Real and effective userids.

[Conor P. Cahill]

In article <291 at bmers58.UUCP>, davem at bmers58.UUCP (Dave Mielke) writes:
> In article <1219 at virtech.UUCP> cpcahil at virtech.UUCP (Conor P. Cahill) writes:
> >The real userid is the numerical id of "the user"
> >that is running a process, as opposed to "the effective" id of the process
> >that is used to determine whether you can open, create, unlink, etc. a file.
>
> This is intuitively the way things should work, but when I open a file
> from within a setuid program it appears to enforce the access rights of
> the real userid and not those of the effective userid. Why is this?


You don't post your program so I can't tell you what is happening, but using
the following code:

	main()
	{
		close(creat("testfile.cpc",0777));
	}

compiling the program, changing the mode to 4755, 

	-rwsr-xr-x   1 cpcahil  opadmin     5281 Oct  3 07:20 /tmp/t

log in as user angie with home directory mode:

	drwxr-x---   7 angie    opadmin      512 Oct  3 07:23 .

Run /tmp/t and NO file is created since only the owner has write access and the
program runs as cpcahil (who is not the owner).

Run "chmod g+w ." and then re-run /tmp/t and the following file is created
since the user's group does have write access to the current directory:

	drwxr-x---   1 cpcahil  opadmin        0 Oct  3 07:24 testfile.cpc

Note that the file is not owned by the current user (angie) but by the owner
of the program that is setuid.

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