Re: [FFmpeg-devel] [PATCH v2] libavformat/vapoursynth: Update to API version 4, load library at runtime

[Stefan Oltmanns via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

Hi Ramiro,

Am 18.07.24 um 15:04 schrieb Ramiro Polla:
>>>>  [...]
>>>>
>>>> +static VSScriptLibrary vs_script_library;
>>>
>>> Does vs_script_library have to be a global?
>>>
>>
>> I think it has to: ffmpeg allows multiple input files, in case you open
>> two VapourSynth files you want to load the Library only once.
>
> It should be possible to dlopen()/LoadLibrary() multiple times, and
> the library only gets really unloaded after the last call to
> dlclose()/FreeLibrary(). In that case you could move that struct to
> the context. If it's unavoidable to keep the global, at least add some
> locks to access it.

Yes, that should be possible. I did a quick search at it seems that
dlopen()/LoadLibrary() is smart and will not open the same library
multiple times, but return the same pointer.
As dlclose won't be used anymore when removing the atexit handler, that
is not an issue at all.

>
>> This is exactly how it's done for AviSynth.
>
> Perhaps AviSynth is not the best example to follow :)
>
>>>> +
>>>> +static int vs_atexit_called = 0;
>>>> +
>>>> +static av_cold void vs_atexit_handler(void);
>>>> +
>>>> +#ifdef _WIN32
>>>> +static av_cold char* get_vs_script_dll_name(void) {
>>>> +     LONG r;
>>>> +     WCHAR vss_path[512];
>>>> +     char *vss_path_utf8;
>>>> +     DWORD buf_size = sizeof(vss_path) - 2;
>>>> +     r = RegGetValueW(HKEY_CURRENT_USER, L"SOFTWARE\\VapourSynth",
>>>> +                      L"VSScriptDLL", RRF_RT_REG_SZ, NULL,
>>>> +                      &vss_path, &buf_size);
>>>> +     if (r == ERROR_SUCCESS && wchartoutf8(vss_path, &vss_path_utf8)
>>>> == 0)
>>>> +         return vss_path_utf8;
>>>> +     r = RegGetValueW(HKEY_LOCAL_MACHINE, L"SOFTWARE\\VapourSynth",
>>>> +                      L"VSScriptDLL", RRF_RT_REG_SZ, NULL,
>>>> +                      &vss_path, &buf_size);
>>>> +     if (r == ERROR_SUCCESS && wchartoutf8(vss_path, &vss_path_utf8)
>>>> == 0)
>>>> +         return vss_path_utf8;
>>>> +     if (wchartoutf8(L"VSScript.dll", &vss_path_utf8) == 0)
>>>> +         return vss_path_utf8;
>>>> +     return 0;
>>>> +}
>>>> +#endif
>>>
>>> Don't fetch the path on the registry on Windows. The user should set the
>>> path outside of FFmpeg.
>>
>> How exactly should the user do that? Additional option to ffmpeg?
>
> By making sure the libraries are accessible in the PATH environment
> variable. For example by adding the VapourSynth path to the PATH
> environment variable, or overriding PATH on the call to FFmpeg on a
> script. Either way that's outside the scope of FFmpeg.

Well, the DLL directory is added to PATH by the VapourSynth installer,
but for safety reasons ffmpeg explictly tells the LoadLibrary function
to only search the application directory and system32, quote from
w32dlfcn.h:

> /**
>  * Safe function used to open dynamic libs. This attempts to improve program security
>  * by removing the current directory from the dll search path. Only dll's found in the
>  * executable or system directory are allowed to be loaded.
>  * @param name  The dynamic lib name.
>  * @return A handle to the opened lib.
>  */
So ffmpeg prevents that solution on purpose. Or should that behavior be
changed in the w32dlfcn.h?

Best regards
Stefan
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