From: Yuqi Jin <jinyuqi@xxxxxxxxxx>
[ Upstream commit a6211caa634da39d861a47437ffcda8b38ef421b ]
Commit adb03115f459 ("net: get rid of an signed integer overflow in ip_idents_reserve()")
used atomic_cmpxchg to replace "atomic_add_return" inside the function
"ip_idents_reserve". The reason was to avoid UBSAN warning.
However, this change has caused performance degrade and in GCC-8,
fno-strict-overflow is now mapped to -fwrapv -fwrapv-pointer
and signed integer overflow is now undefined by default at all
optimization levels[1]. Moreover, it was a bug in UBSAN vs -fwrapv
/-fno-strict-overflow, so Let's revert it safely.
[1] https://gcc.gnu.org/gcc-8/changes.html
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Alexey Kuznetsov <kuznet@xxxxxxxxxxxxx>
Cc: Hideaki YOSHIFUJI <yoshfuji@xxxxxxxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: Jiri Pirko <jiri@xxxxxxxxxxx>
Cc: Arvind Sankar <nivedita@xxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Jiong Wang <jiongwang@xxxxxxxxxx>
Signed-off-by: Yuqi Jin <jinyuqi@xxxxxxxxxx>
Signed-off-by: Shaokun Zhang <zhangshaokun@xxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
net/ipv4/route.c | 14 ++++++--------
1 file changed, 6 insertions(+), 8 deletions(-)
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -477,18 +477,16 @@ u32 ip_idents_reserve(u32 hash, int segs
atomic_t *p_id = ip_idents + hash % IP_IDENTS_SZ;
u32 old = ACCESS_ONCE(*p_tstamp);
u32 now = (u32)jiffies;
- u32 new, delta = 0;
+ u32 delta = 0;
if (old != now && cmpxchg(p_tstamp, old, now) == old)
delta = prandom_u32_max(now - old);
- /* Do not use atomic_add_return() as it makes UBSAN unhappy */
- do {
- old = (u32)atomic_read(p_id);
- new = old + delta + segs;
- } while (atomic_cmpxchg(p_id, old, new) != old);
-
- return new - segs;
+ /* If UBSAN reports an error there, please make sure your compiler
+ * supports -fno-strict-overflow before reporting it that was a bug
+ * in UBSAN, and it has been fixed in GCC-8.
+ */
+ return atomic_add_return(segs + delta, p_id) - segs;
}
EXPORT_SYMBOL(ip_idents_reserve);
