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Date: Sun, 30 Jan 2022 09:19:23 +0100
From: Paul Menzel <pmenzel@...gen.mpg.de>
To: Zhouyi Zhou <zhouzhouyi@...il.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>,
Josh Triplett <josh@...htriplett.org>, rcu@...r.kernel.org,
LKML <linux-kernel@...r.kernel.org>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org
Subject: Re: BUG: Kernel NULL pointer dereference on write at 0x00000000
(rtmsg_ifinfo_build_skb)
Dear Zhouyi,
Am 30.01.22 um 01:21 schrieb Zhouyi Zhou:
> Thank you for your instructions, I learned a lot from this process.
Same on my end.
> On Sun, Jan 30, 2022 at 12:52 AM Paul Menzel <pmenzel@...gen.mpg.de> wrote:
>> Am 29.01.22 um 03:23 schrieb Zhouyi Zhou:
>>
>>> I don't have an IBM machine, but I tried to analyze the problem using
>>> my x86_64 kvm virtual machine, I can't reproduce the bug using my
>>> x86_64 kvm virtual machine.
>>
>> No idea, if it’s architecture specific.
>>
>>> I saw the panic is caused by registration of sit device (A sit device
>>> is a type of virtual network device that takes our IPv6 traffic,
>>> encapsulates/decapsulates it in IPv4 packets, and sends/receives it
>>> over the IPv4 Internet to another host)
>>>
>>> sit device is registered in function sit_init_net:
>>> 1895 static int __net_init sit_init_net(struct net *net)
>>> 1896 {
>>> 1897 struct sit_net *sitn = net_generic(net, sit_net_id);
>>> 1898 struct ip_tunnel *t;
>>> 1899 int err;
>>> 1900
>>> 1901 sitn->tunnels[0] = sitn->tunnels_wc;
>>> 1902 sitn->tunnels[1] = sitn->tunnels_l;
>>> 1903 sitn->tunnels[2] = sitn->tunnels_r;
>>> 1904 sitn->tunnels[3] = sitn->tunnels_r_l;
>>> 1905
>>> 1906 if (!net_has_fallback_tunnels(net))
>>> 1907 return 0;
>>> 1908
>>> 1909 sitn->fb_tunnel_dev = alloc_netdev(sizeof(struct ip_tunnel), "sit0",
>>> 1910 NET_NAME_UNKNOWN,
>>> 1911 ipip6_tunnel_setup);
>>> 1912 if (!sitn->fb_tunnel_dev) {
>>> 1913 err = -ENOMEM;
>>> 1914 goto err_alloc_dev;
>>> 1915 }
>>> 1916 dev_net_set(sitn->fb_tunnel_dev, net);
>>> 1917 sitn->fb_tunnel_dev->rtnl_link_ops = &sit_link_ops;
>>> 1918 /* FB netdevice is special: we have one, and only one per netns.
>>> 1919 * Allowing to move it to another netns is clearly unsafe.
>>> 1920 */
>>> 1921 sitn->fb_tunnel_dev->features |= NETIF_F_NETNS_LOCAL;
>>> 1922
>>> 1923 err = register_netdev(sitn->fb_tunnel_dev);
>>> register_netdev on line 1923 will call if_nlmsg_size indirectly.
>>>
>>> On the other hand, the function that calls the paniced strlen is if_nlmsg_size:
>>> (gdb) disassemble if_nlmsg_size
>>> Dump of assembler code for function if_nlmsg_size:
>>> 0xffffffff81a0dc20 <+0>: nopl 0x0(%rax,%rax,1)
>>> 0xffffffff81a0dc25 <+5>: push %rbp
>>> 0xffffffff81a0dc26 <+6>: push %r15
>>> 0xffffffff81a0dd04 <+228>: je 0xffffffff81a0de20 <if_nlmsg_size+512>
>>> 0xffffffff81a0dd0a <+234>: mov 0x10(%rbp),%rdi
>>> ...
>>> => 0xffffffff81a0dd0e <+238>: callq 0xffffffff817532d0 <strlen>
>>> 0xffffffff81a0dd13 <+243>: add $0x10,%eax
>>> 0xffffffff81a0dd16 <+246>: movslq %eax,%r12
>>
>> Excuse my ignorance, would that look the same for ppc64le?
