[RFC PATCH, binutils, ARM 8/11] Add support for ARMv8-M Secure Gateway veneer generation

[Thomas Preudhomme]

Hi Richard,

On Thursday 07 July 2016 11:52:56 Richard Earnshaw wrote:
> +   As per "ARMv8-M Security Extensions: Requirements on Development Tools"
> +   document, a secure gateway veneer is needed when there exists a
> non-local
> +   function symbol called "normal" symbol (eg. foo) with the same value
> as a
> +   symbol with the same type, binding a name save for a __acle_se_ prefix,
> +   called a "special" symbol (eg. __acle_se_foo).  Entry functions handling
> +   with secure state transition by themselves have these symbols with
> different
> +   values.
> 
> 
> This is very unclear as to what is being/needs to be done.  Can you try
> redrafting it?

What do you think of the attached updated patch?

By the way, I realized I did not copy over the explanation for the changes to 
elf32_arm_get_plt_info from the original patch submission. I'm adding the full 
cover letter again here to have all the information in one email in the 
archive.



This patch is part of a patch series to add support for ARMv8-M security 
extension[1] to GNU ld. This specific patch adds support for creating ARMv8-M 
Secure Gateway veneers.

ARM v8-M security extensions require [3] secure gateway veneers to be 
generated for (secure) entry function in order for code to transition from 
non-secure state to secure state when calling these entry function. Unlike 
other veneers, these veneers are generated independently of relocations, ie a 
veneer can be generated in the absence of relocation. The condition for the 
generation is that the normal symbol (the one whose name is the same as in C) 
of an entry function has the same value as the special symbol (normal symbol 
prefixed with "__acle_se_"). When that happens, the normal symbol is rebound to 
the veneer generated. When the two symbols have different value it indicates 
that the entry function already contains an sg instruction to do the secure 
state transition and the normal symbol points to the sg instruction while the 
special symbol points after that.

This patch also makes use of the infrastructure laid out in previous patches 
to control the address of these veneers and to avoid the presence of the bit 
pattern of the SG instruction in non secure callable memory, as required [4]
[5].

Finally, the patch also contains a small change to elf32_arm_get_plt_info () 
to return FALSE when there is no PLT, ensuring that a NULL splt is not 
dereferenced in the block starting with "If the call goes through" in 
elf32_arm_final_relocate (). This was not necessary before because root_plt-
>offset is set to -1 in elf32_arm_adjust_dynamic_symbol called by 
_bfd_elf_adjust_dynamic_symbol from bfd_elf_size_dynamic_sections when dynobj 
is not NULL. However, dynobj is set in elf32_arm_check_relocs which is not 
called when there is no relocation in the input section. Such a situation is 
possible while still invoking elf32_arm_final_relocate () due to SG veneers 
being created in the absence of relocation but needing themselves relocation.

[1] Software requirements for ARMv8-M security extension are described in 
document ARM-ECM-0359818 [2]
[2] Available on http://infocenter.arm.com in Developer guides and articles > 
Software development > ARM®v8-M Security Extensions: Requirements on 
Development Tools
[3] See section 3.4.3 and requirement 44 of ARM-ECM-0359818 [2]
[4] requirement 14 and following comment of ARM-ECM-0359818 [2]
[5] requirement 12 and 13 and following comment of ARM-ECM-0359818 [2]


ChangeLog entries remain unchanged:

*** bfd/ChangeLog ***

2016-05-04  Thomas Preud'homme  <thomas.preudhomme@arm.com>

        * elf32-arm.c (CMSE_PREFIX): Define macro.
        (elf32_arm_stub_cmse_branch_thumb_only): Define stub sequence.
        (cmse_branch_thumb_only): Declare stub.
        (struct elf32_arm_link_hash_table): Define cmse_stub_sec field.
        (elf32_arm_get_plt_info): Add globals parameter.  Use it to return
        FALSE if there is no PLT.
        (arm_type_of_stub): Adapt to new elf32_arm_get_plt_info signature.
        (elf32_arm_final_link_relocate): Likewise.
        (elf32_arm_gc_sweep_hook): Likewise.
        (elf32_arm_gc_mark_extra_sections): Mark sections holding ARMv8-M
        secure entry functions.
        (arm_stub_is_thumb): Add case for arm_stub_cmse_branch_thumb_only.
        (arm_dedicated_stub_output_section_required): Change to a switch case
        and add a case for arm_stub_cmse_branch_thumb_only.
        (arm_dedicated_stub_output_section_required_alignment): Likewise.
        (arm_stub_dedicated_output_section_name): Likewise.
        (arm_stub_dedicated_input_section_ptr): Likewise and remove
        ATTRIBUTE_UNUSED for htab parameter.
        (arm_stub_required_alignment): Likewise.
        (arm_stub_sym_claimed): Likewise.
        (arm_dedicated_stub_section_padding): Likewise.
        (cmse_scan): New function.
        (elf32_arm_size_stubs): Call cmse_scan for ARM M profile targets.
        Set stub_changed to TRUE if such veneers were created.
        (elf32_arm_swap_symbol_in): Add detection code for CMSE special
        symbols.


*** include/elf/ChangeLog ***

2015-12-16  Thomas Preud'homme  <thomas.preudhomme@arm.com>

        * arm.h (ARM_GET_SYM_CMSE_SPCL): Define macro.
        (ARM_SET_SYM_CMSE_SPCL): Likewise.


*** ld/ChangeLog ***

2016-02-17  Thomas Preud'homme  <thomas.preudhomme@arm.com>

        * ld.texinfo (Placement of SG veneers): New concept entry.
        * testsuite/ld-arm/arm-elf.exp
        (Secure gateway veneers: no .gnu.sgstubs section): New test.
        (Secure gateway veneers: wrong entry functions): Likewise.
        (Secure gateway veneers (ARMv8-M Baseline)): Likewise.
        (Secure gateway veneers (ARMv8-M Mainline)): Likewise.
        * testsuite/ld-arm/cmse-veneers.s: New file.
        * testsuite/ld-arm/cmse-veneers.d: Likewise.
        * testsuite/ld-arm/cmse-veneers.rd: Likewise.
        * testsuite/ld-arm/cmse-veneers.sd: Likewise.
        * testsuite/ld-arm/cmse-veneers-no-gnu_sgstubs.out: Likewise.
        * testsuite/ld-arm/cmse-veneers-wrong-entryfct.out: Likewise.


Best regards,

Thomas
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