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Date: Wed, 10 Oct 2012 18:36:48 +0200
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Subject: Re: [Qemu-trivial] [Qemu-devel] [PATCH] virtfs-proxy-helper: check
 return code of setfsgid/setfsuid
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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