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Date:   Mon, 21 Feb 2022 12:17:40 +0100
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Subject: Re: BUG: Kernel NULL pointer dereference on write at 0x00000000
 (rtmsg_ifinfo_build_skb)
Content-Language: en-US
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     Zhouyi Zhou <zhouzhouyi@gmail.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Josh Triplett <josh@joshtriplett.org>, rcu@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>, netdev@vger.kernel.org,
        Nick Desaulniers <ndesaulniers@google.com>,
        llvm@lists.linux.dev, Fangrui Song <i@maskray.me>
References: <159db05f-539c-fe29-608b-91b036588033@molgen.mpg.de>
 <CAABZP2xampOLo8k93OLgaOfv9LreJ+f0g0_1mXwqtrv_LKewQg@mail.gmail.com>
 <3534d781-7d01-b42a-8974-0b1c367946f0@molgen.mpg.de>
 <CAABZP2zFDY-hrZqE=-c0uW8vFMH+Q9XezYd2DcBX4Wm+sxzK1g@mail.gmail.com>
 <04a597dc-64aa-57e6-f7fb-17bd2ec58159@molgen.mpg.de>
 <CAABZP2yb7-xa4F_2c6tuzkv7x902wU-hqgD_pqRooGC6C7S20A@mail.gmail.com>
 <f41550c7-26c0-cf81-7de9-aa924434a565@molgen.mpg.de>
 <Yg2h2Q2vXFkkLGTh@dev-arch.archlinux-ax161>
From:   Paul Menzel <pmenzel@molgen.mpg.de>
In-Reply-To: <Yg2h2Q2vXFkkLGTh@dev-arch.archlinux-ax161>
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[Cc: +Fangrui]

Dear Nathan,


Am 17.02.22 um 02:16 schrieb Nathan Chancellor:

> On Wed, Feb 16, 2022 at 02:19:51PM +0100, Paul Menzel wrote:
>> [Cc: +LLVM/clang build support folks]

[…]

