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

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/11/2015 16:18, Cole Robinson wrote:
>> > -    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.

You're right.  I misread how to test the change.

The right change is what Markus proposed.

Paolo