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Re: ARM position-independent code in Gas
- From: Matt Thomas <matt at 3am-software dot com>
- To: Alex Matveev <alex dot matveev at samsung dot com>
- Cc: binutils at sourceware dot org
- Date: Wed, 26 Feb 2014 19:45:25 -0800
- Subject: Re: ARM position-independent code in Gas
- Authentication-results: sourceware.org; auth=none
- References: <20140227000201 dot 42c81571 at amatveev>
On Feb 26, 2014, at 12:02 PM, Alex Matveev <email@example.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
> 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.
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]
.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.