ARM long branch stubs: shared libs

[Christophe LYON]

On 02.04.2009 18:11, Daniel Jacobowitz wrote:
> On Thu, Apr 02, 2009 at 05:06:08PM +0200, Christophe LYON wrote:
>> Because I didn't want to change a behaviour I didn't completely master.  
>> As the usual rule applies: what is not tested does not work.
>>
>> But... I have tried your suggestion (ie. mark the undef symbol with .weak 
>> in the testcase), and I get an error message from the linker (relocation 
>> truncated to fit).
>>
>> Now, if I handle undefweak and undef symbols in the same way, I observed  
>> that:
>> - in allocate_dynrelocs, a PLT entry is created for the undefweak,  
>> despite the comment "Make sure this symbol is output as a dynamic symbol. 
>> Undefined weak syms won't yet be marked as dynamic."
> 
> This part is correct.  The comment is not clear, but the goal is to
> mark undefined weak symbols as dynamic at this point.  Branches should
> go to the PLT entry.  I'm not sure what happens if you call that PLT
> entry, though, and there is no definition at runtime.  Ideally,
> nothing would happen, but I'm sure glibc does not implement that.
> 
I have read again the ELF spec and discovered that indeed weak symbols 
are preemtible and thus need to use a PLT entry.

> Is this a hitherto undiscovered requirement on the ARM dynamic linker?
Isn't this requirement common to all targets?


>> Why is there a different handling of a ARM call vs a THUMB call in this  
>> case?
> 
> There shouldn't be.  It sounds like you've found another bug.  The
> Thumb version makes more sense.
> 

I am giving a look at it, and I hope to be able to submit a patch soon.

Thanks for your help,

Christophe.