>> To recap: On a ppc64le machine, building Linux in Ubuntu 21.10 with *llvm*
>> and *clang* 1:13.0-53~exp1
>>
>>      $ clang --version
>>      Ubuntu clang version 13.0.0-2
>>      Target: powerpc64le-unknown-linux-gnu
>>      Thread model: posix
>>      InstalledDir: /usr/bin
>>
>> results in a segmentation fault, while it works when building with GCC.
>>
>>      $ gcc --version
>>      gcc (Ubuntu 11.2.0-7ubuntu2) 11.2.0
> 
> Thank you for keying us in. I am going to have a bit of a brain dump
> here based on the information I have uncovered after a couple of hours
> of debugging.
> 
> TL;DR: It seems like something is broken with __read_mostly + ld.lld
> before 14.0.0.
> 
> My initial reproduction steps (boot-qemu.sh comes from
> https://github.com/ClangBuiltLinux/boot-utils):
> 
> $ clang --version
> clang version 13.0.1 (Fedora 13.0.1-1.fc37)
> Target: x86_64-redhat-linux-gnu
> Thread model: posix
> InstalledDir: /usr/bin
> 
> $ powerpc64le-linux-gnu-as --version
> GNU assembler version 2.37-2.fc36
> Copyright (C) 2021 Free Software Foundation, Inc.
> This program is free software; you may redistribute it under the terms of
> the GNU General Public License version 3 or later.
> This program has absolutely no warranty.
> This assembler was configured for a target of `powerpc64le-linux-gnu'.
> 
> $ curl -LSso .config https://lore.kernel.org/all/f41550c7-26c0-cf81-7de9-aa924434a565@molgen.mpg.de/3-linux-5.17-rc4-rcu-dev-config.txt
> 
> $ scripts/config --set-val INITRAMFS_SOURCE '""'
> 
> $ make -skj"$(nproc)" ARCH=powerpc CROSS_COMPILE=powerpc64le-linux-gnu- LLVM=1 LLVM_IAS=0 all
> 
> $ boot-qemu.sh -a ppc64le -k . -t 45s
> QEMU location: /usr/bin
> 
> QEMU version: QEMU emulator version 6.2.0 (qemu-6.2.0-5.fc37)
> 
> + timeout --foreground 45s stdbuf -oL -eL qemu-system-ppc64 -initrd \
> /home/nathan/cbl/github/boot-utils-ro/images/ppc64le/rootfs.cpio -device \
> ipmi-bmc-sim,id=bmc0 -device isa-ipmi-bt,bmc=bmc0,irq=10 -L \
> /home/nathan/cbl/github/boot-utils-ro/images/ppc64le/ -bios skiboot.lid \
> -machine powernv8 -display none -kernel \
> /home/nathan/cbl/src/linux/arch/powerpc/boot/zImage.epapr -m 2G \
> -nodefaults -serial mon:stdio
> ...
> [    1.478028][    T1] BUG: Kernel NULL pointer dereference on read at 0x00000000
> [    1.478630][    T1] Faulting instruction address: 0xc00000000090bee0
> [    1.479521][    T1] Oops: Kernel access of bad area, sig: 11 [#1]
> [    1.480036][    T1] LE PAGE_SIZE=64K MMU=Hash PREEMPT SMP NR_CPUS=16 NUMA PowerNV
> [    1.480853][    T1] Modules linked in:
> [    1.481265][    T1] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.17.0-rc4-00001-gfa15c7cb550f #1
> [    1.481967][    T1] NIP:  c00000000090bee0 LR: c000000000d96b60 CTR: c0000000000d5b4c
> [    1.482596][    T1] REGS: c000000007443330 TRAP: 0380   Not tainted  (5.17.0-rc4-00001-gfa15c7cb550f)
> [    1.483305][    T1] MSR:  9000000002009033 <SF,HV,VEC,EE,ME,IR,DR,RI,LE>  CR: 22800a87  XER: 00000000
> [    1.484277][    T1] CFAR: c000000000d96b5c IRQMASK: 0
> [    1.484277][    T1] GPR00: c000000000d96b54 c0000000074435d0 c0000000028bc600 0000000000000000
> [    1.484277][    T1] GPR04: ffffffffffffffff ffffffffff1ea558 ffffffffff1ebfe4 c00000000261ae88
> [    1.484277][    T1] GPR08: 0000000000000003 0000000000000004 c00000000261ae88 0000000000000000
> [    1.484277][    T1] GPR12: 0000000000800000 c000000002a60000 c000000000012518 0000000000000000
> [    1.484277][    T1] GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
> [    1.484277][    T1] GPR20: 0000000000000000 c0000000027bff80 0000000000000cc0 0000000000000000
> [    1.484277][    T1] GPR24: 0000000000000010 0000000000000000 0000000000000000 0000000000000000
> [    1.484277][    T1] GPR28: c0000000028fcfd8 c000000007b83000 0000000000000000 c0000000074435d0
> [    1.490325][    T1] NIP [c00000000090bee0] strlen+0x10/0x30
> [    1.490788][    T1] LR [c000000000d96b60] if_nlmsg_size+0x2b0/0x390
> [    1.491319][    T1] Call Trace:
