[PATCH] cxgb3: Use struct_group() for memcpy() region

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memcpy(), memmove(), and memset(), avoid
intentionally writing across neighboring fields.

Use struct_group() in struct rss_hdr around members imm_data and intr_gen,
so they can be referenced together. This will allow memcpy() and sizeof()
to more easily reason about sizes, improve readability, and avoid future
warnings about writing beyond the end of imm_data.

"pahole" shows no size nor member offset changes to struct rss_hdr.
"objdump -d" shows no object code changes.

Signed-off-by: Kees Cook <redacted>
---
 drivers/net/ethernet/chelsio/cxgb3/sge.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/chelsio/cxgb3/sge.c b/drivers/net/ethernet/chelsio/cxgb3/sge.c
index c3afec1041f8..70f528a9c727 100644
--- a/drivers/net/ethernet/chelsio/cxgb3/sge.c
+++ b/drivers/net/ethernet/chelsio/cxgb3/sge.c

@@ -126,8 +126,10 @@ struct rsp_desc {		/* response queue descriptor */
 	struct rss_header rss_hdr;
 	__be32 flags;
 	__be32 len_cq;
-	u8 imm_data[47];
-	u8 intr_gen;
+	struct_group(immediate,
+		u8 imm_data[47];
+		u8 intr_gen;
+	);
 };
 
 /*

@@ -925,7 +927,8 @@ static inline struct sk_buff *get_imm_packet(const struct rsp_desc *resp)
 
 	if (skb) {
 		__skb_put(skb, IMMED_PKT_SIZE);
-		skb_copy_to_linear_data(skb, resp->imm_data, IMMED_PKT_SIZE);
+		BUILD_BUG_ON(IMMED_PKT_SIZE != sizeof(resp->immediate));
+		skb_copy_to_linear_data(skb, &resp->immediate, IMMED_PKT_SIZE);
 	}
 	return skb;
 }

-- 
2.30.2

Hello:

This patch was applied to netdev/net-next.git (master)
by David S. Miller [off-list ref]:

On Thu, 18 Nov 2021 10:41:42 -0800 you wrote:

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memcpy(), memmove(), and memset(), avoid
intentionally writing across neighboring fields.

Use struct_group() in struct rss_hdr around members imm_data and intr_gen,
so they can be referenced together. This will allow memcpy() and sizeof()
to more easily reason about sizes, improve readability, and avoid future
warnings about writing beyond the end of imm_data.

[...]

Here is the summary with links:
  - cxgb3: Use struct_group() for memcpy() region
    https://git.kernel.org/netdev/net-next/c/88181f1d3474

You are awesome, thank you!
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