| From foo@baz Thu 28 May 2020 02:29:37 PM CEST |
| From: Yuqi Jin <jinyuqi@huawei.com> |
| Date: Sat, 16 May 2020 11:46:49 +0800 |
| Subject: net: revert "net: get rid of an signed integer overflow in ip_idents_reserve()" |
| |
| 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); |
| |