Re: [Qemu-devel] [PATCH 4/4] ivshmem: check the value returned by fstat()

[Levente Kurusa <lkurusa@redhat.com>]

----- Original Message -----
> Hi Michael,
> Thanks for your review of this patch!
> 
> > On Mon, Aug 04, 2014 at 04:25:44PM +0800, zhanghailiang wrote:
> >> The function fstat() may fail, so check its return value.
> >>
> >> Signed-off-by: zhanghailiang<zhang.zhanghailiang@huawei.com>
> >> ---
> >>   hw/misc/ivshmem.c | 5 ++++-
> >>   1 file changed, 4 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/hw/misc/ivshmem.c b/hw/misc/ivshmem.c
> >> index 768e528..2667e9f 100644
> >> --- a/hw/misc/ivshmem.c
> >> +++ b/hw/misc/ivshmem.c
> >> @@ -324,7 +324,10 @@ static int check_shm_size(IVShmemState *s, int fd) {
> >>
> >>       struct stat buf;
> >>
> >> -    fstat(fd,&buf);
> >> +    if (fstat(fd,&buf)<  0) {
> >> +        fprintf(stderr, "Cannot stat IVSHMEM: %s\n", strerror(errno));
> >> +        return -1;
> >> +    }
> >>
> >
> > That's a confusing error message:
> > 1. You don't stat ivshmem. You stat a shmem fd. Also best to print fd #.
> I will add this in next version of the patch.
> > 2. Tell the user what action was taken, e.g. IVSHMEM failed to start.
> >
> >>       if (s->ivshmem_size>  buf.st_size) {
> >>           fprintf(stderr,
> >> --
> I have check the places of calling this function, And found that, if
> this function return -1, qemu will call exit(-1). One of the callers is
> ivshmem_read(), the purpose of check_shm_size() is to forbid guest to
> map more memory than the object has allocated. So here is it suitable to
> return -1 if fstat() failed? Or just give a warning message and return 0?
> what's your opinion? Thanks.

Yes, please return -1. All the errors fstat(2) can return in this scenario
will be fatal to ivshmem.

Exiting is probably not the best way to go, but I'm working on a fix for
that at the moment, and for now, let's just exit, like all the other error
paths do.

Thanks,
Levente Kurusa