Re: failure of pructl (atexit/_Block_copy/--no-allow-shlib-undefined)

On 2 Dec 2021, at 06:42, Kyle Evans <kevans@FreeBSD.org> wrote:
> 
> On Wed, Dec 1, 2021 at 8:05 PM John-Mark Gurney <jmg@funkthat.com> wrote:
>> 
>> Hello,
>> 
>> It seems like the recent changes to make --no-allow-shlib-undefined
>> broke pructl.
>> 
>> lib/libc/stdlib/atexit.c uses a weak _Block_copy symbol, but
>> pructl does not use atexit_b, and yet gets the following error:
>> : && /usr/bin/cc -Werror -O2 -pipe  -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing -std=c99 -fstack-protector-strong CMakeFiles/pructl.dir/pructl.c.o -o pructl  -Wl,-rpath,/usr/local/lib:  /usr/local/lib/libpru.so && :
>> ld: error: /lib/libc.so.7: undefined reference to _Block_copy [--no-allow-shlib-undefined]
>> cc: error: linker command failed with exit code 1 (use -v to see invocation)
>> 
>> What is the correct fix?  It seems like atexit.c or the linker should
>> be fixed, as pructl doesn't use atexit_b at all.
>> 
> 
> CC dim@ and jrtc27@... this seems like a toolchain regression? We're
> relying on the address of weak _Block_copy to simply evaluate to NULL
> if it's undefined here, which seems legit and pretty well-defined at
> this point from my recollection.

What do you mean exactly with "here"? In libc? In atexit? In libpru? I
can't make it out from the context, sorry. :)

As far as I can see, _Block_copy has always been a weak undefined symbol
in libc.so:

     4: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _Block_copy

Apparently the "block runtime" is supposed to provide the actual object,
so I guess you have to explicitly link to that runtime?

I know next to nothing about the blocks stuff, so it's all pretty much
unknown territory to me... :)

-Dimitry