[PATCH] iommu: Use C99 flexible array in fwspec

[PATCH] iommu: Use C99 flexible array in fwspec

[Robin Murphy]

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@arm.com>
---

Before the Coccinelle police catch up with me... :)

Deliberately no fixes tag, since the original code predates
struct_size(), and it's really just a cosmetic cleanup that
shouldn't be backported anyway.

Robin.

 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 3e3528436e0b..660eea8d1d2f 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -2405,7 +2405,8 @@ 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;
 
@@ -2432,15 +2433,15 @@ 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;
 
@@ -2450,7 +2451,7 @@ 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 */
-- 
2.23.0.dirty

Re: [PATCH] iommu: Use C99 flexible array in fwspec

[Joerg Roedel]

On Thu, Feb 13, 2020 at 02:00:21PM +0000, Robin Murphy wrote:
> 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@arm.com>
> ---
> 
> Before the Coccinelle police catch up with me... :)

Applied, thanks. You should be safe now :)