[PATCH] ARM: Add support for thumb1 PCROP relocations.

[Ramana Radhakrishnan]

On Mon, Nov 23, 2015 at 8:43 AM, mickael guene <mickael.guene@st.com> wrote:
> Hi all,
>
> This is a proposal for a patch that implements the new ARM relocations
> needed to support execute-only code segment for thumb1 code sequences.
>
> Some STM32 MCUs implement a security feature called 'Proprietary Code
> Read-Out Protection' aka PCROP that forbids data read access to some
> code areas (only fetch access is allowed).
> This protection prevents usage of literal pools (since one cannot load
> data from code sections), so compilers have to use specific code
> sequence to generate constants. This is particularly a problem to
> generate symbol addresses.
>
> For armv7-m architecture compilers can generate movt/movw pair
> instructions relying on R_ARM_THM_MOVW_ABS_NC and R_ARM_THM_MOVT_ABS
> relocations support.
> For armv6-m there is currently no possibility to generate such symbol
> addresses.
> This is why the ARM ABI defines four new relocations that allow
> symbol address generation in armv6-m architecture using a sequence of
> movs/adds/lsls instructions.
>
> This patch implements this feature in gas and bfd.
>
> Unit tests have be written to check correct implementation for both
> gas and bfd.
> No regressions have been observed for arm-linux-gnueabi,
> arm-linux-gnueabihf, arm-none-eabi, arm-none-nacl and
> armeb-linux-gnueabihf targets on 64 bits linux host.
>


I am not happy about directives like low_low, low_high, high_low and
high_high - without thinking much about it, I would much rather see
something like lower0_8 , lower8_16 , upper0_8 and upper8_16 in line
with the pre-existing lower16 and upper16 directives.

regards
Ramana




> Changelogs:
>
> include/ChangeLog:
>
> 2015-11-17  Mickael Guene <mickael.guene@st.com>
>
>         * elf/arm.h: Add new arm relocations.
>
> bfd/ChangeLog:
>
> 2015-11-17  Mickael Guene <mickael.guene@st.com>
>
>         * bfd-in2.h: Regenerate.
>         * reloc.c: Add new relocations.
>         * libbfd.h (bfd_reloc_code_real_names): Add new relocations display
>         names.
>         * elf32-arm.c (elf32_arm_howto_table_1): Add HOWTO for new
> relocations.
>         (elf32_arm_reloc_map): Add bfd/arm mapping for new relocations.
>         (elf32_arm_final_link_relocate): Implement new relocations
> resolution.
>
> gas/ChangeLog:
>
> 2015-11-17  Mickael Guene <mickael.guene@st.com>
>
>         * config/tc-arm.c (group_reloc_table): Add mapping between gas
> syntax
>         and new relocations.
>         (do_t_add_sub): Keep new relocations for add operand.
>         (do_t_mov_cmp): Keep new relocations for mov operand.
>         (insns): Use 'shifter operand with possible group relocation'
> operand
>         parse code for movs operand.
>         (md_apply_fix): Implement mov and add encoding when new relocations
>         on them.
>         (tc_gen_reloc): Add new relocations.
>         (arm_fix_adjustable): Since offset has a limited range ([0:255]) we
>         disable adjust_reloc_syms() for new relocations.
>
> gas/testsuite/ChangeLog:
>
> 2015-11-17  Mickael Guene <mickael.guene@st.com>
>
>         * gas/arm/adds-thumb1-reloc-local.d: New
>         * gas/arm/adds-thumb1-reloc-local.s: New
>         * gas/arm/movs-thumb1-reloc-local.d: New
>         * gas/arm/movs-thumb1-reloc-local.s: New
>
> ld/testsuite/ChangeLog:
>
> 2015-11-17  Mickael Guene <mickael.guene@st.com>
>
>         * ld-arm/arm-elf.exp (armelftests_common): Add new relocations
> tests.
>         * ld-arm/thumb1-adds.d: New
>         * ld-arm/thumb1-adds.s: New
>         * ld-arm/thumb1-movs.d: New
>         * ld-arm/thumb1-movs.s: New