[PATCH bpf-next] bpf: sockmap: initialize sg table entries properly

[Prashant Bhole]

When CONFIG_DEBUG_SG is set, sg->sg_magic is initialized to SG_MAGIC,
when sg table is initialized using sg_init_table(). Magic is checked
while navigating the scatterlist. We hit BUG_ON when magic check is
failed.

Fixed following things:
- Initialization of sg table in bpf_tcp_sendpage() was missing,
  initialized it using sg_init_table()

- bpf_tcp_sendmsg() initializes sg table using sg_init_table() before
  entering the loop, but further consumed sg entries are initialized
  using memset. Fixed it by replacing memset with sg_init_table() in
  function bpf_tcp_push()

Signed-off-by: Prashant Bhole <bhole_prashant...@lab.ntt.co.jp>
---
 kernel/bpf/sockmap.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/sockmap.c b/kernel/bpf/sockmap.c
index 69c5bccabd22..8a848a99d768 100644
--- a/kernel/bpf/sockmap.c
+++ b/kernel/bpf/sockmap.c
@@ -312,7 +312,7 @@ static int bpf_tcp_push(struct sock *sk, int apply_bytes,
                        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;
@@ -763,10 +763,14 @@ 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_table(sg, MAX_SKB_FRAGS);
+       }
 
        /* Catch case where ring is full and sendpage is stalled. */
        if (unlikely(m->sg_end == m->sg_start &&
@@ -774,7 +778,6 @@ 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;
-- 
2.14.3