>> Unfortunately, I didn’t save the problematic `vmlinuz` file, but on a
>> current build (without rcutorture) I have the line below, where strlen
>> shows up.
>>
>> (gdb) disassemble if_nlmsg_size
>> […]
>> 0xc000000000f7f82c <+332>: bl 0xc000000000a10e30 <strlen>
>> […]
>>
>>> and the C code for 0xffffffff81a0dd0e is following (line 524):
>>> 515 static size_t rtnl_link_get_size(const struct net_device *dev)
>>> 516 {
>>> 517 const struct rtnl_link_ops *ops = dev->rtnl_link_ops;
>>> 518 size_t size;
>>> 519
>>> 520 if (!ops)
>>> 521 return 0;
>>> 522
>>> 523 size = nla_total_size(sizeof(struct nlattr)) + /* IFLA_LINKINFO */
>>> 524 nla_total_size(strlen(ops->kind) + 1); /* IFLA_INFO_KIND */
>>
>> How do I connect the disassemby output with the corresponding line?
> I use "make ARCH=powerpc CC=powerpc64le-linux-gnu-gcc-9
> CROSS_COMPILE=powerpc64le-linux-gnu- -j 16" to cross compile kernel
> for powerpc64le in my Ubuntu 20.04 x86_64.
>
> gdb-multiarch ./vmlinux
> (gdb)disassemble if_nlmsg_size
> [...]
> 0xc00000000191bf40 <+112>: bl 0xc000000001c28ad0 <strlen>
> [...]
> (gdb) break *0xc00000000191bf40
> Breakpoint 1 at 0xc00000000191bf40: file ./include/net/netlink.h, line 1112.
>
> But in include/net/netlink.h:1112, I can't find the call to strlen
> 1110static inline int nla_total_size(int payload)
> 1111{
> 1112 return NLA_ALIGN(nla_attr_size(payload));
> 1113}
> This may be due to the compiler wrongly encode the debug information, I guess.
`rtnl_link_get_size()` contains:
size = nla_total_size(sizeof(struct nlattr)) + /*
IFLA_LINKINFO */
nla_total_size(strlen(ops->kind) + 1); /*
IFLA_INFO_KIND */
Is that inlined(?) and the code at fault?
>>> But ops is assigned the value of sit_link_ops in function sit_init_net
>>> line 1917, so I guess something must happened between the calls.
>>>
>>> Do we have KASAN in IBM machine? would KASAN help us find out what
>>> happened in between?
>>
>> Unfortunately, KASAN is not support on Power, I have, as far as I can
>> see. From `arch/powerpc/Kconfig`:
>>
>> select HAVE_ARCH_KASAN if PPC32 && PPC_PAGE_SHIFT <= 14
>> select HAVE_ARCH_KASAN_VMALLOC if PPC32 && PPC_PAGE_SHIFT <= 14
>>
> en, agree, I invoke "make menuconfig ARCH=powerpc
> CC=powerpc64le-linux-gnu-gcc-9 CROSS_COMPILE=powerpc64le-linux-gnu- -j
> 16", I can't find KASAN under Memory Debugging, I guess we should find
> the bug by bisecting instead.
I do not know, if it is a regression, as it was the first time I tried
to run a Linux kernel built with rcutorture on real hardware.
>>> Hope I can be of more helpful.
>>
>> Some distributions support multi-arch, so they easily allow
>> crosscompiling for different architectures.
> I use "make ARCH=powerpc CC=powerpc64le-linux-gnu-gcc-9
> CROSS_COMPILE=powerpc64le-linux-gnu- -j 16" to cross compile kernel
> for powerpc64le in my Ubuntu 20.04 x86_64. But I can't boot the
> compiled kernel using "qemu-system-ppc64le -M pseries -nographic -smp
> 4 -net none -m 4G -kernel arch/powerpc/boot/zImage". I will continue
> to explore it.
Oh, that does not sound good. But I have not tried that in a long time
either. It’s a separate issue, but maybe some of the PPC
maintainers/folks could help.
Kind regards,
Paul
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