netdev - Re: Latest net-next from GIT panic

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Date:   Wed, 20 Sep 2017 21:13:04 +0200
From:   Paweł Staszewski <pstaszewski@...are.pl>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     Wei Wang <weiwan@...gle.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: Latest net-next from GIT panic



W dniu 2017-09-20 o 20:36, Cong Wang pisze:
> On Wed, Sep 20, 2017 at 11:30 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> On Wed, 2017-09-20 at 11:22 -0700, Cong Wang wrote:
>>> but dmesg at this time shows nothing about interfaces or flaps.
>>>
>>> This is very odd.
>>>
>>> We only free netdevice in free_netdev() and it is only called when
>>> we unregister a netdevice. Otherwise pcpu_refcnt is impossible
>>> to be NULL.
>> If there is a missing dev_hold() or one dev_put() in excess,
>> this would allow the netdev to be freed too soon.
>>
>> -> Use after free.
>> memory holding netdev could be reallocated-cleared by some other kernel
>> user.
>>
> Sure, but only unregister could trigger a free. If there is no unregister,
> like what Pawel claims, then there is no free, the refcnt just goes to
> 0 but the memory is still there.
>
About possible mistake from my side with bisect - i can judge too early 
that some bisect was good
the road was:
git bisect start
# bad: [ac7b75966c9c86426b55fe1c50ae148aa4571075] Merge tag 
'pinctrl-v4.13-1' of 
git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-pinctrl
git bisect bad ac7b75966c9c86426b55fe1c50ae148aa4571075
# good: [e24dd9ee5399747b71c1d982a484fc7601795f31] Merge branch 'next' 
of git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security
git bisect good e24dd9ee5399747b71c1d982a484fc7601795f31
# bad: [9cc9a5cb176ccb4f2cda5ac34da5a659926f125f] datapath: Avoid using 
stack larger than 1024.
git bisect bad 9cc9a5cb176ccb4f2cda5ac34da5a659926f125f
# good: [073cf9e20c333ab29744717a23f9e43ec7512a20] Merge branch 
'udp-reduce-cache-pressure'
git bisect good 073cf9e20c333ab29744717a23f9e43ec7512a20
# bad: [8abd5599a520e9f188a750f1bde9dde5fb856230] Merge branch 
's390-net-updates-part-2'
git bisect bad 8abd5599a520e9f188a750f1bde9dde5fb856230
# good: [2fae5d0e647c6470d206e72b5fc24972bb900f70] Merge branch 
'bpf-ctx-narrow'
git bisect good 2fae5d0e647c6470d206e72b5fc24972bb900f70
# good: [41500c3e2a19ffcf40a7158fce1774de08e26ba2] rds: tcp: remove 
cp_outgoing
git bisect good 41500c3e2a19ffcf40a7158fce1774de08e26ba2
# bad: [8917a777be3ba566377be05117f71b93a5fd909d] tcp: md5: add 
TCP_MD5SIG_EXT socket option to set a key address prefix
git bisect bad 8917a777be3ba566377be05117f71b93a5fd909d
# good: [4a6ce2b6f2ecabbddcfe47e7cf61dd0f00b10e36] net: introduce a new 
function dst_dev_put()

And currently have this running for about 4 hours without problems.



git bisect good 4a6ce2b6f2ecabbddcfe47e7cf61dd0f00b10e36
# bad: [a4c2fd7f78915a0d7c5275e7612e7793157a01f2] net: remove 
DST_NOCACHE flag

Here for sure - panic

git bisect bad a4c2fd7f78915a0d7c5275e7612e7793157a01f2
# bad: [ad65a2f05695aced349e308193c6e2a6b1d87112] ipv6: call 
dst_hold_safe() properly
git bisect bad ad65a2f05695aced349e308193c6e2a6b1d87112
# good: [9df16efadd2a8a82731dc76ff656c771e261827f] ipv4: call 
dst_hold_safe() properly
git bisect good 9df16efadd2a8a82731dc76ff656c771e261827f
# bad: [1cfb71eeb12047bcdbd3e6730ffed66e810a0855] ipv6: take 
dst->__refcnt for insertion into fib6 tree

im not 100% sure tor last two
Will test them again starting from
[95c47f9cf5e028d1ae77dc6c767c1edc8a18025b] ipv4: call dst_dev_put() properly


git bisect bad 1cfb71eeb12047bcdbd3e6730ffed66e810a0855
# bad: [b838d5e1c5b6e57b10ec8af2268824041e3ea911] ipv4: mark DST_NOGC 
and remove the operation of dst_free()



git bisect bad b838d5e1c5b6e57b10ec8af2268824041e3ea911
# first bad commit: [b838d5e1c5b6e57b10ec8af2268824041e3ea911] ipv4: 
mark DST_NOGC and remove the operation of dst_free()