“getgroups”的奇怪行为

Question

I'm working on a little learning project,need to call "getgroups(int gidsetsize, gid_t grouplist[])" I'v got "0" result of an id should have a list.

while checking all possibilities,I found out the user's group must not be "0",or the function won't return none 0 result. but i'm only meet this problem on my own computer which running archlinux. I checked virtual machine which use manjaro or ubuntu,none of them has the problem, a co-league has an vps which use arch too do not has the problem.

arch bbs replied "the gid of user should not be 0",but it couldn't explain why my machine is the only one has the problem.

I'v compared id output before the post.

Only the physical machine give me NULL list,and the strace output is different

my machine result

    rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
    getgroups(0, NULL)                     = 0
    getgroups(0, [])                        = 0
    newfstatat(AT_FDCWD, "/etc/nsswitch.conf", {st_mode=S_IFREG|0644, st_size=359, ...}, 0) = 0

other machine's the first getgroups will return none 0 result,and then,the 2nd will use the result as the 1st para to get a list. I couldn't find the difference by myself.

The ubuntu vm result

getgroups(0, NULL)                      = 7
getgroups(7, [0, 4, 24, 27, 30, 46, 110])                        = 7

"id" command only return the none zero result to get "self" result but not other's because of different execution branch.

Tried edit the user's gid to none 0,after reboot,the result going to normal

Change gid back to 0 again,after reboot,the result is NULL again. Might be something about user namespace?

Any suggestions?

edit,more info:

Seems the problem is related to "WHO IS THE FATHER". When the process is child(or grand child etc.) of

/usr/lib/systemd/systemd --user

getgroups will give bad result.

When not(running on i3wm,the process has no father)the result is good

Answer 1

following the trace of systemd,arch bbs give me this.

User service not running with supplementary groups

------------EDIT---------------

It's not the same problem,possible another bug.