[U-Boot] Let's bury CONFIG_NEEDS_MANUAL_RELOC

[U-Boot] Let's bury CONFIG_NEEDS_MANUAL_RELOC

[Graeme Russ]

Hi Guys,

Since I did the initial 'full relocation' for x86 way back when 
(http://git.denx.de/?p=u-boot.git;a=commit;h=1c409bc7101a24ecd47a13a4e851845d66dc23ce), 
there has been a lot of chatter about the ongoing need for 
CONFIG_NEEDS_MANUAL_RELOC. I think it's about time we looked really hard 
at this define and come to a definitive conclusion as to IF and HOW we 
can get rid of it for the remaining architectures that use it (being 
avr32, m68k, nds32, and sparc).

First, a bit of a primer on why and how I finally killed off 
CONFIG_NEEDS_MANUAL_RELOC for x86...

Despite the best intentions of the 'manual' relocation performed by 
U-Boot, there were still instances where the U-Boot binary referenced 
the 'unrelocated' image which typically resides in flash (NOR flash in 
the case of the x86 board I was using). I only finally tracked this down 
through some arduous debugging of random crashes during the writing of a 
new U-Boot image to the NOR flash.

After a lot of digging around, I managed to get a good understanding of 
the ELF binary format for x86 and compiler/linker flags needed in order 
to generate a binary image that could be relocated. It turns out that 
for x86, it was the -pic and --emit-relocs ld flags (Oddly, I cannot 
find the -pic flag in the gnu ld documentation maybe it's depricated?)

The --emit-relocs flags leaves the relocation sections (for x86 the 
section of interest is .rela.dyn) in the final image. These sections 
list all the locations in the final binary that need to be adjusted if 
the image is loaded at a different address than the one it is compiled 
for. You can see the processing in do_elf_reloc_fixups() in 
arch/x86/lib/relocate.c

Keep in mind that the relocation sections are NOT copied to RAM - they 
are only used once after U-Boot is copied to RAM to locate the memory 
locations that need to be adjusted.

So the crux of generic relocation is to generate a stripped binary with 
all necessary ELF relocation information located AFTER the text and data 
sections which are copied to RAM

So the first question that we need to answer is: Do the avr32, m68k, 
nds32, and sparc toolchains support the generation of the relevant 
sections needed to perform relocation in a manner similar to x86 (i.e 
generation of relocation references into sections of the final U-Boot 
binary)?

Regards,

Graeme

[U-Boot] Let's bury CONFIG_NEEDS_MANUAL_RELOC

[Albert ARIBAUD]

Hello Graeme,

On Wed, 04 Feb 2015 18:14:22 +1100, Graeme Russ
<gruss@tss-engineering.com> wrote:
> Hi Guys,
> 
> Since I did the initial 'full relocation' for x86 way back when 
> (http://git.denx.de/?p=u-boot.git;a=commit;h=1c409bc7101a24ecd47a13a4e851845d66dc23ce), 
> there has been a lot of chatter about the ongoing need for 
> CONFIG_NEEDS_MANUAL_RELOC. I think it's about time we looked really hard 
> at this define and come to a definitive conclusion as to IF and HOW we 
> can get rid of it for the remaining architectures that use it (being 
> avr32, m68k, nds32, and sparc).

I second that.

> First, a bit of a primer on why and how I finally killed off 
> CONFIG_NEEDS_MANUAL_RELOC for x86...
>
> Despite the best intentions of the 'manual' relocation performed by 
> U-Boot, there were still instances where the U-Boot binary referenced 
> the 'unrelocated' image which typically resides in flash (NOR flash in 
> the case of the x86 board I was using). I only finally tracked this down 
> through some arduous debugging of random crashes during the writing of a 
> new U-Boot image to the NOR flash.
> 
> After a lot of digging around, I managed to get a good understanding of 
> the ELF binary format for x86 and compiler/linker flags needed in order 
> to generate a binary image that could be relocated. It turns out that 
> for x86, it was the -pic and --emit-relocs ld flags (Oddly, I cannot 
> find the -pic flag in the gnu ld documentation maybe it's depricated?)

When I did the ELF relocation for ARM, only -pie existed for ld; but it
has an alias, --pic-executable. Maybe -pic is short for -pic-executable?

> The --emit-relocs flags leaves the relocation sections (for x86 the 
> section of interest is .rela.dyn) in the final image. These sections 
> list all the locations in the final binary that need to be adjusted if 
> the image is loaded at a different address than the one it is compiled 
> for. You can see the processing in do_elf_reloc_fixups() in 
> arch/x86/lib/relocate.c

For ARM I just specified the placement of relocation sections in the
linker script; I'm surprised that x86 needs this --emit-relocs. Maybe
that's needed when one does not provide a custom linker script?

> Keep in mind that the relocation sections are NOT copied to RAM - they 
> are only used once after U-Boot is copied to RAM to locate the memory 
> locations that need to be adjusted.
> 
> So the crux of generic relocation is to generate a stripped binary with 
> all necessary ELF relocation information located AFTER the text and data 
> sections which are copied to RAM
> 
> So the first question that we need to answer is: Do the avr32, m68k, 
> nds32, and sparc toolchains support the generation of the relevant 
> sections needed to perform relocation in a manner similar to x86 (i.e 
> generation of relocation references into sections of the final U-Boot 
> binary)?

Those arches which use GCC should be able to use -pie / -pic-executable
and possibly --emit-relocs, as none of these is arch-specific.

Then, each arch might have to look into what GCC options are needed.
For the record, on ARM, I did not need any gcc option, but actually
had one -fPIC option /removed/. 

> Regards,
> 
> Graeme

Amicalement,
-- 
Albert.

[U-Boot] Let's bury CONFIG_NEEDS_MANUAL_RELOC

[Graeme Russ]

Hi Albert,

On 04/02/15 21:29, Albert ARIBAUD wrote:
> Hello Graeme,
>
> On Wed, 04 Feb 2015 18:14:22 +1100, Graeme Russ
> <gruss@tss-engineering.com> wrote:


>> So the first question that we need to answer is: Do the avr32, m68k,
>> nds32, and sparc toolchains support the generation of the relevant
>> sections needed to perform relocation in a manner similar to x86 (i.e
>> generation of relocation references into sections of the final U-Boot
>> binary)?
>
> Those arches which use GCC should be able to use -pie / -pic-executable
> and possibly --emit-relocs, as none of these is arch-specific.

That was my understanding too - it's a function of ELF, not the 
architecture.

> Then, each arch might have to look into what GCC options are needed.
> For the record, on ARM, I did not need any gcc option, but actually
> had one -fPIC option /removed/.

Yes, I vaguely remember this. The difference is that -fPIC is used for 
relocatable LIBRARIES whereas -fPIE is used for relocatable EXECUTABLES. 
Since U-Boot is a monolithic executable, -fPIC makes no sense

Regards,


Graeme