Re: linker crash in arm stub generation

[Christophe LYON <christophe.lyon@st.com>]

Hi Daniel,

Thank you for your answer, but I am afraid things are still not clear to 
me, despite reading your answer several times :-(

(part of the confusion probably comes from the fact that I deal with 
different architectures, too...)

>> I have started to look at this problem more closely, and I have one  
>> question: in elf32-arm.c:allocate_dynrelocs(), there is this comment:
>>
>>   /* If this symbol is not defined in a regular file, and we are
>>      not generating a shared library, then set the symbol to this
>>      location in the .plt.  This is required to make function
>>      pointers compare as equal between the normal executable and
>>      the shared library.  */
>>
>> Why is the behaviour different when generating a shared lib?
>>
>> I thought I had understood the comment about function pointers  
>> comparison, but I am wondering now....
> 
> A PLT entry with a non-zero address is used as the canonical location
> of the function.
This "canonical" location is only used by the dynamic linker, when it 
patches the dyn relocs pointing to this symbol, right? (when the address 
of the function is stored in a constant pool for instance, or in GOT)

 > This is only ever required in an executable, never
> in a shared library.  If all accesses to the address are PIC - which
> they must be, in a shared library - then they can be easily adjusted
> to point to the function's address.
Easy because the dyn linker needs to patch the GOT only (ie one entry 
instead of several references)?

 > And it's better to do that,
> because calls through those pointers will go directly to the function
> instead of to the PLT.
So you mean that in a shared lib, PLT are generated, but not executed 
because the dyn linker manages to make these indirect calls go directly 
to the function?

But then, how is symbol preemption handled? I mean, if a shared lib is 
actually shared, ie used by two different executables, and one of them 
preempts the function definition, but not the other, I think the calls 
need to go through the PLT so that different GOTs are used to reach 
different functions.

> In an executable, this might not be the case.  For instance you might
> have the address of the funtion in a constant pool in the text
> segment.  If that happens, the linker must fix the address of the
> function at static link time, even if the definition turns out to be
> in a shared library.
Why couldn't this be performed at load time?

> Such code is (or is supposed to be, anyway) rejected in shared
> objects.
> 

I thought your answer would help me solve my actual problem, but since 
it seems that I need better understanding, I will expose my actual 
question here:
If I am generating a shared lib, let's say that some ARM code references 
a THUMB function in a shared lib.
As the target is in a shared lib, we need to have a PLT generated.
But, we also need to know if we need to change modes, and if we need a 
long branch stub.

However, because of the comment I mentioned earlier, the destination is 
not recorded as being the PLT, so we don't know the actual distance, and 
the symbol type is not switched to ARM type.

This scenario is handled properly when generating an executable, but 
when generating a shared lib, the current generation is broken.

Christophe.

PS: I have noted that Phil Blundell has inadvertently committed a wrong 
patch for the issue being discussed (as part of another commit of his). 
I don't know if it should be cancelled separately or if it can't wait 
until I propose a full patch + testcase.