libc.so, Bsymbolic no longer necessary?

libc.so, Bsymbolic no longer necessary?

[Jon Chesterfield]

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Hey,

The early design notes for musl mention linking with Bsymbolic, which
seemed reasonable. I don't see that in the current Makefile, or the
equivalent protected visibility.

This seems to suggest that LD_PRELOAD can override calls to libc from
within libc. That seems dubious. Is there a use case for this? Or is it
avoided by the control flow in the loader itself?

I'm thinking of building the musl loader/libc elf with protected visibility
on the exported symbols and wondered whether I'm missing something.

Thanks,

Jon

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Re: libc.so, Bsymbolic no longer necessary?

[Fangrui Song]

On 2019-12-02, Jon Chesterfield wrote:
>Hey,
>
>The early design notes for musl mention linking with Bsymbolic, which seemed
>reasonable. I don't see that in the current Makefile, or the equivalent
>protected visibility.
>
>This seems to suggest that LD_PRELOAD can override calls to libc from within
>libc. That seems dubious. Is there a use case for this? Or is it avoided by the
>control flow in the loader itself?
>
>I'm thinking of building the musl loader/libc elf with protected visibility on
>the exported symbols and wondered whether I'm missing something.

-Bsymbolic-functions was changed to --dynamic-list in commit
b9410061e2ad6fe91bb3910c3adc7d4a315b7ce9 (2018-04). 
The file dynamic.list lists the symbols that can be interposed.
Among the list you can find allocator functions (malloc and its
friends) See commit c9f415d7ea2dace5bf77f6518b6afc36bb7a5732

Some notes:

An empty --dynamic-list is identical to -Bsymbolic.

--dynamic-list with a list that specifies all STT_OBJECT symbols is
similar to -Bsymbolic-function. Specifying STT_OBJECT symbols (e.g.
stdin/stdout/environ) is to support COPY relocations.

   // lld/ELF/Writer.cpp
   static bool computeIsPreemptible(const Symbol &b) {
     assert(!b.isLocal());
   
     // Only symbols that appear in dynsym can be preempted.
     if (!b.includeInDynsym())
       return false;
   
     // Only default visibility symbols can be preempted.
     if (b.visibility != STV_DEFAULT)
       return false;
   
     // At this point copy relocations have not been created yet, so any
     // symbol that is not defined locally is preemptible.
     if (!b.isDefined())
       return true;
   
     if (!config->shared)
       return false;
   
     // If the dynamic list is present, it specifies preemptable symbols in a DSO.
     if (config->hasDynamicList)
       return b.inDynamicList;
   
     // -Bsymbolic means that definitions are not preempted.
     if (config->bsymbolic || (config->bsymbolicFunctions && b.isFunc()))
       return false;
     return true;
   }

Re: libc.so, Bsymbolic no longer necessary?

[Jon Chesterfield]

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>
> -Bsymbolic-functions was changed to --dynamic-list in commit
> b9410061e2ad6fe91bb3910c3adc7d4a315b7ce9 (2018-04).
>

Nice! Thanks

>

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