Re: ARM position-independent code in Gas

[Matt Thomas <matt@3am-software.com>]

On Feb 26, 2014, at 12:02 PM, Alex Matveev <alex.matveev@samsung.com> wrote:

> Hello all,
> 
> I'm working on a project which is part C and part assembler (ARM, GCC
> 4.8.2, binutils 2.23.1), and final binary should be
> position-independent.
> 
> I compile stuff with -fpie and link with -pie -fpie. This works out
> fine on the C side - I get R_ARM_RELATIVE and R_ARM_ABS32 relocations,
> fix'em up on startup, all good. But the problem is, there are no relocs
> from assembler objects, thus the result is not PIC. Trying to trick Gas
> into producing relocs, I've found couple of
> not-very-extensively-documented directives - '.reloc' and ARM-specific
> '.word foo(RELOC)', but I haven't had any success with them either.
> 
> Can GAS in principle produce PIC? Or am I missing something obvious?

gas doesn't know PIC.  The arm assembly has to be PIC to start.

> And a side question: I'm somewhat puzzled by the presence of
> R_ARM_ABS32 relocations in the final output - aren't they static
> (AAELF says so)?

They are static.

Not sure which processor you are using but you can do PIC like:

	movw	r0, #:lower16:foo-.LPIC0
	movt	r0, #:upper16:foo-.LPIC0
	add	r0, r0, pc
	nop	@ at this point r0 points at foo and you can replace foo
		@ by another instruction the same size as the add.
.LPIC0:


You can also do symbol relative:

bar:	@ function entry
	adr	ip, bar
	movw	r0, #:lower16:foo-bar
	movt	r0, #:upper16:foo-bar
	add	r0, r0, ip

If you are dealing with shared libraries and global data:

	ldr	r3, .Lgot
	add	r3, r3, pc
	ldr	r0, .Lfoo	@ load got slot offset
.LPIC0:	ldr	r0, [r0, r3]
	...
.Lgot:
	.long	_GLOBAL_OFFSET_TABLE_-.LPIC0
.Lfoo:
	.word	foo(GOT)	@ reserves a slot in the GOT for foo

using -S -c -fPIC on c source and looking at the assembly produced
can prove to be instructive.