> [    1.491573][    T1] [c0000000074435d0] [c000000000d96b54] if_nlmsg_size+0x2a4/0x390 (unreliable)
> [    1.492291][    T1] [c000000007443680] [c000000000d96790] rtmsg_ifinfo_build_skb+0x80/0x1a0
> [    1.492958][    T1] [c000000007443740] [c000000000d97590] rtmsg_ifinfo+0x70/0xd0
> [    1.493559][    T1] [c000000007443790] [c000000000d7d528] register_netdevice+0x5d8/0x670
> [    1.494205][    T1] [c000000007443820] [c000000000d7d94c] register_netdev+0x4c/0x80
> [    1.494823][    T1] [c000000007443850] [c000000000f826d8] sit_init_net+0x1b8/0x200
> [    1.495426][    T1] [c0000000074438d0] [c000000000d63b5c] ops_init+0x14c/0x1c0
> [    1.496014][    T1] [c000000007443930] [c000000000d6314c] register_pernet_operations+0xec/0x1e0
> [    1.496716][    T1] [c000000007443990] [c000000000d633d0] register_pernet_device+0x60/0xd0
> [    1.497372][    T1] [c0000000074439e0] [c000000002085194] sit_init+0x54/0x160
> [    1.497950][    T1] [c000000007443a70] [c000000000011c58] do_one_initcall+0x108/0x3e0
> [    1.498573][    T1] [c000000007443c70] [c000000002006190] do_initcall_level+0xe4/0x1c4
> [    1.499219][    T1] [c000000007443cc0] [c00000000200604c] do_initcalls+0x84/0xe4
> [    1.499799][    T1] [c000000007443d40] [c000000002005da8] kernel_init_freeable+0x160/0x1ec
> [    1.500444][    T1] [c000000007443da0] [c00000000001254c] kernel_init+0x3c/0x270
> [    1.501042][    T1] [c000000007443e10] [c00000000000cd64] ret_from_kernel_thread+0x5c/0x64
> [    1.501721][    T1] Instruction dump:
> [    1.502202][    T1] eb81ffe0 7c0803a6 4e800020 00000000 00000000 00000000 60000000 60000000
> [    1.502934][    T1] 3883ffff 60000000 60000000 60000000 <8ca40001> 28050000 4082fff8 7c632050
> [    1.504028][    T1] ---[ end trace 0000000000000000 ]---
> ...
> 
> First thing was figuring out where the NULL pointer dereference happens,
> which appears to the "strlen(ops->kind)" in rtnl_link_get_size():
> 
> 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 */
> 
> which I confirmed some really rudimentary printk debugging:
> 
> [    1.476862][    T1] nathan: rtnl_link_get_size(): name: sit0, ops: c0000000028fcfd8, ops->kind: (null)
> 
> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
> index 710da8a36729..c8d928e83aec 100644
> --- a/net/core/rtnetlink.c
> +++ b/net/core/rtnetlink.c
> @@ -520,6 +520,9 @@ static size_t rtnl_link_get_size(const struct net_device *dev)
>   	if (!ops)
>   		return 0;
>   
> +	pr_err("nathan: %s(): name: %s, ops: %px, ops->kind: %s\n", __func__,
> +	       dev->name, ops, ops->kind);
> +
>   	size = nla_total_size(sizeof(struct nlattr)) + /* IFLA_LINKINFO */
>   	       nla_total_size(strlen(ops->kind) + 1);  /* IFLA_INFO_KIND */
>   
> 
> Okay... how did sit0 end up with a NULL kind...? It is very clearly
> defined as "sit":
> 
> 1830 static struct rtnl_link_ops sit_link_ops __read_mostly = {
> 1831 	.kind		= "sit",
> 
> Adding some more debug prints to net/ipv6/sit.c:
> 
> diff --git a/net/ipv6/sit.c b/net/ipv6/sit.c
> index c0b138c20992..7b9edbed2fcd 100644
> --- a/net/ipv6/sit.c
> +++ b/net/ipv6/sit.c
> @@ -1920,6 +1920,12 @@ static int __net_init sit_init_net(struct net *net)
>   	 */
>   	sitn->fb_tunnel_dev->features |= NETIF_F_NETNS_LOCAL;
>   
> +	pr_err("nathan: %s(): &sit_link_ops: %px\n", __func__, &sit_link_ops);
> +	pr_err("nathan: %s(): sit_link_ops.kind: %s\n", __func__, sit_link_ops.kind);
> +	pr_err("nathan: %s(): sit_link_ops.maxtype: %u\n", __func__, sit_link_ops.maxtype);
> +	pr_err("nathan: %s(): sitn->fb_tunnel_dev->rtnl_link_ops: %px\n", __func__, sitn->fb_tunnel_dev->rtnl_link_ops);
> +	pr_err("nathan: %s(): sitn->fb_tunnel_dev->rtnl_link_ops->kind: %s\n", __func__, sitn->fb_tunnel_dev->rtnl_link_ops->kind);
> +
>   	err = register_netdev(sitn->fb_tunnel_dev);
>   	if (err)
>   		goto err_reg_dev;
> 
> reveals:
> 
> [    1.471920][    T1] sit: nathan: sit_init_net(): &sit_link_ops: c0000000028fcfd8
> [    1.472534][    T1] sit: nathan: sit_init_net(): sit_link_ops.kind: (null)
