[PATCH] writeback: prefer struct_size over open coded arithmetic

[Len Baker <len.baker@gmx.com>]

As noted in the "Deprecated Interfaces, Language Features, Attributes,
and Conventions" documentation [1], size calculations (especially
multiplication) should not be performed in memory allocator (or similar)
function arguments due to the risk of them overflowing. This could lead
to values wrapping around and a smaller allocation being made than the
caller was expecting. Using those allocations could lead to linear
overflows of heap memory and other misbehaviors.

In this case this is not actually dynamic size: all the operands
involved in the calculation are constant values. However it is best to
refactor this anyway, just to keep the open-coded math idiom out of
code.

So, use the struct_size() helper to do the arithmetic instead of the
argument "size + count * size" in the kzalloc() function.

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments

Signed-off-by: Len Baker <len.baker@gmx.com>
---
 fs/fs-writeback.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 81ec192ce067..f7abff31e026 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -624,8 +624,8 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
 	int nr;
 	bool restart = false;

-	isw = kzalloc(sizeof(*isw) + WB_MAX_INODES_PER_ISW *
-		      sizeof(struct inode *), GFP_KERNEL);
+	isw = kzalloc(struct_size(isw, inodes, WB_MAX_INODES_PER_ISW),
+		      GFP_KERNEL);
 	if (!isw)
 		return restart;

--
2.25.1