setgid在/ tmp /上不起作用

Question

I have this C program:

#include <sys/stat.h>
#include <stdlib.h>

int main(void) {
    if (chmod("/tmp/foo", 0755 | S_ISGID) < 0) {
            exit(1);
    }
    exit(0);
}

When I run it like this:

rm -f /tmp/foo &&
touch /tmp/foo &&
./a.out &&
ls -al /tmp/foo &&

a.out runs with exit code 0 but the output is:

-rwxr-xr-x  1 philipp  wheel  0 Mar 16 06:58 /tmp/foo

Why is the groupid flag not set here? The permissions should be -rwxr-sr-x .

The following things would fix the issue (but I still wonder why I see this effect):

• running the program as root
• running it in a different directory
• running it on Linux
• setting the set-user-id ( S_ISUID )
• I can swear it worked in an earlier version of OSX

What I tried but didn't work:

• chmod g+s /tmp/foo also doesn't work
• disabling csrutil did not change anything
• altering the permissions on /tmp/ to something different, eg 0777 or 4777

So the question remains: what does make /tmp/ different from the other directories if it's not the permissions? The only difference I could see is:

ls -al /

showed tmp as this:

lrwxr-xr-x@   1 root  wheel     11 Dec 11 19:28 tmp -> private/tmp

The @ sign at the end shows that there are some non-unix properties set on the directory. Querying those with ls -l@ /tmp shows:

lrwxr-xr-x@ 1 root  wheel  11 Dec 11 19:28 /tmp -> private/tmp
    com.apple.FinderInfo    32
    com.apple.rootless       0

Update : According to comment feedbacks and a downvote I figured the question is confusing, so I totally revised the question and the title. During revision I found out that I wrongly compared the effects of my program against chmod u+s which was wrong, I need to compare against chmod g+s , I also corrected this in my question.

Answer 1

The chmod() system call sets the permissions on a file to only the value you provide. This means that setting permissions to S_IRUSR | S_ISGID S_IRUSR | S_ISGID clears all other permissions, including user write and execute.

What you probably want is:

chmod("/tmp/foo", 0755 | S_ISGID);

( 0755 being the octal mode for user read+write+execute and group/other read+execute -- it's a lot less typing than the equivalent constants.)