Re: [Qemu-devel] [PATCH] virtfs-proxy-helper: check return code of setfsgid/setfsuid

[Paolo Bonzini <pbonzini@redhat.com>]

Il 10/10/2012 18:23, Stefan Weil ha scritto:
> < 0 would be wrong because it looks like both functions never
> return negative values.
> I just wrote a small test program (see
> below) and called it with different uids with and without root
> rights. This pattern should be fine:
> 
> new_uid = setfsuid(uid);
> if (new_uid != 0 && new_uid != uid) {
>   return -1;
> }

I didn't really care about this case.  I assumed that the authors knew
what they were doing...

What I cared about is: "When glibc determines that the argument is not a
 valid  group  ID,  it will  return  -1  and set errno to EINVAL without
attempting the system call".

I think this would also work:

   if (setfsuid(uid) < 0 || setfsuid(uid) != uid) {
       return -1;
   }

but it seems wasteful to do four syscalls instead of two.

Paolo