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 <lviv...@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 <phi...@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 >
