Re: R_386_RELATIVE question

[Gregory Shtrasberg <shtras@gmail.com>]

Thanks for the quick response.

What I ment to say is if I want to reassemble the code in some way, the
address of __dso_handle will change. Naturally, every other place that has
address in the data section has a linker relocation on it, like R_386_32,
which also tells me that the value is an address and needs to be altered. So
that I can change the value according to new code order, e.g. 08 1c 00 00
will become the new address of __dso_handle.
But here the only relocation here unlike other places is R_386_RELATIVE,
which, as you said does different thing.
So, this looks to me as some sort of bug in gcc. Or am I missing something?


Ian Lance Taylor-3 wrote:
> 
> Gregory Shtrasberg <shtras@gmail.com> writes:
> 
>> Hello. I've got a question about the purpose of R_386_RELATIVE relocation
>> Here's a part of disassemble of a library, built as follows:
>> g++ -fPIC -c test.cpp -o test.o
>> g++ -shared -Wl,-q -o libtest.so test.o
>>
>> At 0x1bcc there is a R_386_RELATIVE relocation, and the data in this
>> address
>> is 0x1c08, which is an address of __dso_handle. As far as I understand,
>> it's
>> not the job of R_386_RELATIVE to fix the value, so there should have been
>> a
>> linker relocation on 0x1bcc, pointing to 0x1c08. Am I right? (There is no
>> other relocation on 0x1bcc besides R_386_RELATIVE)
>> Thanks in advance
>>
>> Disassembly of section .got:
>>
>> 00001bcc <.got>:
>>     1bcc:       08 1c 00                or     %bl,(%eax,%eax,1)
>>                         1bcc: R_386_RELATIVE    *ABS*
> 
> The value 0x1c08 is already there, in the section contents (see the
> bytes "08 1c" and recall that the i386 is little-endian).  The
> R_386_RELATIVE directs the dynamic linker to adjust the value by the
> load address of the shared library, so that at runtime it will be the
> address where __dso_handle winds up.
> 
> Ian
> 
> 

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