> [    1.473088][    T1] sit: nathan: sit_init_net(): sit_link_ops.maxtype: 20
> [    1.473639][    T1] sit: nathan: sit_init_net(): sitn->fb_tunnel_dev->rtnl_link_ops: c0000000028fcfd8
> [    1.474370][    T1] sit: nathan: sit_init_net(): sitn->fb_tunnel_dev->rtnl_link_ops->kind: (null)
> 
> This is super bizarre, as the maxtype member appears to have the correct
> value, but how is kind's initial getting dropped on the floor?
> 
> Removing the __read_mostly annotation "fixes" it:
> 
> [    1.481708][    T1] sit: nathan: sit_init_net(): &sit_link_ops: c0000000027d3f60
> [    1.482319][    T1] sit: nathan: sit_init_net(): sit_link_ops.kind: sit
> [    1.482878][    T1] sit: nathan: sit_init_net(): sit_link_ops.maxtype: 20
> [    1.483429][    T1] sit: nathan: sit_init_net(): sitn->fb_tunnel_dev->rtnl_link_ops: c0000000027d3f60
> [    1.484174][    T1] sit: nathan: sit_init_net(): sitn->fb_tunnel_dev->rtnl_link_ops->kind: sit
> ...
> Linux version 5.17.0-rc4-00001-g956f02ad5c31-dirty (nathan@dev-fedora.archlinux-ax161) (clang version 13.0.1 (Fedora 13.0.1-1.fc37), LLD 13.0.1) #2 SMP PREEMPT Wed Feb 16 13:29:49 MST 2022
> ...
> 
> diff --git a/net/ipv6/sit.c b/net/ipv6/sit.c
> index 7b9edbed2fcd..f109c7a0233b 100644
> --- a/net/ipv6/sit.c
> +++ b/net/ipv6/sit.c
> @@ -70,7 +70,7 @@ static void ipip6_tunnel_setup(struct net_device *dev);
>   static void ipip6_dev_free(struct net_device *dev);
>   static bool check_6rd(struct ip_tunnel *tunnel, const struct in6_addr *v6dst,
>   		      __be32 *v4dst);
> -static struct rtnl_link_ops sit_link_ops __read_mostly;
> +static struct rtnl_link_ops sit_link_ops;
>   
>   static unsigned int sit_net_id __read_mostly;
>   struct sit_net {
> @@ -1827,7 +1827,7 @@ static void ipip6_dellink(struct net_device *dev, struct list_head *head)
>   		unregister_netdevice_queue(dev, head);
>   }
>   
> -static struct rtnl_link_ops sit_link_ops __read_mostly = {
> +static struct rtnl_link_ops sit_link_ops = {
>   	.kind		= "sit",
>   	.maxtype	= IFLA_IPTUN_MAX,
>   	.policy		= ipip6_policy,
> 
> Switching to ld.bfd also resolves it:
> 
> [    1.470405][    T1] sit: nathan: sit_init_net(): &sit_link_ops: c0000000028acfd8
> [    1.471016][    T1] sit: nathan: sit_init_net(): sit_link_ops.kind: sit
> [    1.471534][    T1] sit: nathan: sit_init_net(): sit_link_ops.maxtype: 20
> [    1.472062][    T1] sit: nathan: sit_init_net(): sitn->fb_tunnel_dev->rtnl_link_ops: c0000000028acfd8
> [    1.472790][    T1] sit: nathan: sit_init_net(): sitn->fb_tunnel_dev->rtnl_link_ops->kind: sit
> ...
> Linux version 5.17.0-rc4-00001-g956f02ad5c31 (nathan@dev-fedora.archlinux-ax161) (clang version 13.0.1 (Fedora 13.0.1-1.fc37), GNU ld version 2.37-2.fc36) #3 SMP PREEMPT Wed Feb 16 13:33:42 MST 2022
> ...
> 
> I tested with ToT LLVM (or at least, close to it, since there is an
> unrelated ld.lld regression there) and I could not reproduce it there,
> so I did a reverse bisect to see what commit fixes this issue in LLVM 14
> and I landed on:
> 
> commit 55c14d6dbfd8e7b86c15d2613fea3490078e2ae4
> Author: Fangrui Song <i@maskray.me>
> Date:   Thu Nov 25 14:12:34 2021 -0800
> 
>      [ELF] Simplify DynamicSection content computation. NFC
> 
>      The new code computes the content twice, but avoides the tricky
>      std::function<uint64_t()>. Removed 13KiB code in a Release build.
> 
>   lld/ELF/SyntheticSections.cpp | 117 ++++++++++++++++--------------------------
>   lld/ELF/SyntheticSections.h   |  12 +----
>   2 files changed, 44 insertions(+), 85 deletions(-)
> 
> That's... interesting, given that commit title says No Functional
> Change, even though there clearly is one. That commit has a couple
> mentions of PowerPC synthetic sections, so it is possible that the
> new content calculation lines up with ld.bfd?
> 
> I am not really sure where to go from here, as I don't fully understand
> what the problem was before that LLD change. I'll see if I can do some
> more investigation tomorrow (unless someone wants to beat me to it ;)

