[U-Boot-Users] Changing u-boot relocation scheme

[J. William Campbell <jwilliamcampbell@comcast.net>]

Haavard Skinnemoen wrote:
> On Sat, 26 Jul 2008 14:29:35 -0700
> "J. William Campbell" <jwilliamcampbell@comcast.net> wrote:
>   
>> Haavard Skinnemoen wrote:
>>     
>>> On Fri, 25 Jul 2008 22:51:09 +0200
>>> kenneth johansson <kenneth@southpole.se> wrote:    
>>>       
>>>> Can't see any reason for using this flag over -fPIC for a program like
>>>> u-boot.
>>>>     
>>>>         
>>> You need both. One is a compiler flag, the other is a linker flag. The
>>> linker will probably barf if you try to create a PIC executable from
>>> modules that were not compiled with -fPIC.
>>>   
>>>       
>> No, it won't.
>>     
>
> On some platforms it will. Text relocations are nasty, so some
> platforms (e.g. avr32) just refuse to deal with them. But that's not
> really relevant -- each architecture should decide whether to compile
> with -fPIC or not.
>
>   
>> You just get a module with a lot more relocations to do. I 
>> have verified that all four possible combinations of the compiler -fPIC 
>> and linker -pie work and make sense. FWIW, -fPIC code on IA32 is about 
>> 16% larger than non-PIC code, while on the Blackfin, -fPIC code is about 
>> 2% larger than non-PIC code. This is an average over several large C++ 
>> applications.
>>     
>
> Right...that's counting the whole loadable image or just the .text
> section? Not suprising that a modern architecture like Blackfin likes
> -fPIC a lot better than an old beast like i386 though.
>   
That is the entire loadable image. The percentage will therefore be 
slightly higher as a function of just the .text section. It is pretty 
representative of the effect though. On the "old beast", it is not so good!
>  
>   
>> I agree with this suggestion. This is the only way to ensure a "sane" 
>> environment, because it emulates what the compiler expects to happen. 
>> Looping over all the relocation entries and doing the "right thing" is 
>> architecture specific, but the process is general. The GOT entries can 
>> also be processed this way. Effort spent on this approach will tend to 
>> be more generic than the current PPC specific approach.
>>     
>
> Right...I think the GOT entries already are processed this way, sort of
>   
>>> Ah, of course. The strings are probably read directly from flash.
>>>   
>>>       
>> Maybe not. I have been looking at assembly dumps of short examples on 
>> the IA32 built with -fPIC. It is clear that the method of addressing 
>> static variables and static constants is DIFFERENT from the method used 
>> for global variables. The relationship of the location of the text 
>> segment (executable code), the GOT data, and the static 
>> variables/constants must remain fixed. The location of the entire 
>> program can move, but it must move in one piece. If it does move as one 
>> piece, the lea (load effective address) instructions relative to the GOT 
>> pointer will be relocated to the new address correctly. These references 
>> are based totally on the offset from the point of reference. If the code 
>> is similar on your platform (which I bet it is), then the reference will 
>> not be to the flash but rather the "new" place where the data was 
>> moved..
>>     
>
> Yes, address calculations in the code should be correct, as the whole
> thing was compiled with -fPIC. Data references, however, are usually
> not. The code being discussed here is an array of pointers to strings.
> I'm pretty sure the pointers are still pointing to flash after
> relocation.
>   
You are correct. The contents of initialized pointers are not relocated 
without using the relocation data provided by -pie on the ld script.
>   
>> Global variables, however are referenced indirectly via 32 bit 
>> address pointers in the GOT, and these addresses must be relocated by 
>> the "loader".
>>     
>
> The global variables themselves are accessed through the GOT, yes. But
> the _value_ of a global variable is currently not relocated
> automatically.
>
>   
>> The "loader" also must relocate any initialized pointers, because the 
>> program itself does not. It would be interesting to know how this is 
>> accomplished, via what relocation codes, but it does happen.
>>     
>
> This is what's currently being done manually by adding a fixed offset
> to all the pointers we "know" need to be relocated. When linking with
> -pie, these initialized pointers will get a dynamic relocation entry
> each so that we can replace all these manual fixups by simply iterating
> over the relocations.
>
> To summarize: Address calculations in executable code do not need to
> change since we already compile with -fPIC. Initialized pointers,
> however, are currently handled in a very suboptimal way, and linking
> with -pie might be one piece of the solution to this.
>   
I agree completely. Now we "just" need to add processing the relocation 
data. Also, the relocation vectors differ in contents from platform to 
platform, so a bit of "custom" processing will be needed for each 
architecture.

Bill Campbell
> Haavard
>
>
>