niu: Silence randstruct warnings

Clang randstruct gets upset when it sees struct addresspace (which is
randomized) being assigned to a struct page (which is not randomized):

drivers/net/ethernet/sun/niu.c:3385:12: error: casting from randomized structure pointer type 'struct address_space *' to 'struct page *'
                        *link = (struct page *) page->mapping;
                                ^

It looks like niu.c is looking for an in-line place to chain its allocated
pages together and is overloading the "mapping" member, as it is unused.
This is very non-standard, and is expected to be cleaned up in the
future[1], but there is no "correct" way to handle it today.

No meaningful machine code changes result after this change, and source
readability is improved.

Drop the randstruct exception now that there is no "confusing" cross-type
assignment.

[1] https://lore.kernel.org/lkml/[email protected]/

Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Cc: Paolo Abeni <[email protected]>
Cc: Du Cheng <[email protected]>
Cc: Christophe JAILLET <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: William Kucharski <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Acked-by: Jakub Kicinski <[email protected]>
Link: https://lore.kernel.org/lkml/[email protected]
Signed-off-by: Kees Cook <[email protected]>

drivers/net/ethernet/sun/niu.c

@@ -35,6 +35,25 @@
 
 #include "niu.h"
 
+/* This driver wants to store a link to a "next page" within the
+ * page struct itself by overloading the content of the "mapping"
+ * member. This is not expected by the page API, but does currently
+ * work. However, the randstruct plugin gets very bothered by this
+ * case because "mapping" (struct address_space) is randomized, so
+ * casts to/from it trigger warnings. Hide this by way of a union,
+ * to create a typed alias of "mapping", since that's how it is
+ * actually being used here.
+ */
+union niu_page {
+	struct page page;
+	struct {
+		unsigned long __flags;	/* unused alias of "flags" */
+		struct list_head __lru;	/* unused alias of "lru" */
+		struct page *next;	/* alias of "mapping" */
+	};
+};
+#define niu_next_page(p)	container_of(p, union niu_page, page)->next
+
 #define DRV_MODULE_NAME		"niu"
 #define DRV_MODULE_VERSION	"1.1"
 #define DRV_MODULE_RELDATE	"Apr 22, 2010"
@@ -3283,7 +3302,7 @@ static struct page *niu_find_rxpage(struct rx_ring_info *rp, u64 addr,
 
 	addr &= PAGE_MASK;
 	pp = &rp->rxhash[h];
-	for (; (p = *pp) != NULL; pp = (struct page **) &p->mapping) {
+	for (; (p = *pp) != NULL; pp = &niu_next_page(p)) {
 		if (p->index == addr) {
 			*link = pp;
 			goto found;
@@ -3300,7 +3319,7 @@ static void niu_hash_page(struct rx_ring_info *rp, struct page *page, u64 base)
 	unsigned int h = niu_hash_rxaddr(rp, base);
 
 	page->index = base;
-	page->mapping = (struct address_space *) rp->rxhash[h];
+	niu_next_page(page) = rp->rxhash[h];
 	rp->rxhash[h] = page;
 }
 
@@ -3382,11 +3401,11 @@ static int niu_rx_pkt_ignore(struct niu *np, struct rx_ring_info *rp)
 		rcr_size = rp->rbr_sizes[(val & RCR_ENTRY_PKTBUFSZ) >>
 					 RCR_ENTRY_PKTBUFSZ_SHIFT];
 		if ((page->index + PAGE_SIZE) - rcr_size == addr) {
-			*link = (struct page *) page->mapping;
+			*link = niu_next_page(page);
 			np->ops->unmap_page(np->device, page->index,
 					    PAGE_SIZE, DMA_FROM_DEVICE);
 			page->index = 0;
-			page->mapping = NULL;
+			niu_next_page(page) = NULL;
 			__free_page(page);
 			rp->rbr_refill_pending++;
 		}
@@ -3451,11 +3470,11 @@ static int niu_process_rx_pkt(struct napi_struct *napi, struct niu *np,
 
 		niu_rx_skb_append(skb, page, off, append_size, rcr_size);
 		if ((page->index + rp->rbr_block_size) - rcr_size == addr) {
-			*link = (struct page *) page->mapping;
+			*link = niu_next_page(page);
 			np->ops->unmap_page(np->device, page->index,
 					    PAGE_SIZE, DMA_FROM_DEVICE);
 			page->index = 0;
-			page->mapping = NULL;
+			niu_next_page(page) = NULL;
 			rp->rbr_refill_pending++;
 		} else
 			get_page(page);
@@ -3518,13 +3537,13 @@ static void niu_rbr_free(struct niu *np, struct rx_ring_info *rp)
 
 		page = rp->rxhash[i];
 		while (page) {
-			struct page *next = (struct page *) page->mapping;
+			struct page *next = niu_next_page(page);
 			u64 base = page->index;
 
 			np->ops->unmap_page(np->device, base, PAGE_SIZE,
 					    DMA_FROM_DEVICE);
 			page->index = 0;
-			page->mapping = NULL;
+			niu_next_page(page) = NULL;
 
 			__free_page(page);
 
@@ -6440,8 +6459,7 @@ static void niu_reset_buffers(struct niu *np)
 
 				page = rp->rxhash[j];
 				while (page) {
-					struct page *next =
-						(struct page *) page->mapping;
+					struct page *next = niu_next_page(page);
 					u64 base = page->index;
 					base = base >> RBR_DESCR_ADDR_SHIFT;
 					rp->rbr[k++] = cpu_to_le32(base);
@@ -10176,6 +10194,9 @@ static int __init niu_init(void)
 
 	BUILD_BUG_ON(PAGE_SIZE < 4 * 1024);
 
+	BUILD_BUG_ON(offsetof(struct page, mapping) !=
+		     offsetof(union niu_page, next));
+
 	niu_debug = netif_msg_init(debug, NIU_MSG_DEFAULT);
 
 #ifdef CONFIG_SPARC64

scripts/gcc-plugins/randomize_layout_plugin.c

@@ -46,8 +46,6 @@ struct whitelist_entry {
 };
 
 static const struct whitelist_entry whitelist[] = {
-	/* NIU overloads mapping with page struct */
-	{ "drivers/net/ethernet/sun/niu.c", "page", "address_space" },
 	/* unix_skb_parms via UNIXCB() buffer */
 	{ "net/unix/af_unix.c", "unix_skb_parms", "char" },
 	{ }