[PATCH net-next v2 1/3] sock: convert sk_peek_offset functions to WRITE_ONCE

[Willem de Bruijn]

From: Willem de Bruijn <will...@google.com>

Make the peek offset interface safe to use in lockless environments.
Use READ_ONCE and WRITE_ONCE to avoid race conditions between testing
and updating the peek offset.

Suggested-by: Eric Dumazet <eduma...@google.com>
Signed-off-by: Willem de Bruijn <will...@google.com>
---
 include/net/sock.h | 24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 310c436..09aec75 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -459,26 +459,28 @@ struct sock {
 
 static inline int sk_peek_offset(struct sock *sk, int flags)
 {
-       if ((flags & MSG_PEEK) && (sk->sk_peek_off >= 0))
-               return sk->sk_peek_off;
-       else
-               return 0;
+       if (unlikely(flags & MSG_PEEK)) {
+               s32 off = READ_ONCE(sk->sk_peek_off);
+               if (off >= 0)
+                       return off;
+       }
+
+       return 0;
 }
 
 static inline void sk_peek_offset_bwd(struct sock *sk, int val)
 {
-       if (sk->sk_peek_off >= 0) {
-               if (sk->sk_peek_off >= val)
-                       sk->sk_peek_off -= val;
-               else
-                       sk->sk_peek_off = 0;
+       s32 off = READ_ONCE(sk->sk_peek_off);
+
+       if (unlikely(off >= 0)) {
+               off = max_t(s32, off - val, 0);
+               WRITE_ONCE(sk->sk_peek_off, off);
        }
 }
 
 static inline void sk_peek_offset_fwd(struct sock *sk, int val)
 {
-       if (sk->sk_peek_off >= 0)
-               sk->sk_peek_off += val;
+       sk_peek_offset_bwd(sk, -val);
 }
 
 /*
-- 
2.8.0.rc3.226.g39d4020