archlinux fix

[CastixGitHub]

to replicate it (change just USERNAME with an existing user):

USERNAME="castix"

function mktemptest {
    echo "with $1"
    ls -al $1
    file $1
    cat $1
    ls -al $1
    echo "ok" > $1
    echo "ok"
    chown $USERNAME $1
    chmod 777 $1
    ls -al $1
    echo "okok" > $1
    echo "okok"
    echo
    rm $1
}

mktemptest $(mktemp)
mktemptest $(touch here; echo here)
mktemptest $(touch /tmp/here; echo /tmp/here)
ls -ld /tmp

then as root (su or sudo) run it sudo bash test
my output is

with /tmp/tmp.V8KOHGTLPe
-rw------- 1 root root 0 Jun 22 20:45 /tmp/tmp.V8KOHGTLPe
/tmp/tmp.V8KOHGTLPe: empty
-rw------- 1 root root 0 Jun 22 20:45 /tmp/tmp.V8KOHGTLPe
ok
-rwxrwxrwx 1 castix root 3 Jun 22 20:45 /tmp/tmp.V8KOHGTLPe
test: line 14: /tmp/tmp.V8KOHGTLPe: Permission denied
okok

with here
-rw-r--r-- 1 root root 0 Jun 22 20:45 here
here: empty
-rw-r--r-- 1 root root 0 Jun 22 20:45 here
ok
-rwxrwxrwx 1 castix root 3 Jun 22 20:45 here
okok

with /tmp/here
-rw-r--r-- 1 root root 0 Jun 22 20:45 /tmp/here
/tmp/here: empty
-rw-r--r-- 1 root root 0 Jun 22 20:45 /tmp/here
ok
-rwxrwxrwx 1 castix root 3 Jun 22 20:45 /tmp/here
test: line 14: /tmp/here: Permission denied
okok

drwxrwxrwt 13 root root 540 Jun 22 20:45 /tmp

as you can see it says permission denied (even with chmod 777)
if I run it without root privileges it will execute fine.

tried it also across reboot, and more interesting tried also in raspbian and here's the output

root@biancucia:/home/pi# bash test
with /tmp/tmp.c2cTpdah83
-rw------- 1 root root 0 Jun 22 19:35 /tmp/tmp.c2cTpdah83
/tmp/tmp.c2cTpdah83: empty
-rw------- 1 root root 0 Jun 22 19:35 /tmp/tmp.c2cTpdah83
ok
-rwxrwxrwx 1 pi root 3 Jun 22 19:35 /tmp/tmp.c2cTpdah83
okok

with here
-rw-r--r-- 1 root root 0 Jun 22 19:35 here
here: empty
-rw-r--r-- 1 root root 0 Jun 22 19:35 here
ok
-rwxrwxrwx 1 pi root 3 Jun 22 19:35 here
okok

with /tmp/here
-rw-r--r-- 1 root root 0 Jun 22 19:35 /tmp/here
/tmp/here: empty
-rw-r--r-- 1 root root 0 Jun 22 19:35 /tmp/here
ok
-rwxrwxrwx 1 pi root 3 Jun 22 19:35 /tmp/here
okok

drwxrwxrwt 9 root root 4096 Jun 22 19:35 /tmp

for completeness here's uname -a
Linux triangolo 5.7.3-arch1-1 #1 SMP PREEMPT Wed, 17 Jun 2020 19:42:12 +0000 x86_64 GNU/Linux

so, write the config file, then set the owner

[CastixGitHub]

finally found an answer

https://unix.stackexchange.com/questions/503111/group-permissions-for-root-not-working-in-tmp

fs.protected_regular Linux kernel parameter