Re: [PATCH v3 1/2] net: tap: check if the file descriptor is valid before using it

[Philippe Mathieu-Daudé]

On 7/2/20 10:17 AM, Laurent Vivier wrote:
> On 02/07/2020 10:00, Philippe Mathieu-Daudé wrote:
>> On 7/1/20 9:39 PM, Laurent Vivier wrote:
>>> qemu_set_nonblock() checks that the file descriptor can be used and, if
>>> not, crashes QEMU. An assert() is used for that. The use of assert() is
>>> used to detect programming error and the coredump will allow to debug
>>> the problem.
>>>
>>> But in the case of the tap device, this assert() can be triggered by
>>> a misconfiguration by the user. At startup, it's not a real problem, but it
>>> can also happen during the hot-plug of a new device, and here it's a
>>> problem because we can crash a perfectly healthy system.
>>>
>>> For instance:
>>>  # ip link add link virbr0 name macvtap0 type macvtap mode bridge
>>>  # ip link set macvtap0 up
>>>  # TAP=/dev/tap$(ip -o link show macvtap0 | cut -d: -f1)
>>>  # qemu-system-x86_64 -machine q35 -device 
>>> pcie-root-port,id=pcie-root-port-0 -monitor stdio 9<> $TAP
>>>  (qemu) netdev_add type=tap,id=hostnet0,vhost=on,fd=9
>>>  (qemu) device_add 
>>> driver=virtio-net-pci,netdev=hostnet0,id=net0,bus=pcie-root-port-0
>>>  (qemu) device_del net0
>>>  (qemu) netdev_del hostnet0
>>>  (qemu) netdev_add type=tap,id=hostnet1,vhost=on,fd=9
>>>  qemu-system-x86_64: .../util/oslib-posix.c:247: qemu_set_nonblock: 
>>> Assertion `f != -1' failed.
>>>  Aborted (core dumped)
>>>
>>> To avoid that, add a function, qemu_try_set_nonblock(), that allows to 
>>> report the
>>> problem without crashing.
>>>
>>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>>> ---
>>>  include/qemu/sockets.h |  1 +
>>>  net/tap.c              | 16 +++++++++---
>>>  util/oslib-posix.c     | 26 +++++++++++++------
>>>  util/oslib-win32.c     | 57 ++++++++++++++++++++++++------------------
>>>  4 files changed, 64 insertions(+), 36 deletions(-)
>>>
>>> diff --git a/include/qemu/sockets.h b/include/qemu/sockets.h
>>> index 57cd049d6edd..7d1f8135767d 100644
>>> --- a/include/qemu/sockets.h
>>> +++ b/include/qemu/sockets.h
>>> @@ -18,6 +18,7 @@ int qemu_accept(int s, struct sockaddr *addr, socklen_t 
>>> *addrlen);
>>>  int socket_set_cork(int fd, int v);
>>>  int socket_set_nodelay(int fd);
>>>  void qemu_set_block(int fd);
>>> +int qemu_try_set_nonblock(int fd);
>>>  void qemu_set_nonblock(int fd);
>>>  int socket_set_fast_reuse(int fd);
>>>  
>>> diff --git a/net/tap.c b/net/tap.c
>>> index 6207f61f84ab..fb04c9044ce2 100644
>>> --- a/net/tap.c
>>> +++ b/net/tap.c
>>> @@ -766,6 +766,7 @@ int net_init_tap(const Netdev *netdev, const char *name,
>>>      Error *err = NULL;
>>>      const char *vhostfdname;
>>>      char ifname[128];
>>> +    int ret = 0;
>>
>> No need to zero-initialize.
>>
>> Otherwise:
>> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>>
> 
> I would agree with you but gcc doesn't:
> 
> net/tap.c:769:9: error: ‘ret’ may be used uninitialized in this function
> [-Werror=maybe-uninitialized]
>   769 |     int ret;
>       |         ^~~

I tried to outsmart GCC by manual review :/

> 
> if nfds == 0, we don't enter in the loop and ret is never set.
> 
> Thanks,
> Laurent
>