tcp: sysctl: Fix a race to avoid unexpected 0 window from space

[ Upstream commit c48367427a39ea0b85c7cf018fe4256627abfd9e ]

Because sysctl_tcp_adv_win_scale could be changed any time, so there
is one race in tcp_win_from_space.
For example,
1.sysctl_tcp_adv_win_scale<=0 (sysctl_tcp_adv_win_scale is negative now)
2.space>>(-sysctl_tcp_adv_win_scale) (sysctl_tcp_adv_win_scale is postive now)

As a result, tcp_win_from_space returns 0. It is unexpected.

Certainly if the compiler put the sysctl_tcp_adv_win_scale into one
register firstly, then use the register directly, it would be ok.
But we could not depend on the compiler behavior.

Signed-off-by: Gao Feng <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: gfreewind

include/net/tcp.h

@@ -1099,9 +1099,11 @@ void tcp_select_initial_window(int __space, __u32 mss, __u32 *rcv_wnd,
 
 static inline int tcp_win_from_space(int space)
 {
-	return sysctl_tcp_adv_win_scale<=0 ?
-		(space>>(-sysctl_tcp_adv_win_scale)) :
-		space - (space>>sysctl_tcp_adv_win_scale);
+	int tcp_adv_win_scale = sysctl_tcp_adv_win_scale;
+
+	return tcp_adv_win_scale <= 0 ?
+		(space>>(-tcp_adv_win_scale)) :
+		space - (space>>tcp_adv_win_scale);
 }
 
 /* Note: caller must be prepared to deal with negative returns */