SUID bit ignored when creating files / directories

Question

I've set the SUID & SGID bit on a folder belonging to user foo with sudo chmod g+s myfolder & sudo chmod u+s myfolder

drwsr-sr-x 24 foo www-data 4,0K Okt 25 16:17 myfolder

Then I went inside and created a folder with sudo mkdir xyz, but the user of the folder gets overwritten with root while the group was protected successfully.

drwxr-sr-x  2 root  www-data 4,0K Okt 25 16:24 xyz

I expect the user to be protected, it should stay at foo after executing sudo mkdir xyz. What have I missed?

is setuid supposed to work to override the owner of the created file in some system? — ilkkachu, Oct 25 '22 at 14:53
Does this answer your question? Setting default username and group for files in directory — Chris Davies, Oct 25 '22 at 15:19

score 0 · Answer 1 · answered Oct 25 '22 at 14:50

0

xyz This doesn't look like a problem because you used sudo
which of course should be root instead of foo, and you set chmod g+s myfolder so it's www-data
sudo is only used when needed, not for every command, you can try mkdir xyz

answered Oct 25 '22 at 14:50

山河以无恙

155

score 0 · Answer 2 · answered Oct 25 '22 at 15:20

If you are user foo then without the SUID/SGID bits the permissions will work exactly as you seem to want:

ls -ld myfolder myfolder/xyz
drwxr-xr-x 3 foo www-data 4096 Oct 25 16:00 myfolder
drwxr-xr-x 2 foo www-data 4096 Oct 25 16:00 myfolder/xyz/

SUID doesn't apply to directories, so the directory will be created with you as the owner. With SGID the group carries through, even if the creator/owner is not a member of that group.

However, if ACLs are available on the target filesystem you can use them to allow user foo and/or members of group www-data access to the directories regardless of the visible owner and group.

SUID bit ignored when creating files / directories

2 Answers2