* [PATCH v2] net: revert "net: get rid of an signed integer overflow in ip_idents_reserve()"
@ 2020-05-16 3:46 Shaokun Zhang
2020-05-17 19:34 ` David Miller
0 siblings, 1 reply; 2+ messages in thread
From: Shaokun Zhang @ 2020-05-16 3:46 UTC (permalink / raw)
To: linux-kernel, netdev
Cc: Yuqi Jin, David S. Miller, Alexey Kuznetsov, Hideaki YOSHIFUJI,
Jakub Kicinski, Jiri Pirko, Arvind Sankar, Peter Zijlstra,
Eric Dumazet, Jiong Wang, Shaokun Zhang
From: Yuqi Jin <jinyuqi@huawei.com>
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>
---
ChangLog:
* Revise the commit log
* Add some comments. If it's wholly unnecessary, we
can remove it.
Patch v1: https://patchwork.ozlabs.org/project/netdev/patch/1579058620-26684-1-git-send-email-zhangshaokun@hisilicon.com/
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 788c69d9bfe0..455871d6b3a0 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);
--
2.7.4
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH v2] net: revert "net: get rid of an signed integer overflow in ip_idents_reserve()"
2020-05-16 3:46 [PATCH v2] net: revert "net: get rid of an signed integer overflow in ip_idents_reserve()" Shaokun Zhang
@ 2020-05-17 19:34 ` David Miller
0 siblings, 0 replies; 2+ messages in thread
From: David Miller @ 2020-05-17 19:34 UTC (permalink / raw)
To: zhangshaokun
Cc: linux-kernel, netdev, jinyuqi, kuznet, yoshfuji, kuba, jiri,
nivedita, peterz, edumazet, jiongwang
From: Shaokun Zhang <zhangshaokun@hisilicon.com>
Date: Sat, 16 May 2020 11:46:49 +0800
> From: Yuqi Jin <jinyuqi@huawei.com>
>
> 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>
...
> Signed-off-by: Yuqi Jin <jinyuqi@huawei.com>
> Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Applied, thanks.
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