From a6211caa634da39d861a47437ffcda8b38ef421b Mon Sep 17 00:00:00 2001 From: Yuqi Jin Date: Sat, 16 May 2020 11:46:49 +0800 Subject: [PATCH] net: revert "net: get rid of an signed integer overflow in ip_idents_reserve()" 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 Suggested-by: Eric Dumazet Cc: "David S. Miller" Cc: Alexey Kuznetsov Cc: Hideaki YOSHIFUJI Cc: Jakub Kicinski Cc: Jiri Pirko Cc: Arvind Sankar Cc: Peter Zijlstra Cc: Eric Dumazet Cc: Jiong Wang Signed-off-by: Yuqi Jin Signed-off-by: Shaokun Zhang Signed-off-by: David S. Miller --- net/ipv4/route.c | 14 ++++++-------- 1 file changed, 6 insertions(+), 8 deletions(-) diff --git a/net/ipv4/route.c b/net/ipv4/route.c index fa829f31a3f5..b73f540fa19b 100644 --- a/net/ipv4/route.c +++ b/net/ipv4/route.c @@ -491,18 +491,16 @@ u32 ip_idents_reserve(u32 hash, int segs) atomic_t *p_id = ip_idents + hash % IP_IDENTS_SZ; u32 old = READ_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);