[PATCH] Add a fallback mechanism for undefined atom.

[Shankar Easwaran]

Hi Ruiu,

We need the functionality for ELF too to implement the --wrap functionality.

Thanks

Shankar Easwaran

On 9/6/2013 2:58 PM, Rui Ueyama wrote:
> Guys,
>
> I want to submit this patch. This patch is one of the last few patches that
> I need to link against the standard C++ library on Windows. Symbol aliasing
> does not seems to be needed for ELF. For Mach-O we need the feature, but
> how to implement can be discussed later. This patch is short and not
> intrusive, so it won't hurt anyone.
>
>
> On Tue, Sep 3, 2013 at 1:07 PM, Shankar Easwaran <shankare at codeaurora.org>wrote:
>
>> On 9/3/2013 2:59 PM, Nick Kledzik wrote:
>>
>>> On Sep 3, 2013, at 12:51 PM, Shankar Easwaran <shankare at codeaurora.org>
>>> wrote:
>>>
>>>   On 9/3/2013 2:36 PM, Nick Kledzik wrote:
>>>>> On Sep 3, 2013, at 11:06 AM, Rui Ueyama <ruiu at google.com> wrote:
>>>>>
>>>>>> Nick, does Mach-O need symbol aliasing to be handled in the linker?
>>>>>>
>>>>> Yes.  The darwin linker has command line options for adding aliases.
>>>>>
>>>>> Back when I first started working on the darwin linker, a big complaint
>>>>> from the compiler team was that mach-o did not support aliases like ELF
>>>>> did, so the compiler team kept having to special case lots of test cases
>>>>> and functionality that used __attribute__((alias(“xx”))) on darwin.
>>>>>
>>>>>
>>>>>   looks like ELF also needs the same functionality for implementing
>>>> __wrap.
>>>>
>>>> But in the case of the linker dealing with compiled .o's, the alias
>>>> information is lost.
>>>>
>>> Really?  I thought ELF had a way to encode this.   I’ve received bug
>>> reports saying that mach-o should support it too.
>>>
>> Example :-
>>
>> $cat x.c
>> void __f () { /* Do something. */; }
>>            void f () __attribute__ ((weak, alias ("__f")));
>> $gcc -c x.c
>> $readelf -s x.o
>>
>> Symbol table '.symtab' contains 10 entries:
>>     Num:    Value  Size Type    Bind   Vis      Ndx Name
>>       0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
>>       1: 00000000     0 FILE    LOCAL  DEFAULT  ABS x.c
>>       2: 00000000     0 SECTION LOCAL  DEFAULT    1
>>       3: 00000000     0 SECTION LOCAL  DEFAULT    2
>>       4: 00000000     0 SECTION LOCAL  DEFAULT    3
>>       5: 00000000     0 SECTION LOCAL  DEFAULT    5
>>       6: 00000000     0 SECTION LOCAL  DEFAULT    6
>>       7: 00000000     0 SECTION LOCAL  DEFAULT    4
>>       8: 00000000     5 FUNC    GLOBAL DEFAULT    1 __f
>>       9: 00000000     5 FUNC    WEAK   DEFAULT    1 f
>>
>>
>>
>>>   Does Darwin keep the alias information when the
>>>> __attribute__((alias(“xx”)) is used.
>>>>
>>> What the current darwin linker does is if parses a .o file and there are
>>> two symbols at the same address (and same section), it picks one to be the
>>> atom name (call it “foo”) and the other (call it “bar”) is turned into an
>>> alias atom (zero size and atom foo must layout after atom bar)
>>>
>> We do something very similiar in lld in the ELF reader, except that we
>> dont alias it(as we need the content of the original atom, for the linker
>> to abide the section property).
>>
>> Thanks
>>
>> Shankar Easwaran
>>
>> --
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted
>> by the Linux Foundation
>>
>>


-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by the Linux Foundation