[Question] Does ATfE support Position independent code (PIC) for baremetal aarch32 system?

[sachinpundkar]

Our idea is to have minimal boot code for which both physical and virtual address is same.
But then rest of the code to be have different virtual address at every bootup.

for this call to other code and and data needs to be not fix.

We use C/CPP code in our system.

[smithp35]

Short answer for AArch32 is not out of the box. It is not far away from it if you are willing to build your own runtime libraries. The current AArch32 libraries are not built position independent (although our AArch64 A profile libraries are). Thank you for letting us know of interest in PIC as we have to judge what library variants to include based on user demand.

Here's an outline of what would need to be done:

It should be possible to build a -fpic (or -fpie) variant yourself. I don't think there is any assembly code in Picolibc or llvm-libc that is position dependent so this should be a matter of adding the right compilation flag. Our aarch64 A profile libraries are built with -fPIC as code-size doesn't matter as much for AArch64. See -fpic flag in https://github.com/arm/arm-toolchain/blob/arm-software/arm-software/embedded/arm-multilib/json/variants/aarch64a.json.

For AArch32 support for position independence that works with both C and C++ is limited to -fpic and -fpie. The bare-metal -fropi and -frwpi do not support C++. Assuming static linking, your bootcode will need a "dynamic-loader" that can resolve R_ARM_RELATIVE dynamic relocations. It will also need a means to find those dynamic relocations. One way to do this is to have your boot-code load an ELF file, you can then use the DT tags in the dynamic section to find the relocations. Self relocating programs are also possible, these tend to use linker defined symbols to find the dynamic relocations. To use linker defined symbols will require a modification to the linker script to pull out .rel.dyn and surround with linker defined symbols.

For your use case it should be sufficient to use -fpie, which can be used for executables, but not shared-libraries. You will need to link your application (and libraries compiled with -fpie) with the linker --pie option.

I'm going to be on vacation for the next few days, but hopefully my colleagues can assist you if you have any further comments.