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    Subject[PATCH 5.4 013/142] net: revert "net: get rid of an signed integer overflow in ip_idents_reserve()"
    Date
    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
    @@ -490,18 +490,16 @@ 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);


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