From: Yuqi Jin jinyuqi@huawei.com
[ 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@infradead.org Suggested-by: Eric Dumazet edumazet@google.com Cc: "David S. Miller" davem@davemloft.net Cc: Alexey Kuznetsov kuznet@ms2.inr.ac.ru Cc: Hideaki YOSHIFUJI yoshfuji@linux-ipv6.org Cc: Jakub Kicinski kuba@kernel.org Cc: Jiri Pirko jiri@resnulli.us Cc: Arvind Sankar nivedita@alum.mit.edu Cc: Peter Zijlstra peterz@infradead.org Cc: Eric Dumazet edumazet@google.com Cc: Jiong Wang jiongwang@huawei.com Signed-off-by: Yuqi Jin jinyuqi@huawei.com Signed-off-by: Shaokun Zhang zhangshaokun@hisilicon.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/ipv4/route.c | 14 ++++++-------- 1 file changed, 6 insertions(+), 8 deletions(-)
--- a/net/ipv4/route.c +++ b/net/ipv4/route.c @@ -499,18 +499,16 @@ 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);