Re: using relocs in disassembler

[Lynn Winebarger <owinebar at free-expression dot org>]

On 15 Nov 1999, Ian Lance Taylor wrote:

>       Now, I guess my question also applies to some static relocs: when a
>    reloc appears in data, how can I tell how much data is taken up by the
>    reloc, and exactly what I should replace it with.  I'm guessing, if it's
>    data, the data would become a pointer to the symbol (or the value of the
>    symbol).  
> 
> Given the howto structure, you can call bfd_get_reloc_size to get the
> number of bytes that it affects.
> 

   Yeah, I noticed that when I went back to the manual (doh!). The source
code (reloc.c) does mention the possibility of variable sized relocs,
though it doesn't mention when that could happen.

>    movl eax, (edx)
>    jmp eax
> 
>    Could the value edx points to (an offset from the current EIP) be set up
>    by a reloc (such a thing could be set up by a tail recursion optimizing
>    compiler for a functional language, I think, or perhaps an implementation
>    of object methods).  
> 
> I'm not sure I completely understand your question.  Certainly the
> value at the address to which edx points could be initialized using a
> reloc.  Also, the offset off of edx could be set by a reloc.  I don't
> think a C compiler is likely to ever generate such a case, but it
> could be done in assembly code.
> 

   This kind of code can be generated by a large switch statement in C.
A label is produced for each case of the switch, and the case to jump to
is determined by looking up the case in a table of labels, and jumping.
Depending on what kind of jump is used, the table entry could require a
pc-relative displacement.  (Though I can't think of any time this would
require a reloc - that's sort of the point of the pc-relative
displacement).
   Another (more likely) example is a functional language where tail calls
are optimized.  In this case, I might set a variable to a closure, then
apply that variable in tail position.  Tail calls should never cost more
than a jump in a tail recursion optimized language, so you might very well
want to use a dynamic relocation that sets the variable to the pc-relative
offset of a dynamically loaded piece of code.  Whether or not this is
actually supported by system tools, I don't know.

Thanks for the info.

Lynn