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fPIC with alignment

Note: This was originally posted on 18th July 2012 at http://forums.arm.com

Hi

I am trying to create a PIC elf image with --apcs=/fpic compiler option and --fpic linker option. It creates a PIC image alright with different load regions for code and data, but I would like to have them 4K aligned so that I can map those with appropriate RX and RW permissions. Any idea how can force the alignment?

Thanks in advance.

Elf Elf over 12 years ago

Note: This was originally posted on 19th July 2012 at http://forums.arm.com

The problem is that --fpicis ignored when --scatter is used, so it does not generate PIC image. To generate the pic image --sysv --sharedhad to be added which the ignores the --scatter option. it then generates the PIC image correctly, but does not create aligned load regions. so bottom line, --scatter loader file cannot be used for PIC images. One solution i found was specifying --ro-base and and --rw-base WITH -fPIC linker option. This is not efficient since RO and RW are going to grow and we need to make sure they dont overlap. so it sucks that there are no good options to create PIC image and make the load regions aligned.

Thanks.
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Elf Elf over 12 years ago
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Peter Harris over 12 years ago

Note: This was originally posted on 18th July 2012 at http://forums.arm.com

With the ARM tools this is achieved using the scatter loader:

http://infocenter.arm.com/help/topic/com.arm.doc.dui0474g/CHDIEAJD.html

For GNU tools you'll need to create a linker script (similar concept the ARM scatter loading scripts, but different syntax)

http://www.delorie.com/gnu/docs/binutils/ld_6.html

HTH,
Iso
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Peter Harris over 12 years ago

Note: This was originally posted on 19th July 2012 at http://forums.arm.com

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The problem is that --fpicis ignored when --scatter is used
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[size=2]For RVCT you just need to mark the section as position independent in the scatter file (add the "PI" attribute to the section). I'd be surprised if there was not a similar option for the GNU linker script.[/size]
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[color=#222222][size=2]Iso[/size][/color]
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