Thank you for looking into this, and sharing your analysis.

I built LLVM/clang from the master branch, rebuilt, but can still 
reproduce this.

     $ git clone --depth=1 https://github.com/llvm/llvm-project.git
     $ cd llvm-project/
     $ git log --oneline
     41cb504b7 [mlir][linalg][bufferize][NFC] Move interface impl to 
Linalg Transforms
     $ mkdir build
     $ cd build
     $ cmake -DLLVM_ENABLE_PROJECTS=clang -G "Unix Makefiles" 
-DCMAKE_BUILD_TYPE=Release -DLLVM_INSTALL_UTILS=ON 
-DCMAKE_INSTALL_PREFIX=/scratch/local2/llvm ../llvm
     $ make -j20
     $ make -j20 clang-check
     $ make install
     $ /scratch/local2/llvm/bin/clang --version
     clang version 15.0.0 (https://github.com/llvm/llvm-project.git 
41cb504b7c4b18ac15830107431a0c1eec73a6b2)
     Target: powerpc64le-unknown-linux-gnu
     Thread model: posix
     InstalledDir: /scratch/local2/llvm/bin

Then build Linux after `make clean` with `/scratch/local2/llvm/bin` in 
the path.

     $ LLVM=1 LLVM_IAS=0 eatmydata make -j20

     $ qemu-system-ppc64 -enable-kvm -nographic -smp cores=1,threads=1 
-net none -enable-kvm -M pseries -nodefaults -device spapr-vscsi -serial 
stdio -m 512 -kernel /dev/shm/linux/vmlinux -append 
"debug_boot_weak_hash panic=-1 console=ttyS0 
rcupdate.rcu_cpu_stall_suppress_at_boot=1 torture.disable_onoff_at_boot 
rcupdate.rcu_task_stall_timeout=30000 rcupdate.rcu_self_test=1 
rcutorture.onoff_interval=1000 rcutorture.onoff_holdoff=30 
rcutorture.n_barrier_cbs=4 rcutorture.stat_interval=15 
rcutorture.shutdown_secs=420 rcutorture.test_no_idle_hz=1 
rcutorture.verbose=1"
     […]
     Preparing to boot Linux version 5.17.0-rc5-00178-ga4b9a8fb20e7 
(pmenzel@flughafenberlinbrandenburgwillybrandt.molgen.mpg.de) (clang 
version 15.0.0 (https://github.com/llvm/llvm-project.git 
41cb504b7c4b18ac15830107431a0c1eec73a6b2), LLD 13.0.0) #29 SMP PREEMPT 
Mon Feb 21 10:58:54 CET 2022
     […]
     [    0.465889][    T1] BUG: Kernel NULL pointer dereference on read 
at 0x00000000
     [    0.466749][    T1] Faulting instruction address: 0xc0000000008fc300
     [    0.467507][    T1] Oops: Kernel access of bad area, sig: 11 [#1]
     […]


Kind regards,

Paul