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TM4C129ENCPDT: Implement flash unreadable protection using GCC compiler

Mike Chou
Prodigy 20 points
Part Number: TM4C129ENCPDT

Hi All,

I have a long used project based on GCC compiler(GNU Tools ARM Embedded 4.9 2015q3) directly. (No use the CCS IDE and Arm® code generation tools – compiler  )

Recently, I start to consider the security issue, so the Flash unreadable protection operation is added.

After I configure the specified Flash to "Execute Only", then using the JTag to upload it, the value of the specified range is "0".

I think the protection is work!

But the chip will crash when the program execute the protected range. 

I refer the below document to test the flash protection feature based on CCS 10.

https://www.ti.com/lit/an/spma044a/spma044a.pdf

It works normal and the chip not crash when execute the protected range. 

I think the different of them is that I skip the step of adjusting compiler parameter  in first project. 

Then I recover the "Specify whether constants can be embedded in the code section (--embedded_constants)" to On in the CCS test project.

The chip also crash when executing the protected flash.

I read the description, and I realize this step is necessary. 

After I search the "--embedded_constants", all document are based on "Arm® code generation tools – compiler"

I try to adjust the configuration refer the GCC compiler manual, but it no success.

gcc.gnu.org/.../Option-Summary.html

Could anyone know how to configure the GCC compiler to implement the "--embedded_constants" feature in the "Arm® code generation tools – compiler"?

Best regards.

  • 0
    Guru 92251 points

    Mike Chou said:
    Could anyone know how to configure the GCC compiler to implement the "--embedded_constants" feature in the "Arm® code generation tools – compiler"?

    From looking at https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html try -mpure-code

  • 0 in reply to Chester Gillon
    TI__Guru**** 162305 points

    Chester's answer makes sense to me but I am not a GCC expert. 

    Here are the descriptions of each option to compare.

    GCC

    -mpure-code

    Do not allow constant data to be placed in code sections. Additionally, when compiling for ELF object format give all text sections the ELF processor-specific section attribute SHF_ARM_PURECODE. This option is only available when generating non-pic code for M-profile targets.

    TI Arm compiler (armcl)

    --embedded_constants={on|off}

    By default the compiler embeds constants in functions. These constants can include literals, addresses, strings, etc. This is a problem if you wants to prevent reads from a memory region that contains only executable code. To enable the generation of "execute only code", the compiler provides the --embedded_constants=[on|off] option. If the option is not specified, it is assumed to be on. The option is available on the following devices: Cortex-A8, Cortex-M3, Cortex-M4, and Cortex-R4.

  • 0 in reply to Chester Gillon
    Prodigy 20 points

    Hi Chester & John,

    It works now, thanks a lot.

    I want to remind one thing.

    The TM4C129ENCPDT is belong to ARM Cortex-M.

    "GNU Tools ARM Embedded Toolchain" start to support "-mpure-code" after v6 2016q4 version.

    Please refer:

    https://launchpad.net/gcc-arm-embedded/+announcement/14413

    + ARM PURECODE support for ARMv7-M and ARMv8-M Mainline

    Thanks again for rapidly assistance.

    Best regards.