Re: [Qemu-devel] [PATCH] add fd limitations for avoiding a buffer overflow

[Stefan Hajnoczi]

On Fri, Jan 25, 2013 at 04:14:49PM +0800, Amos Kong wrote:
> FD_SET() and FD_CLR() are used to add and remove one descriptor from a
> set, the 'fd' should be less than FD_SETSIZE. Glibc will give a warning
> and crash the qemu when we set a fd (1024) to a set.
> 
>  # qemu -device virtio-net-pci,netdev=macvtap_netdev,mac=92:ff:8a:11:fe:57
>    -netdev tap,id=macvtap_netdev,fd=1024 1024<>/dev/tap4
> 
> *** buffer overflow detected ***: x86_64-softmmu/qemu-system-x86_64
> terminated
> ======= Backtrace: =========
> /lib64/libc.so.6(__fortify_fail+0x37)[0x7f842a2134a7]
> /lib64/libc.so.6(+0x35e9d08620)[0x7f842a211620]
> /lib64/libc.so.6(+0x35e9d0a417)[0x7f842a213417]
> x86_64-softmmu/qemu-system-x86_64(+0x1901fd)[0x7f842f09f1fd]
> x86_64-softmmu/qemu-system-x86_64(+0x198388)[0x7f842f0a7388]
> x86_64-softmmu/qemu-system-x86_64(main+0xfa9)[0x7f842ef897a9]
> /lib64/libc.so.6(__libc_start_main+0xf5)[0x7f842a12aa05]
> x86_64-softmmu/qemu-system-x86_64(+0x7ed49)[0x7f842ef8dd49]
> ======= Memory map: ========
> ....
> 
> This patch added limitations when init tap device and set fd handler
> for synchronous IO.
> 
> Signed-off-by: Amos Kong <address@hidden>
> ---
>  iohandler.c |    3 +++
>  net/tap.c   |    3 ++-
>  2 files changed, 5 insertions(+), 1 deletions(-)

It would be much nicer to avoid fd_set than to add error code paths for
this API limitation.

With increased use of thread eventfds/pipe notification as seen in
vhost-net and virtio-blk-data-plane, the chance of really hitting 1024
file descriptors is growing.  I've already seen a PCI multifunction test
case where we exceed 1024 fds :(.


Anthony, Paolo: Is there an alternative to select(2)?  I think this was
discussed a bit during the glib event loop integration.

The two requirements I can think of are:

1. Portable so that we don't have to write OS-specific versions (epoll,
kqueue, etc).

2. Sub-millisecond timeouts.

Maybe we can use timerfd (and emulate it on non-Linux hosts) and then
fully use the glib event loop?

Internally glib prefers poll(2) but will fall back to select(2) on weird
OSes.  On Windows it has dedicated code.

Stefan