Merge branch 'bpf-sockmap-sg-api-fixes'

Prashant Bhole says:

====================
These patches fix sg api usage in sockmap. Previously sockmap didn't
use sg_init_table(), which caused hitting BUG_ON in sg api, when
CONFIG_DEBUG_SG is enabled

v1: added sg_init_table() calls wherever needed.

v2:
- Patch1 adds new helper function in sg api. sg_init_marker()
- Patch2 sg_init_marker() and sg_init_table() in appropriate places

Backgroud:
While reviewing v1, John Fastabend raised a valid point about
unnecessary memset in sg_init_table() because sockmap uses sg table
which embedded in a struct. As enclosing struct is zeroed out, there
is unnecessary memset in sg_init_table.

So Daniel Borkmann suggested to define another static inline function
in scatterlist.h which only initializes sg_magic. Also this function
will be called from sg_init_table. From this suggestion I defined a
function sg_init_marker() which sets sg_magic and calls sg_mark_end()
====================

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/linux/scatterlist.h

@@ -248,6 +248,24 @@ static inline void *sg_virt(struct scatterlist *sg)
 	return page_address(sg_page(sg)) + sg->offset;
 }
 
+/**
+ * sg_init_marker - Initialize markers in sg table
+ * @sgl:	   The SG table
+ * @nents:	   Number of entries in table
+ *
+ **/
+static inline void sg_init_marker(struct scatterlist *sgl,
+				  unsigned int nents)
+{
+#ifdef CONFIG_DEBUG_SG
+	unsigned int i;
+
+	for (i = 0; i < nents; i++)
+		sgl[i].sg_magic = SG_MAGIC;
+#endif
+	sg_mark_end(&sgl[nents - 1]);
+}
+
 int sg_nents(struct scatterlist *sg);
 int sg_nents_for_len(struct scatterlist *sg, u64 len);
 struct scatterlist *sg_next(struct scatterlist *);

kernel/bpf/sockmap.c

@@ -341,7 +341,7 @@ retry:
 			md->sg_start++;
 			if (md->sg_start == MAX_SKB_FRAGS)
 				md->sg_start = 0;
-			memset(sg, 0, sizeof(*sg));
+			sg_init_table(sg, 1);
 
 			if (md->sg_start == md->sg_end)
 				break;
@@ -843,7 +843,7 @@ static int bpf_tcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
 	}
 
 	sg = md.sg_data;
-	sg_init_table(sg, MAX_SKB_FRAGS);
+	sg_init_marker(sg, MAX_SKB_FRAGS);
 	rcu_read_unlock();
 
 	lock_sock(sk);
@@ -950,10 +950,14 @@ static int bpf_tcp_sendpage(struct sock *sk, struct page *page,
 
 	lock_sock(sk);
 
-	if (psock->cork_bytes)
+	if (psock->cork_bytes) {
 		m = psock->cork;
-	else
+		sg = &m->sg_data[m->sg_end];
+	} else {
 		m = &md;
+		sg = m->sg_data;
+		sg_init_marker(sg, MAX_SKB_FRAGS);
+	}
 
 	/* Catch case where ring is full and sendpage is stalled. */
 	if (unlikely(m->sg_end == m->sg_start &&
@@ -961,7 +965,6 @@ static int bpf_tcp_sendpage(struct sock *sk, struct page *page,
 		goto out_err;
 
 	psock->sg_size += size;
-	sg = &m->sg_data[m->sg_end];
 	sg_set_page(sg, page, size, offset);
 	get_page(page);
 	m->sg_copy[m->sg_end] = true;

lib/scatterlist.c

@@ -132,14 +132,7 @@ EXPORT_SYMBOL(sg_last);
 void sg_init_table(struct scatterlist *sgl, unsigned int nents)
 {
 	memset(sgl, 0, sizeof(*sgl) * nents);
-#ifdef CONFIG_DEBUG_SG
-	{
-		unsigned int i;
-		for (i = 0; i < nents; i++)
-			sgl[i].sg_magic = SG_MAGIC;
-	}
-#endif
-	sg_mark_end(&sgl[nents - 1]);
+	sg_init_marker(sgl, nents);
 }
 EXPORT_SYMBOL(sg_init_table);