The problem appears to come from pCloudDrive (and possibly other FUSE file systems) not following:
stat, fstat, lstat, fstatat - get file status
These functions return information about a file, in the buffer pointed to by statbuf. No
permissions are required on the file itself, but—in the case of stat(), fstatat(), and
lstat()—execute (search) permission is required on all of the directories in pathname that
lead to the file.
and I’m pretty sure an lstat done by the IsBlockDevice is where the error is, and this is before the filter is applied. I’m not certain the order has to be that way, but it’s been that way for a very long time.
What’s interesting is that many Linux systems do have a FUSE file system, specifically GVfs, creating /run/user/1000/gvfs for my user 1000. This denies root access, and one can see the error exactly:
# ls --color=never /run/user/1000
dconf gvfs keyring pulse systemd
# ls /run/user/1000
ls: cannot access '/run/user/1000/gvfs': Permission denied
dconf gvfs keyring pulse systemd
# stat /run/user/1000/gvfs
stat: cannot stat '/run/user/1000/gvfs': Permission denied
# strace -f -e trace=file -P /run/user/1000/gvfs stat /run/user/1000/gvfs
lstat("/run/user/1000/gvfs", 0x7ffe62aa0440) = -1 EACCES (Permission denied)
stat: cannot stat '/run/user/1000/gvfs': Permission denied
+++ exited with 1 +++
#
User 1000 has an easy time, although there’s not really anything to see (possibly unlike pCloudDrive):
$ ls -ln /run/user/1000
total 0
drwx------ 2 1000 1000 60 Mar 29 19:36 dconf
dr-x------ 2 1000 1000 0 Feb 29 20:06 gvfs
drwx------ 2 1000 1000 100 Feb 29 20:06 keyring
drwx------ 2 1000 1000 80 Feb 29 20:06 pulse
drwxr-xr-x 2 1000 1000 80 Feb 29 20:06 systemd
$ ls -l /run/user/1000/gvfs
total 0
$
pCloud has a console client at https://github.com/pcloudcom/console-client which has an option for
-m [ --mountpoint ] arg Mount point where drive to be mounted.
Does pCloudDrive provide a similar option? That would allow the pCloudDrive folder to be below a folder on disk whose purpose was to allow filter at that level, where file checking by root is allowed.
Alternatively, if Duplicati is run as you rather than as root, it will probably be able to filter “as usual”.
Alternatively, it’s possible to grant root access to FUSE filesystems but it needs application support.
fuse - format and options for the fuse file systems describes the allow_root and allow_other options.
Maybe pCloud support could answer these questions. All you need is enough root access to lstat. Further access to files as root is not needed, I assume, because you’re trying to exclude that folder.