Message-ID: <aV5S7DZkxDZ0ldKE@pathway.suse.cz>
Date: Wed, 7 Jan 2026 13:34:52 +0100
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: syzbot <syzbot+22a26d9b6c0a64335bf7@...kaller.appspotmail.com>,
	linux-kernel@...r.kernel.org, rostedt@...dmis.org,
	senozhatsky@...omium.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [kernel?] Internal error in div_u64_rem (4)

On Wed 2026-01-07 10:54:38, John Ogness wrote:
> On 2025-12-31, syzbot <syzbot+22a26d9b6c0a64335bf7@...kaller.appspotmail.com> wrote:
> > syzbot found the following issue on:
> >
> > HEAD commit:    c8ebd433459b Merge tag 'nfsd-6.19-2' of git://git.kernel.o..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=15caa7da580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=e5753ed355722af
> > dashboard link: https://syzkaller.appspot.com/bug?extid=22a26d9b6c0a64335bf7
> > compiler:       arm-linux-gnueabi-gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> > userspace arch: arm
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > Downloadable assets:
> > disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/98a89b9f34e4/non_bootable_disk-c8ebd433.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/fd848bb7d9d0/vmlinux-c8ebd433.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/439934b22d51/zImage-c8ebd433.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+22a26d9b6c0a64335bf7@...kaller.appspotmail.com
> >
> > Insufficient stack space to handle exception!
> > Task stack:     [0xeda4c000..0xeda4e000]
> > IRQ stack:      [0xdf804000..0xdf806000]
> > Overflow stack: [0x830bc000..0x830bd000]
> > Internal error: kernel stack overflow: 0 [#1] SMP ARM
> > Modules linked in:
> > CPU: 1 UID: 0 PID: 2128 Comm: syz-executor Tainted: G             L      syzkaller #0 PREEMPT 
> > Tainted: [L]=SOFTLOCKUP
> > Hardware name: ARM-Versatile Express
> > PC is at div_u64_rem+0x4/0x4c include/linux/math64.h:91
> > LR is at div_u64 include/linux/math64.h:130 [inline]
> > LR is at ___update_load_avg kernel/sched/pelt.c:265 [inline]
> > LR is at __update_load_avg_se+0x150/0x518 kernel/sched/pelt.c:312
> > pc : [<802abd9c>]    lr : [<802b54f0>]    psr: 20000193
> > sp : df804010  ip : df804010  fp : df80406c
/> > r10: 00000000  r9 : 00000029  r8 : 0000b993
> > r7 : 85f68c00  r6 : 00000538  r5 : 00000000  r4 : 846c3e80
> > r3 : df804038  r2 : 0000b993  r1 : 00000000  r0 : 00000000
> > Flags: nzCv  IRQs off  FIQs on  Mode SVC_32  ISA ARM  Segment none
> > Control: 30c5387d  Table: 8612bb00  DAC: 00000000
> > Register r0 information: NULL pointer
> > Register r1 information: NULL pointer
> > Register r2 information: non-paged memory
> > Register r3 information: 2-page vmalloc region starting at 0xdf804000 allocated at start_kernel+0x6b0/0x860 init/main.c:1111
> > Register r4 information: slab task_struct start 846c3c00 pointer offset 640 size 3072
> > Register r5 information: NULL pointer
> > Register r6 information: non-paged memory
> > Register r7 information: slab task_struct start 85f68c00 pointer offset 0 size 3072
> > Register r8 information: non-paged memory
> > Register r9 information: non-paged memory
> > Register r10 information: NULL pointer
> > Register r11 information: 2-page vmalloc region starting at 0xdf804000 allocated at start_kernel+0x6b0/0x860 init/main.c:1111
> > Register r12 information: 2-page vmalloc region starting at 0xdf804000 allocated at start_kernel+0x6b0/0x860 init/main.c:1111
> > Process syz-executor (pid: 2128, stack limit = 0xeda4c000)
> > Stack: (0xdf804010 to 0xdf806000)
> > 4000:                                     00000018 00000000 9837f050 00000000
[...]
> > 44c0: df804594 df8044e0 815cdeec 80203e84 8245bc84 85db88d8 00001501 85f68c00
                            ^^^^^^^^
[...]
> > 5fc0: 8025be48 8025b9cc df805ffc df805fd8 81aaeb14 8025be44 81abcf40 60000013
> > 5fe0: ffffffff eda4dbac 82aed0d0 85f68c00 eda4db74 df806000 81a7eaa8 81aaeaa4
> > Call trace: frame pointer underflow
[...]
> > [<802e463c>] (vprintk) from [<80203ea8>] (_printk+0x34/0x58 kernel/printk/printk.c:2451)
> > [<80203e74>] (_printk) from [<815cdeec>] (__dev_queue_xmit+0xcb4/0x1244 net/core/dev.c:4834)
                                  ^^^^^^^^

I wanted to double check whether printk() was responsible for
eating the stack. It might use some buffers somewhere...

If I get it correctly then "815cdeec" is the return
address for printk(). And if I cound it correctly then printk was
called when almost 7k from the 8k stack has already been used:

stack size:  0x6000-0x4000 = 0x2000 = 8192 = 8k
printk at:   0x6000-0x44c0 = 0x1b40 = 6976
remaining:   0x44c0-0x4000 = 0x4c0  = 1216

My conclusion is that printk() is _not_ the sinner here.

> >  r3:85f68c00 r2:00001501 r1:85db88d8 r0:8245bc84
> 
> Note that net/core/dev.c:4834 from __dev_queue_xmit() is:
> 
> 			/* Recursion is detected! It is possible,
> 			 * unfortunately
> 			 */
> recursion_alert:
> 			net_crit_ratelimited("Dead loop on virtual device %s, fix it urgently!\n",
> 					     dev->name);
> 

Yeah, this seems to be the culprit. If I get it correctly then
"81888f18" is the return address (__dev_queue_xmit) and I see
it repeated more times on the stack...

> > [<815cd238>] (__dev_queue_xmit) from [<81888f18>] (dev_queue_xmit include/linux/netdevice.h:3381 [inline])
                                          ^^^^^^^^^
> > [<815cd238>] (__dev_queue_xmit) from [<81888f18>] (neigh_hh_output include/net/neighbour.h:540 [inline])

Best Regards,
Petr

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