Coda File System

ruid or euid/fsuid?

From: Tapio Suihko <>
Date: 14 Apr 2003 14:19:18 +0300

I have a question concerning the usage of real uid vs. effective (or
file system) uid in venus.

In coda release 5.3.20, venus seems to use real uid (ruid) in file
access because it extracts the real uid from struct coda_cred by using
CRTORUID(cred). However, in venus/vproc.h, there is the comment

>   For all filesystem use CRTORUID is used: for Linux we definitely
>   want to fsuid to be used for filesystem access.  This however
>   breaks the old AFS semantics that if an "su" is performed you
>   retain tokens.

Accordingly, for Linux, there is the definition

> #define CRTORUID(cred)  ((vuid_t)((cred).cr_fsuid))

which, indeed, makes venus to use fsuid instead of ruid.

However, in the later CVS revisions, credentials handling has been
streamlined and coda_cred has collapsed into a single uid value. So,
in the kernel file .../fs/coda/upcall.c, in function alloc_upcall(), we

> inp->ih.uid = current->uid;

This is the real uid. So, in the case of Linux, access control
semantics seem to be changing since 5.3.20.

My application works with 5.3.20 (I am running Linux 2.4.20). In order
to get it to work with a newer CVS release of Coda, I changed the
above line to

inp->ih.uid = current->fsuid;

This is about whose Coda tokens are used in file access, I assume (so,
it is not directly or only related to your discussion about local
access control at the Coda client).

If by chance my observations mentioned above happened to make any
sense, I would like to know the rationale behind this
change. Personally, for now, I would be happy if 5.3.20 semantics were
retained (until someone points out a huge security hole in using fsuid
or euid :-))



Tapio Suihko               Technical Research Centre of Finland (VTT)                            Information Technology
Tel. + 358 9 456 6078                  P.O.Box 1203 (Tekniikantie 4B)
GSM. + 358 40 552 9646                         FIN-02044 VTT, FINLAND
Fax. + 358 9 456 7028

Received on 2003-04-14 07:22:17