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Date: Wed, 10 Oct 2012 18:23:27 +0200
From: Stefan Weil <Stefan.Weil@weilnetz.de>
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To: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-trivial] [Qemu-devel] [PATCH] virtfs-proxy-helper: check
 return code of setfsgid/setfsuid
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Am 10.10.2012 18:17, schrieb Paolo Bonzini:
> Il 10/10/2012 18:14, Stefan Weil ha scritto:
>>>
>>> diff --git a/fsdev/virtfs-proxy-helper.c b/fsdev/virtfs-proxy-helper.c
>>> index f9a8270..b34a84a 100644
>>> --- a/fsdev/virtfs-proxy-helper.c
>>> +++ b/fsdev/virtfs-proxy-helper.c
>>> @@ -290,8 +290,12 @@ static int setfsugid(int uid, int gid)
>>>            CAP_DAC_OVERRIDE,
>>>        };
>>>    -    setfsgid(gid);
>>> -    setfsuid(uid);
>>> +    if (setfsgid(gid) != 0) {
>>> +        return -1;
>>> +    }
>> Wouldn't setfsgid(gid) == gid be also ok?
> Of course, it should be < 0.  I have no idea how to test this thing...
>
> Paolo

< 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;
}

Stefan

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h> /* glibc uses <sys/fsuid.h> */
#include <sys/fsuid.h>

int main(int argc, char *argv[])
{
   uid_t fsuid = strtoul(argv[1], 0, 0);
   int r = setfsuid(fsuid);
   printf("setfsuid(%u) returned %u\n", fsuid, r);
   return 0;
}