position independent executable using shared libraries on arm

position independent executable using shared libraries on arm

[mathias palmqvist]

Hey

When I compile simple test code with -fPIC and no pie link parameter
or shared libs I can see
prologue code similar to this in each function setting up R10 and adding PC.

.text:0000804C                 MOV     R12, SP
.text:00008050                 STMFD   SP!, {R10-R12,LR,PC}
.text:00008054                 SUB     R11, R12, #4
.text:00008058                 LDR     R10, =0x3D8
.text:0000805C                 ADD     R10, PC, R10
.text:00008060                 LDR     R3, =0xC

If I add -pie as a link parameter, or link the executable with a
shared library, the

ADD R10,PC,R10

instruction is replaced with a NOP, and the code looks like this:

.text:0000034C                 MOV     R12, SP
.text:00000350                 STMFD   SP!, {R10-R12,LR,PC}
.text:00000354                 SUB     R11, R12, #4
.text:00000358                 LDR     R10, =_GLOBAL_OFFSET_TABLE_ ; PIC mode
.text:0000035C                 NOP
.text:00000360                 LDR     R3, =0xC


The object file generated by gcc is the same. Only link parameters change.

Am I missing something here? What should I do to get ld to position
GOT at a relative
offset instead of at the absolute address when linking with shared libs?

We are currently implementing dynamic linking support for elf and
simple so:s on our embedded
platform, so we have complete control of the loading, but want to use
gcc/ld for building.

Using: ld 2.16.1 with gcc 4.1.0. Building for armv5.

Thanks in advance
/Mathias

Re: position independent executable using shared libraries on arm

[Nick Clifton]

Hi Mathias,

> Using: ld 2.16.1 with gcc 4.1.0. Building for armv5.

Try using ld 2.18 or else the current mainline version.

If you are not building for an ARM EABI target then I suggest doing that as well.

Cheers
   Nick