From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55965) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZtGsM-00086n-9L for qemu-devel@nongnu.org; Mon, 02 Nov 2015 10:18:30 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZtGsJ-00039E-1y for qemu-devel@nongnu.org; Mon, 02 Nov 2015 10:18:26 -0500 References: <1446473439-10701-1-git-send-email-pbonzini@redhat.com> From: Cole Robinson Message-ID: <56377EBC.5030402@redhat.com> Date: Mon, 2 Nov 2015 10:18:20 -0500 MIME-Version: 1.0 In-Reply-To: <1446473439-10701-1-git-send-email-pbonzini@redhat.com> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] qemu-sockets: do not test path with access() before unlinking List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini , qemu-devel@nongnu.org Cc: qemu-trivial@nongnu.org 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 > --- > 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