Dan Baldor<danbaldor@gmail.com> writes:
I have been trying to see if I can implement a minimized dynamic
linker to locate the globals necessary to load the code (using the
Dynamic section along with the symbol table) and fix up the
relocations that are specified in the rel.dyn section. I already use
a liner script with the AT modifier and have tried to relocate the GOT
section the way you mentioned but when I do so all the global
variables are calculating the location of the .GOT based on the
current PC which causes a problem since the distance between the .text
segment and the .data segment has changed based on where I load the
code. Is there no way to use -mpic-register and gcc-help@gcc.gnu.org to
specify the location of the GOT? I am not sure how these compiler
options work but looking at the assembly that the code generates when
I use them I noticed that the references to the PC seem to be removed.
Please reply to the mailing list, not just to me. Thanks.
I'm not clear on whether you need fully relocatable code at runtime, or
whether you just need the data section to be loaded at a different
address. That is, do you know the final address where your program will
run, or not?
-mpic-register sets the register to use for the GOT, it does not set the
address for the GOT.
-msingle-pic-base assumes that the GOT register is fixed. You could use
that to put the GOT wherever you want, as long as you set the register
correctly.
However, any use of the GOT, or the -fpic option in general, implies
some sort of dynamic linker.
Ian
On 27/02/2011 2:24 PM, Ian Lance Taylor wrote:
Dan Baldor<danbaldor@gmail.com> writes:
I am working on a project using a arm cortex-m3 processor that needs
relocatable code. The relocation is such that the .text and .data
sections will have a different offset when loaded than what they were
compiled with (the .text section can be loaded to different sections
of internal flash while the .data section will always reside in the
same location of sram). If I keep the .got with .text section I can
get the global variables to have the correct addresses but my static
local variables have the wrong address (they are offset from where
they should be the amount that the .text segment shifted). I think the
static locals are referenced from the start of the .GOT section which
is why I am having problems. I am compiling all my c code with âfpic
and linking with âfpic and âpie. Is there any way to either force the
static locals (and maybe global variables) to absolute addresses or to
relocate the .GOT section dynamically? I tried doing by placing the
GOT section with the data but when I do so code does not locate the
GOT section correctly. I thought that perhaps I should be using
|-msingle-pic-base and -mpic-register=|/reg /but so far all attempts
to use this did not work at all.
If the runtime address is fixed, this is normally done using a linker
script using the AT() modifier; see
http://sourceware.org/binutils/docs-2.21/ld/Output-Section-LMA.html .
If the runtime address varies, then I don't know that there is a way to
handle this. Using -fpic/-pie implies the existence of a dynamic
linker.
Ian