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

[Markus Armbruster]

Cole Robinson <address@hidden> writes:

> 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 <address@hidden>
>> ---
>>  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.

As Paolo says, access() is almost never a solution to anything.  Here,
the proper solution should be something like

    if (unlink(un.sun_path) < 0 && errno != ENOENT) {