Hi Marc-André,

By looking into chardev and yank_register_function logic,  this old chardev
is registered according to the chardev label.
So it's been in yank_instance_list. yank instance only has a chardev label,
and the new chardev's label is the same as the old chardev.
So it doesn't need to register it again when changing the chardev backend.
Otherwise, it will report  duplicated yank instances.
I think the chardev logic has no problems. And it works with yank
functions.

Thanks
Li

On Mon, Mar 15, 2021 at 7:51 PM Marc-André Lureau <
marcandre.lureau@gmail.com> wrote:

> Hi
>
> On Mon, Mar 15, 2021 at 9:22 PM Li Zhang <zhlcindy@gmail.com> wrote:
>
>> From: Li Zhang <li.zhang@cloud.ionos.com>
>>
>> When executing the QMP commands "chardev-change" to change the
>> backend device to socket, it will cause a segment fault because
>> it assumes chr->label as non-NULL in function yank_register_instance.
>> The function qmp_chardev_change calls chardev_new, which label
>> is NULL when creating a new chardev. The label will be passed to
>> yank_register_instance which causes a segment fault. The callchain
>> is as the following:
>>         chardev_new ->
>>             qemu_char_open ->
>>                 cc->open ->
>>                 qmp_chardev_open_socket ->
>>                     yank_register_instance
>>
>> Signed-off-by: Li Zhang <li.zhang@cloud.ionos.com>
>> ---
>>  chardev/char-socket.c | 8 +++++---
>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/chardev/char-socket.c b/chardev/char-socket.c
>> index c8bced76b7..26d5172682 100644
>> --- a/chardev/char-socket.c
>> +++ b/chardev/char-socket.c
>> @@ -1421,10 +1421,12 @@ static void qmp_chardev_open_socket(Chardev *chr,
>>          qemu_chr_set_feature(chr, QEMU_CHAR_FEATURE_FD_PASS);
>>      }
>>
>> -    if (!yank_register_instance(CHARDEV_YANK_INSTANCE(chr->label),
>> errp)) {
>> -        return;
>> +    if (chr->label) {
>> +        if (!yank_register_instance(CHARDEV_YANK_INSTANCE(chr->label),
>> errp)) {
>> +            return;
>> +        }
>> +        s->registered_yank = true;
>>      }
>> -    s->registered_yank = true;
>>
>>      /* be isn't opened until we get a connection */
>>      *be_opened = false
>>
>
> Looks wrong to me, the new chardev will get the same label, and it should
> still be possible to call the yank functions then. The registration logic
> needs to be reworked during chardev-change.
>
> --
> Marc-André Lureau
>