netlink: fix snl_writer and linear_buffer re-allocation logic

- Use the correct base pointer after re-allocation to avoid buffer
  overflows.

- Maintain correct snl_writer.size, which avoids redundant memory
  allocation, e.g. a need for ~1k bytes may end up with ~32k
  linear_buffer actually allocated.

This fixes a pfctl regression at least for armv7 after the addrule logic
migration to netlink:
  ffbf25951e7b ("pf: convert rule addition to netlink")

The add rule command creates a bigger than default size netlink requests
which triggers the re-allocation logic.

Reviewed by:	kp
MFC after:	2 weeks
Differnetial Revision:	https://reviews.freebsd.org/D43003

(cherry picked from commit 0c511bafdd5b309505c13c8dc7c6816686d1e103)

1 files changed, 11 insertions, 7 deletions

diff --git a/sys/netlink/netlink_snl.h b/sys/netlink/netlink_snl.h
index 8bb7b076b7b7..7bdf5424ddf3 100644
--- a/sys/netlink/netlink_snl.h
+++ b/sys/netlink/netlink_snl.h
@@ -1036,19 +1036,23 @@ snl_realloc_msg_buffer(struct snl_writer *nw, size_t sz)
 	if (nw->error)
 		return (false);
 
-	void *new_base = snl_allocz(nw->ss, new_size);
-	if (new_base == NULL) {
+	if (snl_allocz(nw->ss, new_size) == NULL) {
 		nw->error = true;
 		return (false);
 	}
+	nw->size = new_size;
 
-	memcpy(new_base, nw->base, nw->offset);
-	if (nw->hdr != NULL) {
-		int hdr_off = (char *)(nw->hdr) - nw->base;
+	void *new_base = nw->ss->lb->base;
+	if (new_base != nw->base) {
+		memcpy(new_base, nw->base, nw->offset);
+		if (nw->hdr != NULL) {
+			int hdr_off = (char *)(nw->hdr) - nw->base;
 
-		nw->hdr = (struct nlmsghdr *)(void *)((char *)new_base + hdr_off);
+			nw->hdr = (struct nlmsghdr *)
+			    (void *)((char *)new_base + hdr_off);
+		}
+		nw->base = new_base;
 	}
-	nw->base = new_base;
 
 	return (true);
 }