Re: [Qemu-devel] [PATCH v10 5/8] module: implement module loading

[Wenchao Xia <xiawenc@linux.vnet.ibm.com>]

于 2013/9/16 18:38, Paolo Bonzini 写道:
> Il 16/09/2013 12:24, Alex Bligh ha scritto:
>> At risk of heresy, can I suggest a rather simpler scheme that requires
>> a total of zero infrastructure changes?
>>
>> Here's a patch against qemu 1.0 (sorry) Ubuntu dist (sorry) that
>> uses weak binding to load and compile against any version of
>> librbd:
>>    https://github.com/flexiant/qemu/commit/6fa2e9c95bdaca7c814881e27f04424fb6cc2960
>>
>> This requires librbd-dev headers of some sort in order to build. But
>> librbd does not need to be there on a deployment, and indeed multiple
>> versions work (and the patch does different things depending on
>> librbd version).
> No, librbd does need to be there for the other symbols that are not weak
> (e.g. rbd_aio_read).  This approach cannot be "taken to the limit", i.e.
> removing the librbd dependency altogether.  For example:
>
> xx.c:
> int f(void)
> {
> 	return 42;
> }
>
> yy.c:
> #pragma weak f
> extern int f(void);
> int main()
> {
> 	printf("%p %d", f, f ? f(): 67);
> }
>
> $ gcc xx.c -shared -o xx.so
> $ LD_RUN_PATH=$PWD gcc yy.c xx.so -o yy
> $ ./yy
> 0x4005b0 42
> $ rm xx.so
> $ ./yy
> ./yy: error while loading shared libraries: xx.so: cannot open shared
> object file: No such file or directory
>
Is there a way to tell the program loader: don't resolve the dynamic 
symbol, f,
then code can run into main(), and we add code do 
dlopen("/another_dir/xx.so")? (Assume xx.so
exist in /another_dir/?

> Also, the code _is_ ugly.  Do it once and it's perhaps acceptable.  Do
> it for libiscsi, librbd, libcurl, libssh2, SPICE, GTK+, SDL etc. and it
> becomes unmaintainable.
>
> Paolo
>
>> This would seem to achieve the stated objective (qemu package can
>> be installed without librbd dependencies) without any reliance
>> on modules at all. So we could scrap the whole modules infrastructure
>> and just tell people developing third party libraries to use weak
>> binding. As you can see, for librbd anyway, the changes required
>> to support weak binding are pretty minimal.
>
>