Re: Specify how undefined weak symbol should be resolved in executable

["H.J. Lu" <hjl.tools@gmail.com>]

On Tue, Feb 23, 2016 at 9:10 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Tue, Feb 23, 2016 at 9:01 AM, Michael Matz <matz@suse.de> wrote:
>> Hi,
>>
>> On Tue, 23 Feb 2016, H.J. Lu wrote:
>>
>>> >> Not only we need to change defined weak symbol behavior, we also need
>>> >> to change undefined non-weak symbol behavior when taking its address.
>>> >
>>> > Why?  An undefined non-weak symbol reference leads to a linker error,
>>> > done.  Why change that?
>>>
>>> Make it "external non-weak symbol", which is a symbol defined in a
>>> shared object.
>>
>> What would you like to change in behaviour for them (and why must it be
>> done at the same time as changing something for weak symbols)?  A
>> reference to the address of such a function symbol from an executable
>> resolves to the plt slot currently (which in turn is exported, so that
>> address references from other modules resolve to the same).  A reference
>> to a non-function symbol is loaded from the got.  In all cases must this
>> symbol be defined somewhere (otherwise linker error), so I don't see what
>> we need to change, nor why that would force DT_TEXTREL (or the compiler to
>> emit PIC like code).
>>
>
> At run-time, there is no difference between weak defined and non-weak
> defined symbols.  If we change defined weak symbol behavior, we also
> need to change defined non-weak symbol behavior.
>

I will check this patch:

https://sourceware.org/ml/binutils/2016-02/msg00339.html

into binutils master branch to make behaviors of static executable,
dynamic executable and position independent executable consistent with
references to undefined weak and defined symbols, regardless if the
relocatable input is compiled with PIC or not.

I created users/hjl/pr19704 branch to treat undefined weak symbol in
the executable as dynamic, requiring dynamic relocation, on x86 via
a command line switch:

-z dynamic-undefined-weak   Treat undefined weak symbol as dynamic

We need to fully investigate its implications before moving forward
with it.

-- 
H.J.