Re: [Qemu-trivial] [Qemu-devel] [PATCH] qemu-sockets: do not test path with access() before unlinking

[Cole Robinson <crobinso@redhat.com>]

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 <pbonzini@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