On 11/02/2015 09:10 AM, Paolo Bonzini wrote:
> Using access() is a time-of-check/time-of-use race condition. It is
> okay to use them to provide better error messages, but that is pretty
> much it.
>
> This is not one such case, so just drop the call.
>
> Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
> ---
> util/qemu-sockets.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
> index 9142917..2833c70 100644
> --- a/util/qemu-sockets.c
> +++ b/util/qemu-sockets.c
> @@ -751,8 +751,7 @@ int unix_listen_opts(QemuOpts *opts, Error **errp)
> qemu_opt_set(opts, "path", un.sun_path, &error_abort);
> }
>
> - if ((access(un.sun_path, F_OK) == 0) &&
> - unlink(un.sun_path) < 0) {
> + if (unlink(un.sun_path) < 0) {
> error_setg_errno(errp, errno,
> "Failed to unlink socket %s", un.sun_path);
> goto err;
>
This is a serious semantic change, after this patch you will get:
$ ./x86_64-softmmu/qemu-system-x86_64 -vnc unix:/tmp/idontexist.sock
qemu-system-x86_64: -vnc unix:/tmp/idontexist.sock: Failed to start VNC
server: Failed to unlink socket /tmp/idontexist.sock: No such file or directory
Previously it would 'just work'. Common libvirt usage depends on this as well
Yeah there's a TOCTTOU race here, but it's very minor: if sun_path is created
after the access() check, qemu is just going to fail to start since bind()
will barf if the unix socket path exists.
- Cole
