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SubjectIssues with a rather unusual configured NFS server
so that the server either crashes (if it is a user mode linux image) or at least its reboot functionality got broken
- if the NFS server is hammered with scary NFS calls using a fuzzy tool running at a remote NFS client under a non-privileged user id.

It can re reproduced, if
- the NFS share is an EXT3 or EXT4 directory
- and it is created at file located at tempfs and mounted via loop device
- and the NFS server is forced to umount the NFS share
- and the server forced to restart the NSF service afterwards
- and trinity is used

I could find a scenario for an automated bisect. 2 times it brought this commit
commit 68a3396178e6688ad7367202cdf0af8ed03c8727
Author: J. Bruce Fields <>
Date: Thu Mar 21 11:21:50 2013 -0400

nfsd4: shut down more of delegation earlier

to be the one after which the user mode linux server crashes with a back trace like this:

$ cat /mnt/ramdisk/bt.v3.11-rc4-172-g8ae3f1d
[New LWP 14025]
Core was generated by `/home/tfoerste/devel/linux/linux earlyprintk ubda=/home/tfoerste/virtual/uml/tr'.
Program terminated with signal 6, Aborted.
#0 0xb77ef424 in __kernel_vsyscall ()
#0 0xb77ef424 in __kernel_vsyscall ()
#1 0x083a33c5 in kill ()
#2 0x0807163d in uml_abort () at arch/um/os-Linux/util.c:93
#3 0x08071925 in os_dump_core () at arch/um/os-Linux/util.c:138
#4 0x080613a7 in panic_exit (self=0x85a1518 <panic_exit_notifier>, unused1=0, unused2=0x85d6ce0 <buf.15904>) at arch/um/kernel/um_arch.c:240
#5 0x0809a3b8 in notifier_call_chain (nl=0x0, val=0, v=0x85d6ce0 <buf.15904>, nr_to_call=-2, nr_calls=0x0) at kernel/notifier.c:93
#6 0x0809a503 in __atomic_notifier_call_chain (nr_calls=<optimized out>, nr_to_call=<optimized out>, v=<optimized out>, val=<optimized out>, nh=<optimized out>) at kernel/notifier.c:182
#7 atomic_notifier_call_chain (nh=0x85d6cc4 <panic_notifier_list>, val=0, v=0x85d6ce0 <buf.15904>) at kernel/notifier.c:191
#8 0x08400ba8 in panic (fmt=0x0) at kernel/panic.c:128
#9 0x0818edf4 in ext4_put_super (sb=0x4a042690) at fs/ext4/super.c:818
#10 0x081010d2 in generic_shutdown_super (sb=0x4a042690) at fs/super.c:418
#11 0x0810209a in kill_block_super (sb=0x0) at fs/super.c:1028
#12 0x08100f6a in deactivate_locked_super (s=0x4a042690) at fs/super.c:299
#13 0x08101001 in deactivate_super (s=0x4a042690) at fs/super.c:324
#14 0x08118e0c in mntfree (mnt=<optimized out>) at fs/namespace.c:891
#15 mntput_no_expire (mnt=0x0) at fs/namespace.c:929
#16 0x0811a2f5 in SYSC_umount (flags=<optimized out>, name=<optimized out>) at fs/namespace.c:1335
#17 SyS_umount (name=134541632, flags=0) at fs/namespace.c:1305
#18 0x0811a369 in SYSC_oldumount (name=<optimized out>) at fs/namespace.c:1347
#19 SyS_oldumount (name=134541632) at fs/namespace.c:1345
#20 0x080618e2 in handle_syscall (r=0x49e919d4) at arch/um/kernel/skas/syscall.c:35
#21 0x08073c0d in handle_trap (local_using_sysemu=<optimized out>, regs=<optimized out>, pid=<optimized out>) at arch/um/os-Linux/skas/process.c:198
#22 userspace (regs=0x49e919d4) at arch/um/os-Linux/skas/process.c:431
#23 0x0805e65c in fork_handler () at arch/um/kernel/process.c:160
#24 0x00000000 in ?? ()

A real system however would not crash bug would give a kernel BUG as reported here:
Furthermore the server won't be able any longer to reboot - it would hang infinitely in the reboot phase.
Just the magic sysrq keys still works then.

Steps to reproduce at two 32 bit Gentoo Linux user mode linux images:

1. prepare the server :
<mount a tempfs onto /mnt/ramdisk>
mkdir /mnt/ramdisk/victims
dd if=/dev/zero of=/mnt/ramdisk/disk1 bs=1M count=257 2>/dev/null
yes | mkfs.ext4 -q /mnt/ramdisk/disk1 1>/dev/null
mount -o loop /mnt/ramdisk/disk1 /mnt/ramdisk/victims
chmod 777 /mnt/ramdisk/victims
/etc/init.d/nfs restart

2. prepare the client
mount the NFS share onto the local mount point /mnt/ramdisk/victims/ with NFSv4

3. run the fuzzy tool trinity at the client:
while [[ : ]]; do
<(re-)create and fill /mnt/ramdisk/victims/v1/v2 with 100 empty files and 100 empty directories>
trinity -V /mnt/ramdisk/victims/v1/v2 -C 1 -N 10000 -q
sleep 3

4. after 15 min kill the user mode linux client with -9

5. now run at the server
umount /mnt/ramdisk/victims || /etc/init.d/nfs restart && umount /mnt/ramdisk/victims && echo ' no issue so far'

You might need this patch too from Oleg Nesterov <> (not in mainline currently) .

--- x/kernel/exit.c
+++ x/kernel/exit.c
@@ -783,8 +783,8 @@ void do_exit(long code)
- exit_task_namespaces(tsk);
+ exit_task_namespaces(tsk);

Toralf Förster
pgp finger print: 7B1A 07F4 EC82 0F90 D4C2 8936 872A E508 7DB6 9DA3
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