[PATCH v2][next] PCI/P2PDMA: Fix potential undefined behavior bug in struct pci_p2pdma_pagemap

[Gustavo A. R. Silva]

`struct dev_pagemap` is a flexible structure, which means that it
contains a flexible-array member at the bottom. This could potentially
lead to an overwrite of the objects following `pgmap` in `struct
pci_p2pdma_pagemap`, when `nr_range > 1`. This is currently not the
case (notice that `nr_range` is hardcoded to `1`), however as commit
b7b3c01b1915 ("mm/memremap_pages: support multiple ranges per invocation")
mentions in the subject line, this code can `support multiple
ranges per invocation`. So, we'd better prevent any problems that may
arise in the future.

Fix this by placing the declaration of object `pgmap` at the end of
`struct pci_p2pdma_pagemap`.

-Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
ready to enable it globally.

Reviewed-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
---
Changes in v2:
 - Remove `Fixes:` tags. (Logan Gunthorpe)
 - Update changelog text. Make it clear that `nr_range` is hardcoded to `1`
   (Logan Gunthorpe)
 - Update subject.
 - Add Logan's RB.

v1:
 Link: https://lore.kernel.org/linux-hardening/ZRnf6wVOu0IJQ2Ok@work/

 drivers/pci/p2pdma.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
index fa7370f9561a..ab34d3d36a64 100644
--- a/drivers/pci/p2pdma.c
+++ b/drivers/pci/p2pdma.c
@@ -28,9 +28,9 @@ struct pci_p2pdma {
 };
 
 struct pci_p2pdma_pagemap {
-	struct dev_pagemap pgmap;
 	struct pci_dev *provider;
 	u64 bus_offset;
+	struct dev_pagemap pgmap;
 };
 
 static struct pci_p2pdma_pagemap *to_p2p_pgmap(struct dev_pagemap *pgmap)
-- 
2.34.1