Strange bug with a folder doc in /home/firefox

[Caramel]

When Firefox is open, the folder /home/firefox/.cache/doc is inaccessible

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ls /home/firefox/.cache/doc
ls: impossible d'accéder à '/home/firefox/.cache/doc': Permission denied

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# ps | grep fuse
19474 root      0:00 fusermount3 -o rw,nosuid,nodev,fsname=portal,auto_unmount,subtype=portal -- /home/firefox/.cache/doc
29479 root      0:00 grep fuse

and see the last line in /etc/mtab

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# cat /etc/mtab 
none /proc proc rw,relatime,gid=118,hidepid=noaccess 0 0
none /sys sysfs rw,relatime 0 0
devtmpfs /dev devtmpfs rw,relatime,size=2009716k,nr_inodes=502429,mode=755 0 0
/dev/sda35 /mnt/sda35 ext4 rw,relatime,commit=30,data=ordered 0 0
tmpfs /mnt/.easy_ro tmpfs rw,relatime,size=812192k 0 0
/dev/loop0 /mnt/.easy_ro/easy_sfs squashfs ro,noatime,errors=continue 0 0
aufs / aufs rw,relatime,si=debe19d2717487bb 0 0
securityfs /sys/kernel/security securityfs rw,relatime 0 0
devpts /dev/pts devpts rw,relatime,mode=600,ptmxmode=000 0 0
shmfs /dev/shm tmpfs rw,relatime,size=999456k 0 0
tmpfs /tmp tmpfs rw,relatime,size=1998908k 0 0
portal /home/firefox/.cache/doc fuse.portal rw,nosuid,nodev,relatime,user_id=1007,group_id=1005 0 0

Very strange.

Edit : The parent process ID (PPID) of the fusermount3 command is /usr/libexec/xdg-document-portal

Related links :
https://bugs.launchpad.net/ubuntu/+sour ... ug/1906744
https://bugs.launchpad.net/ubuntu/+sour ... ug/1905623
https://github.com/flatpak/xdg-desktop- ... issues/512

[BarryK]

@Caramel
Like you, I was immensely puzzled.
I see Chromium also creates /home/chromium/.cache/doc, with the same problem.

But then I read this:

https://github.com/flatpak/xdg-desktop- ... issues/512

The document portal is a fuse mount, so it is denied access to by any uid other than the owning uid, including root. So, I expect the regular /run/user/1000/doc to be inaccessible by root.

So, did this:

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# login -f firefox
# whoami
firefox
# ls /home/firefox/.cache/doc
by-app
# 

...works!