof(*fwspec), GFP_KERNEL);
+	/* Preallocate for the overwhelmingly common case of 1 ID */
+	fwspec = kzalloc(struct_size(fwspec, ids, 1), GFP_KERNEL);
 	if (!fwspec)
 		return -ENOMEM;
 
@@ -2437,15 +2438,15 @@ EXPORT_SYMBOL_GPL(iommu_fwspec_free);
 int iommu_fwspec_add_ids(struct device *dev, u32 *ids, int num_ids)
 {
 	struct iommu_fwspec *fwspec = dev_iommu_fwspec_get(dev);
-	size_t size;
-	int i;
+	int i, new_num;
 
 	if (!fwspec)
 		return -EINVAL;
 
-	size = offsetof(struct iommu_fwspec, ids[fwspec->num_ids + num_ids]);
-	if (size > sizeof(*fwspec)) {
-		fwspec = krealloc(fwspec, size, GFP_KERNEL);
+	new_num = fwspec->num_ids + num_ids;
+	if (new_num > 1) {
+		fwspec = krealloc(fwspec, struct_size(fwspec, ids, new_num),
+				  GFP_KERNEL);
 		if (!fwspec)
 			return -ENOMEM;
 
@@ -2455,7 +2456,7 @@ int iommu_fwspec_add_ids(struct device *dev, u32 *ids, int num_ids)
 	for (i = 0; i < num_ids; i++)
 		fwspec->ids[fwspec->num_ids + i] = ids[i];
 
-	fwspec->num_ids += num_ids;
+	fwspec->num_ids = new_num;
 	return 0;
 }
 EXPORT_SYMBOL_GPL(iommu_fwspec_add_ids);
diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index d1b5f4d98569..4d1ba76c9a64 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -592,7 +592,7 @@ struct iommu_fwspec {
 	u32			flags;
 	u32			num_pasid_bits;
 	unsigned int		num_ids;
-	u32			ids[1];
+	u32			ids[];
 };
 
 /* ATS is supported */
-- 
2.25.1