[EXTERNAL] Re: Calling OpenSSL_thread_stop() multiple times

[Martin Bonner]

Yes, I read all that documentation.  I didn't interpret that as saying:
"It is safe to call this OPENSSL_thread_stop(_ex) from DLLMain in
response to DLL_THREAD_DETACH".

I think I was confused by the wording:
"in response to a DLL_THREAD_DETACH message being sent to the
libcrypto32.dll entry point"

Firstly, I thought the word "message" meant a WM_MESSAGE, and secondly,
I didn't realize "libcrypto32.dll entry point" meant "DLLMain".

I think that quoted text would be better written as:
"in response to the DLLMain for libcrypto32.dll being called with
dwReason==DLL_THREAD_DETACH"

This does raise the question: if libcrypto is automatically calling
OPENSSL_thread_stop(), why does Deepak B need to do so (but that is
curiosity, not safety)?

Finally:
> OPENSSL_thread_stop() is used for freeing resources and I wouldn't
> generally expect more complex processing to occur in it.

I wouldn't expect complex processing, but if some provider kept a
count of threads using an auxiliary DLL, and did a FreeLibrary in
OPENSSL_thread_stop if that call falls to zero, that would not be
good.



Martin Bonner


On 24/08/2023 08:38, Martin Bonner via openssl-users wrote:

OPENSSL_thread_stop() is used for freeing resources and I wouldn't
generally expect more complex processing to occur in it.

If running on Windows and linking to OpenSSL dynamically then
OPENSSL_thread_stop() is automatically called (the application does not
need to do anything special). OpenSSL itself does this from DllMain and
using DLL_THREAD_DETACH:

https://github.com/openssl/openssl/blob/84a149254f977f502dd2314169812fc6eae8c309/crypto/dllmain.c*L28-L44

The documentation for OPENSSL_thread_stop() does explicitly mention this:

"Resources local to a thread are deallocated automatically when the
thread exits (e.g. in a pthreads environment, when pthread_exit() is
called). On Windows platforms this is done in response to a
DLL_THREAD_DETACH message being sent to the libcrypto32.dll entry point.
Some windows functions may cause threads to exit without sending this
message (for example ExitProcess()). If the application uses such
functions, then the application must free up OpenSSL resources directly
via a call to OPENSSL_thread_stop() on each thread. Similarly this
message will also not be sent if OpenSSL is linked statically, and
therefore applications using static linking should also call
OPENSSL_thread_stop() on each thread. Additionally if OpenSSL is loaded
dynamically via LoadLibrary() and the threads are not destroyed until
after FreeLibrary() is called then each thread should call
OPENSSL_thread_stop() prior to the FreeLibrary() call."



Matt

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