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

[mickael guene <mickael dot guene at st dot com>]

Hi Ramana,

 Thanks for your feedback. I agree with you about gas
directives names. Your names proposal are better than
mines, but I'm still not happy with them since for instance
'lower0_8' seems to imply  bit  numbers (but then should be lower0_7)
whereas pre-existing 'lower16' seems to imply a size. If someone
has a better idea for those names, feel free to
contribute !!!

Regards
Mickael

On 12/01/2015 04:31 PM, Ramana Radhakrishnan wrote:
> 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