[Pcsclite-muscle] About SHARED/EXCLUSIVE and starvation Was: Portable way to detect when a smart card is inserted

[Ludovic Rousseau]

2014-09-17 11:58 GMT+02:00 helpcrypto helpcrypto <helpcrypto at gmail.com>:
> On Mon, Sep 15, 2014 at 6:43 PM, Ludovic Rousseau
> <ludovic.rousseau at gmail.com> wrote:
>>
>>
>> Please, please do NOT use a loop of proposed by Bruno Jesus.
>> I invested a lot of time in removing all polling loops in pcsc-lite
>> and the CCID driver. So please do not add a polling loop in your
>> application when you can do without one.
>
>
> Talking about this, Ludovic, I'll like to ask you:
>
> IIUC, using SHARED + BeginTransaction/EndTransaction could lead to
> starvation:
>
> Process A opens a shared connection and invokes beginTransaction
> Process B opens a shared connection and invokes beginTransaction (blocked)
> If evil process A never releases transaction, B will starve

Yes. This is the expected behaviour.

> A "complex" father-child process will be needed to detect if process B is
> being blocked to be able to resume, right?

To resume what?

> Using EXCLUSIVE with retry-loop seems to be better:
> Process A opens an exclusive connection
> Process B try to open an exclusive connection and fails (now it is aware of
> other process using SC)
>     process B could retry a few times, or even detect if its a deadlock
> (process A died before releasing the connection)
> Am I right?

No.
If A dies then the lock will be released automatically. If that does
not occur then it is a bug in pcsc-lite and you should report it.

> Said so: are exclusive connections better in a non-controlled/evil
> applications environment?

It is the application choice to share or not the reader.

Once application A (evil or not) has an exclusive access to the reader
then application B can just wait.

PC/SC has not been designed to give a fair access to the smart card to
all the running applications.

Bye

-- 
 Dr. Ludovic Rousseau