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

[Stefan Weil]

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;
}