arm64 code looking for variable rodata_enabled when STRICT_KERNEL_RWX = n

[Chan Kim]

Hello all,

This is what I see when I search "KERNEL_RWX" during "make menuconfig" for
arm64 kernel(5-10.0-rc5).  

The Kconfig file says CONFIG_STRICT_KERNEL_RWX is for setting text and
rodata read-only.

       

     Symbol: ARCH_OPTIONAL_KERNEL_RWX [=n]   

     Type  : bool   

     Defined at arch/Kconfig:928   

 

     Symbol: ARCH_OPTIONAL_KERNEL_RWX_DEFAULT [=n]   

     Type  : bool   

     Defined at arch/Kconfig:931   

 

     Symbol: STRICT_KERNEL_RWX [=y]   

     Type  : bool   

     Defined at arch/Kconfig:937   

       Prompt: Make kernel text and rodata read-only   

       Depends on: ARCH_HAS_STRICT_KERNEL_RWX [=y]   

       Visible if: ARCH_HAS_STRICT_KERNEL_RWX [=y] &&
ARCH_OPTIONAL_KERNEL_RWX [=n]   

       Location:   

     (1) -> General architecture-dependent options   

 

I wanted to try setting STRICT_KERNEL_RWX to =n. It says this option is
visible when ARCH_OPTIONAL_KERNEL_RWX is =y which is now =n. 

So I modified to ARCH_OPTIONAL_KERNEL_RWX=y in arch/Kconfig line 928. (BTW,
is it correct to modify this Kconfig file directly? I'm not sure at the
moment)  

Then I could see the STRICT_KERNEL_RWX menu in the menuconfig and I set it
to =n as I wanted.  

But when I build the kernel, I see this errors.  

 

    ckim at ckim-ubuntu:~/ProjX/LinuxDevDrv/kernel-release-RD-INFRA-2020.11.30$
makeit

      CALL    scripts/atomic/check-atomics.sh

      CALL    scripts/checksyscalls.sh

      CHK     include/generated/compile.h

      CC      arch/arm64/mm/mmu.o

    arch/arm64/mm/mmu.c: In function 'parse_rodata':

    arch/arm64/mm/mmu.c:595:28: error: 'rodata_enabled' undeclared (first
use in this function)

      595 |  int ret = strtobool(arg, &rodata_enabled);

          |                            ^~~~~~~~~~~~~~

    arch/arm64/mm/mmu.c:595:28: note: each undeclared identifier is reported
only once for each function it appears in

    arch/arm64/mm/mmu.c: In function 'map_entry_trampoline':

    arch/arm64/mm/mmu.c:614:18: error: 'rodata_enabled' undeclared (first
use in this function)

      614 |  pgprot_t prot = rodata_enabled ? PAGE_KERNEL_ROX :
PAGE_KERNEL_EXEC;

          |                  ^~~~~~~~~~~~~~

    arch/arm64/mm/mmu.c: In function 'map_kernel':

    arch/arm64/mm/mmu.c:669:23: error: 'rodata_enabled' undeclared (first
use in this function)

      669 |  pgprot_t text_prot = rodata_enabled ? PAGE_KERNEL_ROX :
PAGE_KERNEL_EXEC;

          |                       ^~~~~~~~~~~~~~

    make[2]: *** [scripts/Makefile.build:283: arch/arm64/mm/mmu.o] Error 1

    make[1]: *** [scripts/Makefile.build:500: arch/arm64/mm] Error 2

    make: *** [Makefile:1799: arch/arm64] Error 2

 

variable "rodata_enabled" is defined in init/main.c as below. 

 

    #if defined(CONFIG_STRICT_KERNEL_RWX) ||
defined(CONFIG_STRICT_MODULE_RWX)

    bool rodata_enabled __ro_after_init = true;

    static int __init set_debug_rodata(char *str)

    {

        return strtobool(str, &rodata_enabled);

    }

    __setup("rodata=", set_debug_rodata);

    #endif

 

But now since CONFIG_STRICT_KERNEL_RWX=n, it is not defined
here(CONFIG_STRICT_MODULE_RWX=n too). 

and arch/arm64/mm/mmu.c code is still using `rodata_enabled`. Is this a bug
of the code? Or am I missing something?

 

I can modify init/main.c and include/linux/init.h so that this
rodata_enabled and related functions be defined regardless of these CONFIG
values and make the errors go away, but I'm curious if this a kind of kernel
bug raising compiler error.

Any comment will be really appreciated.

Thank you!

 

Chan Kim

 

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