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 <fgao@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/include/net/tcp.h b/include/net/tcp.h
index caf35e0..18f029b 100644 (file)
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1265,9 +1265,11 @@ 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 */