netfilter: ebtables: fix fortify warnings in size_entry_mwt()

When compiling with gcc 13 and CONFIG_FORTIFY_SOURCE=y, the following
warning appears:

In function ‘fortify_memcpy_chk’,
inlined from ‘size_entry_mwt’ at net/bridge/netfilter/ebtables.c:2118:2:
./include/linux/fortify-string.h:592:25: error: call to ‘__read_overflow2_field’
declared with attribute warning: detected read beyond size of field (2nd parameter);
maybe use struct_group()? [-Werror=attribute-warning]
592 | __read_overflow2_field(q_size_field, size);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The compiler is complaining:

memcpy(&offsets[1], &entry->watchers_offset,
sizeof(offsets) - sizeof(offsets[0]));

where memcpy reads beyong &entry->watchers_offset to copy
{watchers,target,next}_offset altogether into offsets[]. Silence the
warning by wrapping these three up via struct_group().

Signed-off-by: GONG, Ruiqi <[e-mail]>
Reviewed-by: Gustavo A. R. Silva <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>

include/uapi/linux/netfilter_bridge/ebtables.h

···
 	unsigned char sourcemsk[ETH_ALEN];
 	unsigned char destmac[ETH_ALEN];
 	unsigned char destmsk[ETH_ALEN];
-	/* sizeof ebt_entry + matches */
-	unsigned int watchers_offset;
-	/* sizeof ebt_entry + matches + watchers */
-	unsigned int target_offset;
-	/* sizeof ebt_entry + matches + watchers + target */
-	unsigned int next_offset;
+	__struct_group(/* no tag */, offsets, /* no attrs */,
+		/* sizeof ebt_entry + matches */
+		unsigned int watchers_offset;
+		/* sizeof ebt_entry + matches + watchers */
+		unsigned int target_offset;
+		/* sizeof ebt_entry + matches + watchers + target */
+		unsigned int next_offset;
+	);
 	unsigned char elems[0] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
 };
 

net/bridge/netfilter/ebtables.c

···
 		return ret;
 
 	offsets[0] = sizeof(struct ebt_entry); /* matches come first */
-	memcpy(&offsets[1], &entry->watchers_offset,
-			sizeof(offsets) - sizeof(offsets[0]));
+	memcpy(&offsets[1], &entry->offsets, sizeof(entry->offsets));
 
 	if (state->buf_kern_start) {
 		buf_start = state->buf_kern_start + state->buf_kern_offset;