* Re: [Qemu-devel] [PATCH] qemu-sockets: do not test path with access() before unlinking
@ 2015-11-02 15:18 ` Cole Robinson
0 siblings, 0 replies; 8+ messages in thread
From: Cole Robinson @ 2015-11-02 15:18 UTC (permalink / raw)
To: Paolo Bonzini, qemu-devel; +Cc: qemu-trivial
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
^ permalink raw reply [flat|nested] 8+ messages in thread* Re: [Qemu-trivial] [Qemu-devel] [PATCH] qemu-sockets: do not test path with access() before unlinking
2015-11-02 15:18 ` Cole Robinson
@ 2015-11-02 15:50 ` Markus Armbruster
-1 siblings, 0 replies; 8+ messages in thread
From: Markus Armbruster @ 2015-11-02 15:50 UTC (permalink / raw)
To: Cole Robinson; +Cc: qemu-trivial, Paolo Bonzini, qemu-devel
Cole Robinson <crobinso@redhat.com> 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 <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.
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) {
^ permalink raw reply [flat|nested] 8+ messages in thread* Re: [Qemu-devel] [PATCH] qemu-sockets: do not test path with access() before unlinking
@ 2015-11-02 15:50 ` Markus Armbruster
0 siblings, 0 replies; 8+ messages in thread
From: Markus Armbruster @ 2015-11-02 15:50 UTC (permalink / raw)
To: Cole Robinson; +Cc: qemu-trivial, Paolo Bonzini, qemu-devel
Cole Robinson <crobinso@redhat.com> 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 <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.
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) {
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [Qemu-trivial] [Qemu-devel] [PATCH] qemu-sockets: do not test path with access() before unlinking
2015-11-02 15:18 ` Cole Robinson
@ 2015-11-02 15:52 ` Paolo Bonzini
-1 siblings, 0 replies; 8+ messages in thread
From: Paolo Bonzini @ 2015-11-02 15:52 UTC (permalink / raw)
To: Cole Robinson, qemu-devel; +Cc: qemu-trivial
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
^ permalink raw reply [flat|nested] 8+ messages in thread* Re: [Qemu-devel] [PATCH] qemu-sockets: do not test path with access() before unlinking
@ 2015-11-02 15:52 ` Paolo Bonzini
0 siblings, 0 replies; 8+ messages in thread
From: Paolo Bonzini @ 2015-11-02 15:52 UTC (permalink / raw)
To: Cole Robinson, qemu-devel; +Cc: qemu-trivial
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
^ permalink raw reply [flat|nested] 8+ messages in thread