note that samba gets rid of leading, trailing, or multiple sequential '/' or '\\' characters, so this reproducer will require a windows server

Tested against rawhide (kernel-5.16.0-0.rc4.20211207gitcd8c917a56f2.30.fc36.x86_64), and it's the same situation.

The patches against cifs between rc4 and rc6 (current upstream) don't show this bug correction.

$ git log --oneline v5.16-rc4..HEAD -- fs/cifs/
9de0737d5ba0 cifs: fix ntlmssp auth when there is no key exchange

So I believe that upstream has this bug.

Also, it seems that any mount where the prepath has more than one '\' between directories is affected, e.g.

# mount -t cifs -o username=user //192.168.0.3/share/subfolder//subsub /mnt
# touch /mnt/a
# mv /mnt/a /mnt/b
mv: cannot move '/mnt/a' to '/mnt/b': No such file or directory

Patch upstream: https://patchwork.kernel.org/project/cifs-client/patch/20211215030831.24058-1-trbecker@gmail.com/

v2 of the patch submitted. https://patchwork.kernel.org/project/cifs-client/patch/20211217182022.589857-1-trbecker@gmail.com/

I've found that multiple delimiters between UNC and prepath (//<unc>////<prepath) failed to mount the share. Also, there's a possible memory leak in the code that should be fixed now.

Test script that I've been using.

~~~
#!/bin/bassh
SERVER=192.168.0.3
UNC=//$SERVER/Users
PATH1=user
PATH2=Videos
USER=user
PASS='p4ssw0rd'

umount /mnt

echo 7 > /proc/fs/cifs/cifsFYI
echo "func build_path_from_dentry_optional_prefix +p" > /sys/kernel/debug/dynamic_debug/control
dmesg -C

rm -fr /tmp/{cifs.pcap,streams.txt,dmesg.txt}
tcpdump -i eth0 -w /tmp/cifs.pcap host $SERVER &
tcpdump_pid=$!

function testfn {
	echo Testing with DEVICE=$1
	mount -t cifs -o username=$USER,pass="$PASS" $1 /mnt
	touch /mnt/a
	mv /mnt/a /mnt/b
	rm /mnt/a /mnt/b 2>/dev/null
	umount /mnt
}

testfn $UNC/$PATH1
testfn $UNC//$PATH1
testfn $UNC/$PATH1/
testfn $UNC/$PATH1/$PATH2/
testfn $UNC/$PATH1//$PATH2
testfn $UNC///$PATH1////$PATH2///

echo 0 > /proc/fs/cifs/cifsFYI
echo "func build_path_from_dentry_optional_prefix -p" > /sys/kernel/debug/dynamic_debug/control
kill $tcpdump_pid
dmesg > /tmp/dmesg.txt
for stream in $(tshark -r /tmp/cifs.pcap -Y smb2 -T fields -e tcp.stream | sort -u) ; do
	echo ===== $stream =====
	tshark -r /tmp/cifs.pcap -Y tcp.stream==$stream 
done > /tmp/streams.txt
~~~

All tests passing on v2

Patch accepted upstream, hash a31080899d5fdafcccf7f39dd214a814a2c82626.
Merge request opened for rhel-8, https://gitlab.com/redhat/rhel/src/kernel/rhel-8/-/merge_requests/1857

Setting the severity. This bug happens on upgrade, under specific conditions, breaking the proper function of systems that where working previously, so I feel like this is good for 'urgent'. Feel free to change it appropriately.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (Important: kernel security, bug fix, and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2022:1988